A SUMMARY OF 

KNOWLEDGE FROM THE

United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical 

Report

PNW-GTR-854

November 2011

Synthesis of Knowledge of 

Extreme Fire Behavior: 

Volume I for Fire Managers

Paul A. Werth, Brian E. Potter, Craig B. Clements, Mark A. Finney, 

Scott L. Goodrick, Martin E. Alexander, Miguel G. Cruz, Jason A. 

Forthofer, and Sara S. McAllister
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Authors

Paul A. Werth is a fire weather meteorologist (retired), Northwest Interagency 

Coordination Center, 150 SW Harrison Street, Suite 400, Portland, OR 97201. Brian E. 

Potter is a research meteorologist, U.S. Department of Agriculture, Forest Service, Pacific 

Wildland Fire Sciences Laboratory, 400 N 34th St., Suite 201, Seattle, WA 98103. Craig 

B. Clements is an assistant professor of meteorology, San Jose State University, San José, 

CA 95192. Mark A. Finney is a research forester, Jason A. Forthofer is a mechanical 

engineer, and Sara S. McAllister is a research mechanical engineer, U.S. Department of 

Agriculture, Forest Service, Missoula Fire Sciences Laboratory, 5775 Highway 10 West, 

Missoula, MT 59808. Scott L. Goodrick is a research meteorologist, U.S. Department 

of Agriculture, Forest Service, Southern Research Station, 320 Green Street, Athens, 

GA 30602. Martin E. Alexander is an adjunct professor of wildland fire science and 

management, Department of Renewable Resources and Alberta School of Forest Science 

and Management, University of Alberta, Edmonton, AB T6G 2H1, Canada. Miguel 

G. Cruz is a senior research scientist, Bush Fire Dynamics and Applications, CSIRO 

Ecosystems Sciences–Climate Adaptation Flagship, GPO Box 284, Canberra, ACT 2601, 

Australia.

Cover Photos

Cover photos by Rick Trembath, USDA Forest Service (retired).
Synthesis of Knowledge 

of Extreme Fire Behavior: 

Volume I for Fire 

Managers 

Paul A. Werth, Brian E. Potter, 

Craig B. Clements, Mark A. Finney, 

Scott L. Goodrick, Martin E. Alexander, 

Miguel G. Cruz, Jason A. Forthofer, 

and Sara S. McAllister

U.S. Department of Agriculture, Forest Service

Pacific Northwest Research Station

Portland, Oregon 

General Technical Report, PNW-GTR-854

November 2011

Published in cooperation with: 

U.S. Department of Agriculture, Forest Service 

Southern Research Station
GENERAL TECHNICAL REPORT PNW-GTR-841

ii

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Abstract

Werth, Paul A.; Potter, Brian E.; Clements, Craig B.; Finney, Mark A.; Goodrick, 

Scott L.; Alexander, Martin E.; Cruz, Miguel G.; Forthofer, Jason A.; McAllister, 

Sara S. 2011. Synthesis of knowledge of extreme fire behavior: volume I for fire 

managers. Gen. Tech. Rep. PNW-GTR-854. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Research Station. 144 p.

The National Wildfire Coordinating Group definition of extreme fire behavior (EFB) indicates 

a level of fire behavior characteristics that ordinarily precludes methods of direct control 

action. One or more of the following is usually involved: high rate of spread, prolific crowning/spotting, presence of fire whirls, and strong convection column. Predictability is difficult 

because such fires often exercise some degree of influence on their environment and behave 

erratically, sometimes dangerously. Alternate terms include “blow up” and “fire storm.”

Fire managers examining fires over the last 100 years have come to understand many 

of the factors necessary for EFB development. This work produced guidelines included in 

current firefighter training, which presents the current methods of predicting EFB by using 

the crown fire model, which is based on the environmental influences of weather, fuels, and 

topography. 

Current training does not include the full extent of scientific understanding. Material 

in current training programs is also not the most recent scientific knowledge. National Fire 

Plan funds have sponsored newer research related to wind profiles’ influence on fire behavior, 

plume growth, crown fires, fire dynamics in live fuels, and conditions associated with vortex 

development. Of significant concern is that characteristic features of EFB depend on conditions undetectable on the ground, relying fundamentally on invisible properties such as wind 

shear or atmospheric stability.

Obviously no one completely understands all the factors contributing to EFB because of 

gaps in our knowledge. These gaps, as well as the limitations as to when various models or 

indices apply should be noted to avoid application where they are not appropriate or warranted. This synthesis will serve as a summary of existing extreme fire behavior knowledge 

for use by fire managers, firefighters, and fire researchers.

The objective of this project is to synthesize existing EFB knowledge in a way that 

connects the weather, fuel, and topographic factors that contribute to development of EFB. 

This synthesis will focus on the state of the science, but will also consider how that science is 

currently presented to the fire management community, including incident commanders, fire 

behavior analysts, incident meteorologists, National Weather Service office forecasters, and 

firefighters. It will seek to clearly delineate the known, the unknown, and areas of research 

with the greatest potential impact on firefighter protection.

Keywords: Extreme fire behavior, fuels, fire behavior.
iv

Preface

In 2008, the National Wildfire Coordinating Group (NWCG) Fire Behavior Committee 

(FBC) asked the Joint Fire Science Program (JFSP) to fund a synthesis and review of the 

scientific literature pertaining to extreme fire behavior (EFB). In September 2008, the JFSP 

announced a call for proposals that included a request for “an examination of the state of 

the science underlying predictions of extreme fire behavior, and an assessment of the appropriate uses and limits of this information.” This document is the result of that request.

In performing the review, it became progressively clearer that the concept of extreme 

fire behavior (EFB) is vaguely defined and means something different to everyone. The 

authors examined the official NWCG definition and solicited input from the management 

community to develop a definition that was both operationally useful and scientifically 

tractable. This definition and the initial stages of the review eventually led to the recognition that some relevant topics had not been included in the original outline. Other topics 

from the original outline were expanded to include sections of their own.

The authors communicated these changes to both the JFSP and the FBC as they arose. 

In those conversations, it became apparent that these two groups had different needs. The 

JFSP needed something for fire managers and others without the technical background of 

a fire behavior analyst. The FBC needed a document for fire behavior analysts that would 

allow them to better understand the use and limitations of the tools they now have and 

may have in the near future. To meet these two needs, this review has two parts. Volume 

1 summarizes the state of the science for fire managers and firefighters with pertinent 

references to scientific papers. It is intended to be of use to anyone who works at or near the 

fireline. Volume 2 covers the same topics (with one exception) in more detail and includes 

information necessary for fire behavior analysts to understand what is scientifically known, 

what science lies behind the tools they have, and what the limitations are on scientific 

knowledge and tools. It includes more references to scientific literature. The one difference 

in topical content between the volumes is that volume 2 includes a chapter on fuel dynamics 

and volume 1 does not. As the study progressed, the scope of this topic led to the need to 

include more experts, and the short time available precluded that section from publication 

in volume 1.
v

Summary

A working definition of extreme fire behavior (EFB) was necessary for development of this 

synthesis. Because the subjective nature of four of the five properties of the EFB definition 

established by the National Wildfire Coordination Group makes the definition intractable 

for scientific purposes, the lead authors asked the fire behavior community for input on 

possible definitions of EFB and examples of phenomena they considered EFB. The only 

coherent theme was that EFB is not steady state. After discussing responses, the authors 

agreed on the following working definition for this project:

Fire spread other than steady surface spread, especially when it involves rapid 

increases.

This definition of EFB does not emphasize any one element of the behavior triangle.

Complexity

It is imperative that fire managers understand that much of what is referred to as “extreme 

fire behavior” is happening where it cannot be seen. Multiple factors come into play and not 

all factors need be present for EFB to occur. No one factor must be present in every case.

A number of interactions among the elements are noted, but the number of possible 

interactions between elements is practically unlimited, making research and the resulting 

tool development a key step in achieving successful forest management and safety.

Myths and Lore

There are many myths and lore with limited scientific basis. Anecdotal evidence sometimes 

takes the place of science and comes to be accepted as fact even when little scientific information exists to validate it. Extreme fire behavior can occur on any scale, great or small, in 

any fuel type, and at any time of the day or night. There is no time or circumstance when 

fire managers can safely assume EFB will not occur.

Over-Arching Gaps

The authors of this synthesis have identified areas in each chapter where understanding of 

the science is lacking and more research is needed. These knowledge gaps may pertain to 

just one chapter’s topic, but they are nonetheless important areas in which further research 

would be of value to the operational community. There are, however, certain over-arching 

gaps where additional research of one element will advance the science for other elements 

as well.

• A greater recognition of the importance of plume dynamics to EFB and spotting.

• Advances in the understanding of fuel structure, especially as it relates to ember 

production and crown fire.

• Better high-resolution observations on windflow in complex terrain to improve wind 

models used in fire behavior and spotting tools, and to identify fire whirl potential. For 

example, upper air soundings on project-size fires.

• The influence of ambient winds or topography on fire interactions.
vi

•	 More research beyond the Haines Index to quantify the effects of atmospheric stability

on fire behavior.

New and expanded research into these areas will increase the understanding of the

science on which they are based and are a necessary starting point for enhanced wildland

fire management and advances in firefighter training and safety.

Operational implications

Even the most advanced tools and models are limited by their design and assumptions.

They can never, nor should they be expected to, take the place of direct observations one

makes on the fireline, such as the “L” in LCES (Lookouts-Communications-Escape RoutesSafety Zones) and the concept of ”situational awareness.” Scientifically sound application of

tools and models requires that the tools/models be used within their design limitations and

in accordance with the tool assumptions.

Current training identifies circumstances that can result in extreme fire behavior,

where increased awareness of multiple factors can guide fire managers to make decisions.

Research can lead to development of additional or improved tools to help fire managers

better identify those situations where extreme fire behavior may occur. The lack of a tool

or model for a situation seen in the field does not mean EFB cannot occur. Knowing what

conditions can lead to EFB, and knowing that you do not know whether those conditions

exist, can be more important than any tools or models. Extreme fire behavior can occur on

any fire.

The state of the science at present can

be summed up as follows:

•	 Fire is three dimensional and is not

steady state.

•	 The tools available to us today are

two dimensional and are predominantly steady state.

•	 Additional research into EFB may

one day result in development of

three-dimensional tools.
vii

Contents

1 Chapter 1: Introduction

	 	 	 Brian E. Potter and Paul A. Werth

1 Definition

2 Methods

3 Liturature Cited

5 Chapter 2: Effects of Complex Terrain on Extreme Fire Behavior

	 	 Craig B. Clements

5 Introduction

5 Wind Systems in Mountainous Terrain

6 Dynamically Driven Winds

10 Thermally Driven Winds

16 Wind Modeling Tools: WindNinja

17 Summary

18 Future Needs

19 Literature Cited

25 Chapter 3: Critical Fire Weather Patterns

	 	 Paul A. Werth

25 Introduction

25 Weather Elements That Promote Extreme Fire Behavior

27 Critical Fire Weather Patterns 

29 Regional Critical Fire Weather Patterns

30 Northern Plains, Great Lakes, and the Northeastern United States

31 Southeastern United States

32 Southwestern United States

34 Rocky Mountain and Intermountain Regions

37 Pacific Northwest Region 

39 California Region

42 Alaska

43 Models and Predictive Tools

43 The National Wildland Significant Fire Potential Outlook

43 GACC 7-Day Significant Fire Potential

45 Fuel and Fire Behavior Advisories

45 Other GACC Products and Services

45 Fire Weather Watches and Red Flag Warnings

45 Spot Weather Forcasts/Digital Web Services

45 Summary/Knowledge Gaps

46 References

49 Chapter 4: Fire Interactions and Mass Fires

	 	 Mark A. Finney and Sara S. McAllister

49 Introduction

50 Background: Time-Dependent Fire Behaviors

50 Fire Acceleration
GENERAL TECHNICAL REPORT PNW-GTR-854

viii

51 Length of Fire Front

52 Flame Tilt

53 Spread Thresholds

53 Conditions Where Fire Interactions Occur

54 Specific Effects of Fire Interation

54 Burning Rate

56 Flame Dimensions

58 Flame Temperatures and Pollutants

59 Indraft Velocity

61 Pulsation

62 Convection Column

63 Summary of Interaction Effects

63 Needs for Further Research and Application

64 Literature Cited

73 Chapter 5: Column/Plume Dynamics

	 	 Brian E. Potter

73 Introduction

74 Plume-Dominated and Wind-Driven Fires

74 Adverse Wind Profiles and Low-Level Jets

75 Stability and Instability

76 Downbursts and Plume Collapse

78 Summary

78 Literature Cited

81 Chapter 6: Spot Fires

	 	 Brian E. Potter

81 Introduction

81 The Spotting Process

83 Management Tools

85 Knowledge Gaps

86 Literature Cited

89 Chapter 7: Vortices and Wildland Fire

	 	 Jason A. Forthofer and Scott L. Goodrick

89 Introduction

89 Vorticity Basics

90 Fire Whirls

91 Fire Whirl Physics

91 Vorticity Sources

92 Vortex Stretching

93 Increased Combustion Rates

93 Scaling Fire Whirls

93 Fire Whirls in the Real World: Common Features

93 Whirl Shedding on the Lee Side of a Plume 
ix

	 94	 L-Shaped Heat Source in Crossflow

94	 Vorticity Associated With Cold Fronts

	 94	 Multiple Interacting Plumes

	 95	 Lee Side of a Hill/Mountain

	 95	 Horizontal Vortices

	 96	 Transverse Vortices

	 96	 Longitudinal Vortices

	 99	 Tree Crown Streets

	100	 Summary

	101	 Knowledge Gaps

	102	 Literature Cited

	107 Chapter 8: Crown Fire Dynamics in Conifer Forests

	 	 Martin E. Alexander and Miguel G. Cruz

	107 Introduction

	108	 Types of Crown Fires

	111	 Crown Fire Initiation 

	115	 Crown Fire Propagation

	115	 Crown Fire Rate of Spread

	116	 Crown Fire Intensity and Flame Zone Characteristics

	117	 Crown Fire Area and Perimeter Growth

119	 Crown Fire Spotting Activity

	121	 Models, Systems, and Other Decision Aids for Predicting Crown Fire Behavior

121 Rothermel Guide to Predicting Size and Behavior of Crown Fires

	122	 U.S. Fire Modeling Systems

	124	 Canadian Forest Fire Behavior Prediction System

	125	 Other Empirically Based Approaches

	126	 Physically Based Models

	129	 Example of a Practical Application of Linking Empirical and Physically Based

	 	 Models

	130	 Implications for Fire and Fuel Management

	134	 Future Outlook 

	135	 Literature Cited

	143	 Acknowledgments

	143	 Unit Conversion Factors

	144 Common and Scientific Names
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Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

1

Chapter 1: Introduction

Brian E. Potter and Paul A. Werth1

The idea of “extreme fire behavior” (EFB) is commonplace 

in the U.S. wildland fire community. It goes back, arguably, 

to the 1950s and the idea of a “blow-up” fire presented by 

George Byram. Byram (1954) listed the terms “blow up,” 

“conflagration” and “erratic” as descriptors of “unusual 

high-intensity fires.” He also used the phrase “extreme 

fire behavior” in both his 1954 paper and in his chapters 

in Davis (1969). Larger fires may be more likely to display 

these characteristics, he noted, but they can occur on a fire 

of any size. Since then, the concept and terms have become 

widely used.

In spite of this widespread use and implied understanding of what constitutes EFB, there is no documented, 

critical examination of the types of fire behavior people 

consider “extreme.” Furthermore, whereas there is little 

question that the behavior labeled as EFB by observers 

occurs, there are numerous explanations for that behavior 

that are now conventional wisdom, yet without any scientific support—the phenomenon is rarely in question, but the 

explanation may be. Actions based on incorrect explanations of EFB can result in death or injury.

The primary goal of this synthesis is to summarize 

what is known scientifically about matters considered 

EFB. That summary is presented to provide the most value 

possible to the operational fire management community. 

Research papers, although increasingly available to everyone, are not necessarily understandable by everyone. They 

contain substantial jargon and math, and may only summarize their findings in terms of basic science. This synthesis 

distills the scientific information and provides references 

to the research papers. Note that science is a process of 

proposing possible explanations, and subsequently ruling 

out those explanations that contradict evidence. It is easy 

to propose explanations, but proving them wrong can be 

easy or difficult. An explanation that is repeatedly tested, 

compared to observations, and never contradicted is not 

necessarily true, but the more it is tested, the more confidence scientists have that it may be, true. In the case of 

EFB, hard, reliable data are rare, making it very difficult 

to confidently refute a proposed explanation. Rather, it is 

much more common to be able to cite scientific reasons for 

greater or lesser confidence in the proposed explanation. In 

this synthesis and review, the authors hope to present what 

hard evidence there is, and when there is none, to provide 

an understanding of the strong and weak points in a given 

explanation.

Definition

A working definition was necessary to begin and execute 

the synthesis. Without it, the task of gathering and summarizing would be unbounded and impossible to complete. 

There is no single scientific paper that laid out a scientific 

definition of EFB. The only official or specific definition of 

EFB is established by the National Wildfire Coordination 

Group (NWCG) glossary of wildland fire terminology 

(NWCG, n.d.): 

“Extreme” implies a level of fire behavior 

characteristics that ordinarily precludes methods 

of direct control action. One or more of the following 

is usually involved: high rate of spread, prolific 

crowning and/or spotting, presence of fire whirls, 

strong convection column. Predictability is difficult 

because such fires often exercise some degree of 

influence on their environment and behave erratically, 

sometimes dangerously.

Of the five properties “usually involved,” four are 

subjectively described as “high,” “prolific,” or “strong.” 

This makes the definition intractable for scientific purposes. 

Furthermore, the definition implies a need to fail at direct 

control in order to designate the fire behavior as “extreme.” 

1 Brian E. Potter, U.S. Department of Agriculture, Forest Service, 

Pacific Wildland Fire Sciences Laboratory, Seattle, WA.

Paul A. Werth, Weather Research and Consulting Services, LLC, 

Battle Ground, WA.
GENERAL TECHNICAL REPORT PNW-GTR-854

2

that there were too many individual phenomena considered 

EFB for a definition to include any sort of list. Furthermore, 

most tractable definitions included some level of subjectivity. In the end, the agreed definition for this project was:

Fire spread other than steady surface spread, 

especially when it involves rapid increases.

This definition includes most or all of the phenomena listed 

above, although admittedly indirectly in some cases. It 

does include some subjectivity, as “rapid” can be a matter 

of opinion. However, this is not the core of the definition—

it is included to emphasize the safety and operational 

importance of increasing spread as opposed to decreasing 

or unusually slow spread. Furthermore, whereas the NWCG 

definition heavily leans toward atmospheric conditions and 

may underrepresent the importance of fuels and topography, 

this definition does not emphasize any one element of the 

behavior triangle.

Methods

The authors divided the work of synthesis and review based 

on expertise. The division is necessary to the synthesis, 

but it is also artificial, and the various sections overlap 

substantially. Many areas of overlap are explicitly noted, 

and readers will undoubtedly see other areas.

The review incorporated three primary sources of 

information. First and foremost was the peer-reviewed 

scientific literature. This is the most authoritative source 

of information to support or refute any explanation of what 

causes EFB. Second was feedback from and interaction 

with practitioners. The project Web site allowed reader 

comments and discussion, and, when appropriate, these 

guided the review. The third source was documents that are 

not peer reviewed—often referred to as “grey literature.” 

Peer review was the exception to the rule for many years in 

the field of forest fire research, so there is an extensive body 

of literature that was not peer reviewed. The problem with 

grey literature is that it has not been tested or widely available, so the scientific rigor of its content is unknown. It can, 

however, provide insight and information, and the authors 

did not want to ignore it.

This makes EFB a function of control success or failure, not 

an objective, physical process.

At the initiation of this project, the lead authors asked 

the fire behavior community for input on possible definitions of EFB and examples of phenomena they considered 

EFB, whether those examples matched the NWCG definition or not. Several people responded—mostly with 

examples—either via email or through MyFireCommunity.

net, and the authors used that feedback in their initial discussion of the working definition. The phenomena listed in 

these responses included:

• Mass ignition.

• Actual plume dominance.

• Rapid exponential growth of spot fires.

• Spotting distances in miles.

• Things that just made me go, “Huh ... didn't expect 

that.” 

• Fire activity that has that momentum feedback 

character, like Jimi Hendrix putting the guitar up to 

the amp, and it just builds and builds feeding back 

on itself.

• When the fire and convection column induce high 

levels of turbulence into the wind field; when the 

momentum flow into the convection column is of 

the same order of magnitude as the momentum in 

the wind field.

• Very rapid fire spread.

• Three-dimensional fire.

• Fire behavior in which large changes take place 

rapidly.

• Flame attachment (the laying over and direct contact 

of flame with new fuels when there are steep slopes 

and strong winds).

The responses made it quite clear that operational users 

had thought about EFB well beyond any formal definition. 

They also recognized the difficulty of creating a precise 

definition that could be applied predictively, or a definition 

more concrete than “I know it when I see it.” After reviewing and discussing practitioner responses, the authors felt 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

3

Literature Cited

Byram, G.M. 1954. Atmospheric conditions related 

to blowup fires. Sta. Pap. 35. Asheville, NC: U.S. 

Department of Agriculture, Forest Service, Southeastern 

Forest Experiment Station. 34 p.

Davis, R.T. 1969. Atmospheric stability forecast and fire 

control. Fire Control Notes. 30(3): 3–4, 15. 

National Wildfire Coordinating Group [NWCG]. [N.d.].

Glossary of wildland fire terminology. http://www.nwcg.

gov/pms/pubs/glossary/index.htm. (June 1, 2011).
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Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

5

Introduction

Atmospheric processes in regions of complex terrain have 

received considerable interest in the research community for 

decades. Traditionally, the term “complex terrain” has been 

used to differentiate mountainous terrain from relatively flat 

and simple terrain. Research in mountain meteorology has 

its foundation in the Alps, and our present understanding 

of mountain circulations and the mountain atmosphere 

in general came from the early observational studies of 

Wagner (1938), Ekhart (1944), and Defant (1949).

The mountain meteorology research community most 

likely adopted the term “complex terrain” from the Atmospheric Studies in Complex Terrain the (ASCOT program) 

or which focused on observational campaigns of thermally 

driven circulations in valleys and, in particular, Colorado’s 

Brush Creek Valley (Whiteman 1990). 

A new classification of mountainous terrain by Meybeck et al. (2001) provided 15 relief patterns based on relief 

roughness and elevation. Relief roughness is defined as 

the difference between maximum and minimum elevation 

divided by half the length of cell used in the elevation 

data set (e.g., digital elevation model [DEM]). This terrain 

parameter is similar to the average slope typical of terrain 

classifications. Although Meybeck et al. determined many 

terrain types, they did not define any as complex terrain. 

Meybeck et al. classified mountains as terrain with elevations higher than 500 m and relief roughness greater than 

20 percent. One problem with this classification is that 

high plateaus are not mountains. Major river valleys can 

be incised into a high plateau such as the Grand Canyon. 

Although this is not “mountainous terrain,” it is complex. 

Most applicable to meteorological use of the term 

“complex terrain” is when defining the effect that land 

shape or topography has on meteorological measurements 

(Brode et al. 1987). These terrain effects include aerodynamic wakes, density-driven slope flows, channeling effects 

of upper level winds, and flow accelerations over the crest 

of mountain ridges. These flows affect the windspeed and 

wind direction measurements made in mountainous regions. 

For fire behavior applications, the term “complex 

terrain” is used to describe regions of relative relief and, in 

most cases, mountain topography. 

Wind Systems in Mountainous Terrain

Wind systems in mountainous terrain can be classified 

into two main types based on their forcing mechanisms: 

dynamically driven and thermally driven winds. Although 

thermally driven circulations occur more regularly in 

mountain terrain and are commonly experienced by 

hikers and climbers during fair weather conditions, it is 

the dynamically driven winds that can play a larger role in 

producing extreme fire behavior owing to their generally 

stronger surface wind velocities. However, the thermally 

driven circulations are subject to diurnal transition periods 

where atmospheric stability changes twice daily, potentially 

leading to extreme changes in observed fire behavior. This 

chapter will review the main mesoscale and local-scale 

wind systems observed in mountainous terrain that can 

potentially lead to extreme fire behavior. 

Dynamically Driven Winds

Dynamically driven winds are generally considered the 

strongest of the wind systems in mountainous terrain and 

include downslope windstorms such as foehn and Santa 

Ana winds, strong surface winds associated with mountain 

wave development, gap winds, and channeling of synopticscale winds. The factors that affect these terrain-forced 

winds as summarized by Whiteman (2000) are (1) the 

Chapter 2: Effects of Complex Terrain on Extreme Fire 

Behavior

Craig B. Clements1

1 Craig B. Clements, Department of Meteorology and Climate 

Science, San José State University, San José, CA.
GENERAL TECHNICAL REPORT PNW-GTR-854

6

stability of the air approaching the mountains, (2) the speed 

of the airflow, and (3) the characteristics of the underlying 

topography or mountain barrier. 

Foehn Winds—

One of the most important dynamically driven winds 

affecting fire behavior in mountainous terrain is the 

Chinook or foehn wind. Foehn winds (pronounced “firn”) 

are downslope wind events and are often associated with 

extreme fire behavior because of their near-surface high 

windspeeds, warm temperatures, and low relative humidities (Durran 1990, Whiteman 2000). As foehn develops, 

its onset can cause rapid changes in temperature and 

humidity because of adiabatic compression as air descends 

the lee side of mountain ranges. Extreme fire behavior can 

potentially occur during nighttime at the onset of a foehn 

event; strong winds will prevent nocturnal inversions from 

forming allowing nighttime temperatures to remain warmer 

(Whiteman 2000). Foehn winds can also start and stop suddenly, called a foehn pause (Whiteman 2000). The alternating wind break-in and cessation during a foehn event can 

cause air temperatures to oscillate sharply and can thus 

affect fire behavior. The foehn pause has been associated 

with changes in upstream conditions, including stability 

and cross-barrier windspeed that cause the wavelength of 

the waves to change (Whiteman 2000), and to lifting of 

the foehn wind by other local drainage flows (BaumannStanzer and Piringer 2004). 

Foehn winds are common in most mountainous regions 

around the world. In the lee of the Rocky Mountains of 

North America they are called Chinooks. The Chinook 

is most prevalent in winter months when strong westerly 

winds cross the Rockies (Whiteman 2000); however, when 

the synoptic conditions are right, Chinooks do occur during 

fire season (see the next chapter, “Critical Fire Weather 

Patterns”). 

In northern California, foehn winds that flow from the 

Great Basin over the Sierra Nevada to the Central Valley 

are known as north winds and in the region of Yosemite 

are called mono winds (Ruscha 1976). Even more localized 

in the San Francisco Bay area, these winds are known as 

Diablo winds. Foehn winds in the Cascade Mountains of the 

Pacific Northwest are called east winds as they blow from 

the east of the Cascades and descend becoming warmer and 

drier over the west slope of the mountain range. In Utah, 

the local foehn is known as the Wasatch wind as it descends 

from the higher elevations east of the Wasatch Mountains 

to the Salt Lake Valley. A comprehensive review of foehn 

winds of the Western United States is found in Whiteman 

(2000). 

Santa Ana winds—

The most notable foehn wind associated with extreme fire 

events is the Santa Ana of southern California. High windspeeds and extreme dryness associated with these episodes 

have been characterized as causing extreme fire behavior 

during fall in southern California. Barry (2008) stated that 

the Santa Ana develops as a result of high pressure over 

the Great Basin and development of a surface low off the 

southern California coast. An upper level trough to the east 

and a ridge in the eastern North Pacific cause the development of northerly flow.

Hughes and Hall (2009) suggested that the surface 

winds associated with Santa Ana events are produced by 

two mechanisms. When strong mid-tropospheric winds 

impinge on mountaintops in a stably stratified environment, 

gravity waves transfer midlevel momentum to the surface, 

causing strong lee-side surface winds. However, Hughes 

and Hall (2009) found strong variability in Santa Ana events 

with many days exhibiting strong offshore flow and weak 

synoptic forcing. They suggested local thermodynamic 

forcing must also cause offshore surface flow. When cold air 

is trapped in the Great Basin by topography, a hydrostatic, 

desert-ocean pressure gradient forms resulting in a negatively buoyant gravity current to flow through mountain 

gaps at the surface. 

Numerical modeling results by Huang et al. (2009) 

showed that a coupling between the synoptic scale and 

mesoscale exists leading to the development of Santa Ana 

winds. The coupling effects of the synoptic scale with the 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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mesoscale are classified in three stages. During stage I, 

mesoscale subsidence occurs in the exit region of the jet 

stream causing an initial surge of dry air to the surface as 

a result of moisture divergence behind a surface cold front 

in the Southwestern United States. During stage II, anticyclonic curvature of the jet stream increases, and strong 

northeasterly winds in the jet exit region advects dry air 

toward the California coast. During stage III, the extremely 

dry mid-tropospheric air is transported to the boundary 

layer on the east side of the coast range caused by wave 

breaking and strong turbulence that lead to the formation 

of a hydraulic jump creating the Santa Ana winds. 

There have been many studies focused on the largescale dynamics of Santa Ana events, but few studies have 

investigated extreme fire behavior associated with these 

events. One recent study was made by Maranghides and 

Mell (2009) who conducted a postincident analysis of the 

fire behavior that occurred during the Witch and Guejito 

Fires near San Diego, California, in 2007. Surface winds in 

the region were approximately 11 m/s with gusts of 15 m/s. 

A home weather station in the region reported a maximum 

windspeed of 25 m/s. Relative humidity dropped from 

90 to 8 percent during the onset of the Santa Ana wind 

event. Spread rates during the Guejito Fire were estimated 

between 1.7 and 2.5 m/s (3.7 and 5.6 mi/h). Spotting distances were estimated to be approximately 4.5 km (2.8 mi) 

from the Guejito Fire front. The surface wind measurements 

were limited to just a few sites in the region of these fires, 

but indicate very strong surface winds and rapid fire spread. 

Better measurements of fire-atmosphere interactions during 

Santa Ana events would lead to improved understanding of 

extreme fire behavior during such events. 

Esperanza Fire—The Esperanza Fire occurred on 26 

October 2006 near Cabazon, California, and was an event 

where extreme fire behavior was associated with five firefighter fatalities. The extreme fire behavior was caused by 

the fire spread up a narrow canyon enhanced by flow channeling created by the onset of a Santa Ana wind (Coen and 

Riggan 2010, Esperanza Investigation Team 2006). One key 

finding (finding 29, Esperanza Investigation Team 2006) 

was that none of the fire shelters for the five firefighters that 

were killed by the burnover were deployed, indicating that 

the head fire must have accelerated as it came up the creek 

drainage and caught all firefighters by surprise leaving them 

no time to deploy their shelters. 

One of the major contributing factors was the Santa 

Ana winds coming into alignment with the “unnamed creek 

drainage” as a channeled flow increasing the surface winds 

in the canyon. Additionally, the inversion was penetrated 

by the convection column from the fire run up the canyon, 

resulting in extreme fire behavior and area ignition. Coen 

and Riggan (2010) confirmed the presence of strong winds 

that aligned with the canyon; however, these surface winds 

were a result of atmospheric gravity waves bringing highmomentum east-northeasterly winds to the surface. 

Sundowner winds—

Another foehn wind that has played a major role in observed 

extreme fire behavior is the sundowner wind of Santa Barbara, California. The sundowner is a localized downslope 

wind that flows from the Santa Ynez Mountains down to the 

narrow coastal plain of Santa Barbara. The topography is 

unique, as it is a section of coastline and mountains that are 

aligned west to east. The winds are a result of perpendicular 

flow at ridgetop, typically associated with warmer and drier 

air near the mountaintops and cooler, higher humidity air 

at the coast. The extreme effects of the winds include the 

onset of severe wind velocities and abrupt warming. The 

abrupt observed warming is a result of the adiabatic descent 

of mid-tropospheric air to the surface and the replacement 

of cooler marine air at the coast with the foehn wind (Blier 

1998). The sundowner name is due to the time of onset, 

typically during the later afternoon or evening hours (Ryan 

1994). One synoptic regime associated with sundowner 

events includes the alignment of an inverted ridge off 

the California coast and inverted trough in the interior of 

the Great Basin allowing for northerly winds along the 

California coast (Blier 1998). Additionally, as with other 

foehn events, the presence of a stable layer at ridge height 

enhances the flow and formation of mountain waves (Blier 

1998). Sundowners have been associated with extreme fire 
GENERAL TECHNICAL REPORT PNW-GTR-854

8

behavior. For example, during the Painted Rock Fire in 

June 1990, an extreme sundowner event caused devastating 

winds and fire spread rates. Additionally, downslope winds 

can cause severe downslope fire spread as noted by Weise 

and Biging (1996).

Washoe zephyr— 

The eastern Sierra Nevada is associated with strong 

Chinook wind events in the winter and spring (Zhong et al. 

2008a). During the summer and fall, however, the Washoe 

zephyr occurs regularly. The Washoe zephyr is a daytime, 

down-canyon wind that occurs on the lee side of the Sierra 

Nevada (Clements 1999, Zhong et al. 2008a) often initiating 

afternoon thunderstorms in western Nevada (Hill 1980). 

Zhong et al. (2008a) defined the Washoe zephyr as a westerly wind with a sustained windspeed greater than 7 m/s 

starting after noon Local Standard Time (LST). Climatology of the zephyr indicates that 85 percent of the time, these 

events start between 1300 and 2000 LST with 70 percent 

onset between 1500 and 1800 LST. Half of the events have 

a duration of 3 to 6 hr, and few events last more than 9 hr 

(5 percent). Although zephyr events do occur all year, they 

are most frequent during the summer months. A frequency 

of less than 10 percent was observed from November to 

February.

The characteristics of the Washoe zephyr are somewhat 

opposite of what is generally observed in mountainous 

terrain where up-valley winds dominate in the afternoon. 

The zephyr develops in late afternoon during the summer 

and fall, and blows strongly down canyon with velocities 

regularly exceeding 5 m/s. The vertical wind profile of 

the zephyr is characterized by a strong low-level jet that 

produces strong wind shear and turbulence (Clements 1999, 

Kingsmill 2000) at the surface. Wind shear can be defined 

as the change in windspeed or wind direction with height 

(vertical wind shear). The strong and gusty nature of the 

zephyr lasts throughout the night and finally diminishes, 

allowing thermally driven down-valley winds to persist 

until morning (Clements 1999). 

The dynamics of the Washoe zephyr have often been 

questioned. Zhong et al. (2008a) showed through mesoscale 

numerical modeling and climatological analyses that the 

Washoe zephyr is driven by the cross-barrier pressure 

gradient formed in response to the thermal low of the Great 

Basin. 

One incident in the lee of the Sierra Nevada that could 

be attributed to a Washoe zephyr-like event occurred 

during the Seven Oak Fire of the Inyo Complex (California 

Department of Forestry and Fire Protection 2008). On the 

afternoon of 7 July 2007 at 1400 Pacific Daylight Time 

(PDT), a strike team was assigned to burn out an area in 

order to protect the historical Mount Whitney Fish Hatchery 

on the western side of Owens Valley near the town of Independence, California. The site was just below the eastern 

escarpment of the Sierra crest. At 1430 PDT, the wind had 

changed and caused the fire to cross the planned control 

line. It is reported that at 1445 PDT, the fire intensified and 

the winds increased and began changing directions. At this 

time, the firefighters realized they were losing control and 

retreated toward their designated safety zone and deployed 

their shelters while waiting out the burnover in a small 

pond. The entrapment resulted in burn and respiratory injuries to all nine firefighters and the total loss of one engine 

and damage to another. The incident report indicated that 

skies were clear with no cumulus buildup. The day before, 

when the fire started, there were frequent lightning strikes 

in the higher elevations of the Sierra with strong, gusty and 

erratic winds. A 26 m/s (58 mi/h) wind gust was recorded 

by fire personnel using a Kestrel handheld anemometer. 

Daytime temperatures on July 7th ranged from low 32 

to 38 °C (90s to 100 °F) at 1247 PDT. Relative humidity 

(RH) values ranged from a high of 13 percent to a low of 

4 percent at 1447 PDT. At the Oak Creek remote automated 

weather station (RAWS), a wind gust of 22 m/s (50 mi/h )

also occurred at 1447 PDT. Winds in the afternoon were 

sustained 4.5 to 6.7 m/s (10 to 15 mi/h) gusting to 13 m/s (30 

mi/h). At the time of the burnover, winds were 9 to 13 m/s 

(20 to 30 mi/h) out of the southwest. 

Although the southeastern Sierra is not usually associated with Washoe zephyr events because of the higher terrain and fewer gaps in the crest, the observed characteristics 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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have some similarities to the zephyr. Southwesterly winds 

with recorded velocities of 4.5 to 6.7 m/s (10 to 15 mi/h) 

are similar to what has been observed in Lee Vining and 

Reno to the north. The onset of the stronger winds occurring between 1400 and 1500 is typical for zephyr events. 

However, the Washoe zephyr typically has a more westerly 

component, but this could possibly be effects of flow channeling along the foothills of the Sierra eastern escarpment 

as found by Zhong et al. (2008b). 

Terrain channeling effects— 

Forced channeling or pressure-driven channeling of upper 

level, larger scale winds can cause drastic changes in 

windspeed and direction to occur in valleys (Whiteman 

2000). These high wind events can be produced by (1) 

downward momentum transport, (2) terrain channeling, and 

(3) pressure-driven channeling (Whiteman 2000, Zhong et 

al. 2008b). For a more detailed review on terrain channeling 

effects in mountainous regions, please refer to the paper by 

Sharples (2009). 

The downward transport of momentum occurs when 

winds within a valley are strongly coupled to winds aloft 

(Zhong et al. 2008b). For this condition to occur, there must 

be vertical mixing associated with unstable or neutral stability allowing upper level winds to penetrate to the surface. 

When winds in a valley are driven by this mechanism, they 

are expected to align with the wind direction aloft. Downward transport of momentum in valleys occurs often.

Another channeling effect is “forced channeling,” 

which occurs when strong winds aloft blow directly along 

the valley’s axis (Whiteman 2000). According to Whiteman, forced channeling occurs more regularly during 

daytime because the valley atmosphere is usually neutral or 

unstable during the day. It typically begins in later morning 

after the breakup of the nocturnal inversion, resulting in 

abrupt changes in windspeed and gustiness. Forced channeling is strongest when the pressure gradient aloft is weak 

in the along-valley direction. Upper level winds can also be 

channeled when they blow at oblique angles to the valley 

axis, either flowing up or down the valley. 

Thirtymile Fire—The Thirtymile Fire investigative report 

indicates that fire-induced winds were associated with the 

deaths of four firefighters who deployed at a site located 30 

m upslope from the valley floor. The analysis suggests that 

the deployment site happened to be located at a point where 

the convection column had impinged on the valley sidewall, 

causing extensive convective heat to pass over the deployment site leading to the asphyxiation of the entrapped firefighters. While initially, winds in the canyon were relatively 

light during the early afternoon, strong fire-induced winds 

were reported to be on the order of 22 m/s (50 mi/h) during 

the onset of the deployment (Brown 2002). 

Tree needle heatset observations made at the deployment sites (Brown 2002) indicated that the fire-induced 

winds were in the up-canyon and upslope direction suggesting that the convection column was being channeled up 

the canyon rather than rising vertically from the surface. 

The fact that the convection column near the surface was 

being advected up canyon suggests that the surface winds 

were blowing through the fire-front boundary. Additionally, 

observed spread rates at this time increased and caught the 

firefighters off guard (Brown 2002). The increase in fire 

spread rate was a result of the fire running in the crowns, 

driven by the up-canyon winds. At the same time, upper 

level winds were from the southwest and in alignment 

with the canyon’s axis providing a source for increased 

wind velocities. The upper level winds may have been 

mixed downward from aloft to the surface owing to the 

dynamics of the convection plume. The downward mixing 

of horizontal momentum could help explain why the fire 

front accelerated and caused the burnover to happen so 

quickly. These events can be surgelike and last for only a 

few minutes. Another mechanism that could be responsible 

for the convection column to have impinged on the canyon 

sidewall might be strong downdrafts that exist in plumes or 

convection columns. These downdrafts can be responsible 

for the strong fire-induced winds that are often observed at 

the fire front and may drive fire spread (Clark et al. 1996, 

Clements et al. 2007). 
GENERAL TECHNICAL REPORT PNW-GTR-854

10

Another mechanism possibly responsible for the intense 

fire-induced winds could be a developing low pressure field. 

This may have existed in the upper elevations of the canyon 

ahead of the fire front. This type of pressure perturbation 

ahead of the fire front has been found in numerical simulations done over flat terrain (Clark et al. 1996) and observed 

over slopes with crosswinds (Clements and Heilman 

2010). A region of low pressure develops as a result of a 

hydrostatic pressure gradient that forms at the base of the 

convection column (Clark et al. 1996). Within a canyon during daytime, low pressure exits owing to the solar heating of 

the canyon volume causing up-canyon winds to occur. With 

the additional heating caused by the advection of the plume 

up the canyon, acceleration in the wind field could result 

and be the cause for the extreme fire-induced winds that 

blew through the fire front advecting hot gases along the 

sidewalls of the canyon. Although these mechanisms could 

be responsible for the plume impingement on the canyon 

sidewall, none has been confirmed. 

Pressure-driven channeling— 

Pressure-driven channeling occurs when there exists a 

larger scale pressure gradient above the valley that is superimposed on the valley below. The direction of the winds 

in the valley depends on the along-valley component of the 

horizontal pressure gradient. Pressure-driven channeling 

causes winds to always blow along the valley axis from 

high pressure (end of valley) to low (end of valley) (Whiteman 2000, Zhong et al. 2008b). Pressure-driven channeling 

is strongest when the along-valley pressure gradient is 

strongest in the along-valley direction. 

Thermally Driven Winds

Thermally driven wind systems are very common because 

they are diurnally driven (daytime vs. nighttime) and are 

probably experienced more by wildland firefighters and 

backcountry hikers. These winds include the classic valley 

and slope winds. There is a distinct diurnal structure to the 

evolution of the thermally driven flows where their direction 

typically reverses daily owing to changes in the pressure 

gradient and buoyancy. 

Two main circulations exist in the valley atmosphere: 

valley winds and slope winds. The valley winds consist of 

two diurnal regimes: the up-valley wind during the daytime 

and the down-valley wind at night. The slope winds consist 

of a similar diurnal structure with downslope winds occurring during the nighttime periods and upslope winds during 

the daytime (Ekhart 1944; Vergeiner and Dreiseitl 1987; 

Whiteman 1990, 2000). The strength of thermally driven 

circulations is a function of aspect, time of day, and time of 

year (Whiteman 2000). Of the two wind systems, the valley 

winds play a larger role in fire behavior because of their 

overall stronger velocities and horizontal extent. 

Slope winds— 

Slope winds are the most intermittent of the thermally 

driven flows found in mountain environments (Vergeiner 

and Dreiseitl 1987, Whiteman 1990). This is due to both 

slope length and depth. Although there have been numerous 

studies focused on the downslope flows (Horst and Doran 

1986, Mahrt 1982, Manins and Sawford 1979, Papadopoulos 

and Helmise 1999, Whiteman and Zhong 2008), limited 

work has been focused on the upslope winds. Vergeiner and 

Dreiseitl suggested that this is due to their intermittency and 

overall difficulty in obtaining useful measurements. They 

also concluded that any field study focused on measuring 

upslope flows will “give random inconclusive results from 

which representative values of mass and heat transport in 

the slope layer cannot be derived” (Vergeiner and Dreiseitl 

1987). 

Fire behavior studies on slopes and especially field 

studies are limited, and therefore it is difficult to determine 

whether or not diurnal slope flows help drive the fire along 

the slope rather than being dominantly driven by the fuels 

and the effect of radiative and convective transfer from 

the fire front to the fuels (flame attachment). However, as 

will be discussed in a later section, the interaction of slope 

winds and valley winds can create shear layers producing 

turbulence along the slopes that can potentially lead to 

extreme fire behavior scenarios. 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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Upslope winds— 

According to Whiteman (2000), upslope flows have depths 

of 10 to 50 m above ground level (AGL) and velocities on 

the order of 1 to 5 m/s2 (fig. 2-1). Upslope flows react instantly to changes in insolation and begin immediately after 

sunrise (Vergeiner and Dreiseitl 1987). Two main forcing 

mechanisms drive the flow upslope: the pressure gradient 

force and the buoyancy force (Atkinson 1981). The air over 

a slope is heated by insolation leading an air parcel adjacent 

to the slope to have a higher potential temperature and 

lower density than air at the same altitude, but away from 

the slope. It is this temperature perturbation that drives 

the pressure gradient to force air toward the slope from the 

center of the valley (at the same altitude). Buoyancy drives 

the air parcel vertically above the slope, and the sum of both 

buoyancy and the horizontal pressure gradient causes the 

air parcel to accelerate up the slope while being replaced by 

air from over the valley center. This is the classic upslope 

circulation during ideal, fair weather conditions and is 

responsible for transporting heat and mass to the valley 

atmosphere (Vergeiner and Dreiseitl 1987). 

One of the more recent observations of upslope flows 

was made by Reuten et al. (2005) who observed upslope 

flows at the foot of a mountain range with a slope angle of 

19° and a ridge height of 780 m above sea level (ASL) in 

coastal British Columbia. Their observations indicate that 

the daytime upslope flows were strong with velocities up 

to 6 m/s and occurred over a depth of nearly 500 m AGL. 

Equally strong and deep return circulations occurred within 

the convective boundary layer (CBL). The transport of mass 

of the upslope flow and return flow approximately balanced 

during the morning period suggesting a closed-cell slope 

flow circulation within the boundary layer. This is the first 

observational evidence of the closed cell slope flow circulation. 

The intermittency of daytime upslope flows may 

influence the upslope fire behavior by possibly increasing 

upslope rate of spread at random intervals. However, this 

influence is more likely limited owing to the weak nature 

of the upslope velocities. Valley winds may have a larger 

impact on fire behavior on slopes owing to the cross-slope 

wind component of the valley winds. As the valley wind 

develops, the valley wind can overcome the slope wind 

layer along the slope and create cross-slope flow (Whiteman 2000). Fire spread will be upslope, but depending on 

the strength of the valley wind, can likely be reduced and 

spread laterally along the slope. Synoptically forced winds 

that penetrate the valley atmosphere would intensify this 

effect. 

Fire behavior on sloped terrain— 

Slope-driven fire spread has been studied for decades 

because many wildfires occur in regions of mountainous terrain, and fire spread on slopes is associated with 

increased acceleration leading to extreme fire behavior 

(Cheney and Sullivan 2008). Understanding of fire behavior 

on slopes is derived mostly from laboratory-scale experiments conduced in wind tunnels (e.g., Weise and Biging 

1996); however, recently a number of numerical simulations 

have been conducted (Linn et al. 2010). The effect of slope 

Figure 2-1—Schematic diagram indicating vertical slope flow 

structure for daytime upslope flows (dashed line) and nighttime 

downslope flows (solid line). Adapted from Whiteman (2000). 

AGL = above ground level.

* m/s = meters per second
GENERAL TECHNICAL REPORT PNW-GTR-854

12

has been viewed as an added component of wind velocity 

since 1946 (Weise and Biging 1997). There have been attempts to determine both the separate and combined effects 

of wind velocity and slope angle on spread rate and flame 

length (Weise and Biging 1997). Results from Weise and 

Biging indicate that as slope and wind velocity increase, fire 

behavior, including flame length and spread rate, increases 

significantly as compared to no-wind and downslope conditions. Backing fires on slopes can result in weak to no 

spread. Weise and Biging suggested that the wind acts to 

cool the unburnt fuel in advance of the fire front.

Santoni et al. (1999) formulated a model to account for 

upslope fire spread and compared the solution to experimental results obtained using a tilted, combustion table. They 

suggested that the flame’s heat that is radiated ahead of the 

fire front toward the fuel is more important under slope 

conditions. They found that the rate of spread increases 

with slope. They also found that the fire front shape distorts 

toward the slope as the fire spreads upslope becoming more 

pointed. The fire front distortion increases with increasing 

slope angle.

Chimney effects— 

An important aspect of upslope wind on fire behavior would 

be the effect that chimneys or steep gullies have on driving 

wind up the mountainside. Gullies can help channel upslope 

flows if the chimney is not lined with dense vegetation. 

Within the canopy, the air is usually cooler than the free 

atmosphere and can result in drainage winds flowing below 

the canopy top while upslope winds occur above the canopy 

(Belcher et al. 2008, Whiteman 2000). Few, if any, observations of wind velocities in steep gullies exist. 

Explosive fire behavior— 

Eruptive fire behavior has been reviewed by Viegas and 

Simeoni (2010) where they define extreme fire acceleration 

as fire blowup characterized by a sudden change of spread 

rate and energy release rate. This designation was first 

proposed by Viegas (2005), and such fire eruptions, especially those associated with canyons, are not rare (Viegas 

and Simeoni 2010). Laboratory studies using a combustion 

chamber and a fuel bed configured on a tilting, V-shaped 

table to replicate a steep chimney were conducted by Viegas 

and Pita (2004) and Viegas (2005). Their conclusions suggest that forest fire blowup depends mainly on fuel-bed 

properties and on the initial fire spread conditions dictated 

by topography or wind. Viegas (2005) also found that if 

the slope is not sufficiently long, blowup may not occur; 

however, a fire in the same fuel bed on a very steep slope 

will start with a high rate of spread (ROS), and blowup may 

be obtained quickly. These studies do provide some insight, 

but they are limited by the experimental design as are most 

chamber-table studies owing to the limited table length. 

Dold and Zinoviev (2009) and Dold (2010) suggested that 

air ahead of a fire front that is spreading upslope flows up 

the slope, causing plume attachment with the fuels on the 

slope and leading to acceleration of fire spread upslope. 

Additionally, this leads to potentially dangerous acceleration of the fireline. They suggested that the airflow is generated by the fire and is independent of the ambient wind. 

Wu et al. (2000) conducted a series of laboratory experiments and successfully visualized experimental fire plumes 

interacting with an inclined surface by using a grid schlieren system. They found that plumes were characterized by 

two parameters, plume attachment length and plume angle, 

and these were used to determine a critical inclination angle 

for flame attachment to occur. Their results suggest that 24° 

is a critical angle for attachment to occur. Additionally, Wu 

et al. found that the critical inclination angle is not sensitive 

to the heat release rate or surface conditions. 

Dupuy and Maréchal (2011) conducted a series of 

laboratory fire experiments to determine the contribution 

of radiation and convection to fuel bed preheating on slopes 

of 0°, 10°, 20°, and 30°. Their results indicate that radiative 

heating is the dominant heat transfer mechanism on slopes 

between 0° and 20°, but close to the fireline. Convective 

heating was also found to be significant, becoming onethird of the total heat flux on the 20° slope. When the slope 

angle increased from 20° to 30°, the rate of spread increased 

by a factor of 2.5 owing to an increase in convective heating; also at this angle, radiative heating stopped increasing. 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

13

Their results also showed that far from the fireline, cooling 

by convection was found to be substantial except on slopes 

of 30° in angle. 

Sharples et al. (2010a) suggested that the trench effect 

or flame attachment phenomena observed in structure fires 

of stairwells can be used as a surrogate for wildland fires 

exhibiting explosive behavior. The trench effect produces 

rapid fire spread in enclosed slopes such as escalator or 

stairwells by the interaction of the buoyant plume and an 

inclined trench of the stairwell. Plume impingement on 

an inclined surface enhances preheating and pyrolysis of 

the fuel resulting in accelerated fire spread. Sharples et al. 

(2010a) suggested that the trench effect is a misnomer and 

the effect is really due to the trenchlike configuration of the 

fuels that limited lateral entrainment into the plume. They 

suggested that plume attachment or flame attachment are 

more appropriate to describe the phenomenon. This conceptual model applies to steep gullies or canyons, as these 

terrain features can potentially limit the lateral entrainment 

into the plume and result in eruptive or accelerated fire 

spread up the canyon. 

Sharples et al. (2010a) also noted that confined slopes 

over 25° are the most prone to flame attachment and the 

reason observed eruptive wildfire behavior is more prevalent on steep slopes and in steep canyons. This observation 

is in agreement with the results from Wu et al. (2000) who 

suggested 24° as a critical slope angle for flame attachment 

to occur. 

Modeling of fire behavior on slopes— 

To date, most studies aimed at determining the role of slope 

on fire behavior have based their models on wind tunnel 

experiments. More recently there have been attempts using 

physics-based, coupled fire-atmosphere modeling systems 

to evaluate the role of slope on fire behavior (Linn et al. 

2007, 2010). Using the FIRETEC modeling system (Linn 

et al. 2002, Linn and Cunningham 2005), Linn et al. (2010) 

simulated fire behavior on a 30° slope with different fuel 

types and found that the slope alone has a significant effect 

on spread rate and spread pattern. This result confirms 

the results of Weise and Biging (1997) and Santoni et al. 

(1999), but the most significant finding from the FIRETEC 

simulations was that the spread rate of all simulations is not 

the same at a point near the bottom of the hill and a point 

near the top, even though the slope is the same at each point. 

Linn et al. (2010) remarked that this result indicates that 

simply having a single value of local slope angle of a hill 

and a single nominal windspeed is not adequate to predict 

the spread rates on slopes. 

Linn et al. (2007) also showed that under certain 

conditions, the local slope had a more pronounced effect 

on spread rate than ambient wind. For example, numerical 

simulations showed that fire spread was dominated by the 

topography at locations on the middle of a slope when ambient winds were 6 m/s, whereas at other locations upwind of 

the slope, the fire behavior was strongly influenced by the 

coupling between the topography and ambient wind. This 

result indicates the importance of understanding the local 

winds that are influenced by the topography. The local wind 

field drives the fire behavior, and the topography has a more 

pronounced effect on the wind field rather than directly on 

the fire. Additionally, Linn et al. (2007) found a relationship among fire behavior, topography, and atmosphere that 

showed importance when the topographically influenced 

winds are not complementary to the slope effects such as 

those reported by Weise and Biging (1997). 

Because present knowledge of fire behavior on slopes 

and in gullies is a result of laboratory experiments and 

numerical modeling studies, there is still a large gap in 

understanding the role of slope-scale winds on fire spread 

on slopes. Therefore, there is an immediate need for welldesigned field experiments.

Downslope winds— 

Downslope winds, also known as katabatic and drainage 

winds, develop once the slope becomes shaded as the sun 

sets. This reversal in the heating causes a shallow layer of 

cold air to develop along the slope, and this cold layer of 

air is now more dense than the surrounding air. As a result, 

it flows or drains downslope. As in the upslope winds, 

downslope winds are driven primarily by temperature differences between the air on the surface of the slope and that 
GENERAL TECHNICAL REPORT PNW-GTR-854

14

at the same elevation away from the slope. Observations of 

downslope flows over simple slopes indicate that the velocities range from 1 to 4 m/s and occur within a depth of 10 to 

40 m above the slope (Horst and Doran 1986, Papadopoulos 

and Helmise 1999, Whiteman 2000). 

Because downslope winds have limited vertical extent 

and are typically much weaker in velocity, their effect on 

fire behavior may be limited. The downvalley winds most 

likely play a larger role on fire behavior-over the slopes in 

a mountain valley. Valley winds typically “overrun” the 

weaker slope flows once the down-valley winds become 

established throughout the night. 

Once the surface winds become decoupled owing to 

the buildup of a nocturnal inversion at the valley floor, fire 

behavior can change dramatically with a change in direction 

or a decrease in spread rates, flame lengths, and intensity. 

These changes can also be attributed to relative humidity 

recovery near the surface. 

Valley winds— 

Valley winds, also known as along-valley winds, are a 

much more consistent wind regime than slope flows and 

are typically associated with much stronger velocities. The 

dynamic forcing is similar to that for the slope winds with 

the exception that the forcing is driven by a valley volume 

effect. During daytime, the air in the valley is warmer than 

over the plain because its volume is less and it thus warms 

faster than the air over the plain (Schmidli and Rotunno 

2010, Whiteman 1990). As a result, pressure is reduced in 

the valley while it is higher over the plain at an altitude that 

is the same elevation as the valley. The pressure gradient 

force is then directed from the plain to the valley (Whiteman 1990). During the night, the pressure gradient reverses 

and the winds blow down valley. Up-valley winds have 

velocities on the order of 3 to 8 m/s and down-valley winds 

about 3 to 6 m/s. Typically there exists an oscillation in 

the winds at night (Porch et al. 1991), which can affect fire 

behavior. The oscillations are thought to be caused by the 

interactions of air flowing out from tributary valleys into 

the main valley causing surges in the winds to occur at 

regular intervals on the order of 10 to 20 min. These surges 

can lead to changes in fire spread rate if the surface wind 

accelerates to the surface. However, there have been no 

quantitative studies on how the valley wind can affect fire 

behavior during daytime or night. 

Valley winds can sometimes be overcome by other 

mesoscale wind circulations especially in regions near 

coastlines. Seto and Clements (in press) observed the formation of a small fire whirl that formed during a prescribed 

fire when the prevailing up-valley wind was overcome by 

sea breeze. Observations from a micrometeorological measurement tower placed in the burn unit showed that the fire 

whirl formed immediately after the sea breeze entered the 

valley at the surface. The fire whirl was first observed in the 

flaming front but moved behind the fire line as it stretched 

about 200 m in the vertical. The fire whirl caused the fire 

crew to quickly reposition themselves away from the fireline 

to remain safe. After the fire whirl dissipated, firing operations resumed. Seto and Clements (in press) ascertained that 

the fire whirl was caused by horizontal vorticity that was 

generated as a result of near-surface wind shear formed by 

the interaction of the sea breeze and the up-valley wind. 

Inversion destruction in valleys— 

The diurnal evolution of vertical temperature structure in 

mountain valleys has been well established by extensive 

field and modeling studies (Whiteman 1982, Whiteman 

and McKee 1982). During the night, cold air forms over 

the slopes and at the valley bottoms forming a temperature 

inversion, where the temperature increases with altitude 

(Whiteman 2000). Inversion breakup occurs in the morning 

when the sun begins heating the surface, and during the 

transitional period that follows, it can produce significant 

changes in surface conditions such as increased windspeed, 

wind direction, temperature, and humidity. Inversions 

can also break up in the middle of the night when stronger 

upper level winds push out the valley inversion (Clements 

et al. 2003). This can occur in shallow valleys that are more 

exposed to upper level winds or when upper level winds are 

excessively strong. For these reasons, inversion breakup is a 

period likely to produce periods of extreme fire behavior. 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

15

The breakup of temperature inversions can occur 

within 2 to 3 hr depending on valley geometry and season 

(Whiteman 1990). The most dangerous situation for 

increased fire behavior is when there is a strong downvalley wind that is decoupled from the surface by the surface inversion’s capping inversion top (fig. 2-2). Once 

the inversion breaks, the stronger winds can mix quickly 

downward to the surface bringing drier and warmer air to 

the surface. Often the wind velocity can easily double and 

be associated with a 180o shift in direction. This situation 

is very common in valleys and can be anticipated on fires, 

but the rate of the inversion breakup and the decoupling of 

winds aloft should be estimated from smoke observations 

or a sequence of soundings taken on site. 

The inversion breakup model of Whiteman (1982) is 

not observed in all valleys. During the Riviera Project in 

the Swiss Alps (Rotach et al. 2004), the thermodynamic 

structure and evolution was different than what was found 

in the valleys of the Colorado Rockies. Rotach et al. (2004) 

described a valley atmosphere that is stable throughout 

the afternoon rather than being well mixed as suggested 

by Whiteman (1982). However, there exists a multilayered 

structure to the temperature profiles, which has been found 

in other valleys of the Alps. Thus, the stability regime can 

be quite different from valley to valley. To determine the 

local stability for fire behavior and fire weather predictions 

requires an in situ sounding at the time of interest. Inversions in valleys may or may not be horizontally homogenous in extent, but rather developing in isolated pockets 

along the valley’s axis. Locations along the valley floor may 

have areas of weaker surface inversions that could result in 

a faster inversion breakup and could potentially lead to different fire behavior only hundreds of meters away. Another 

aspect of valley inversions is the role they have on the 

thermal belt. Thermal belts are areas along valley sidewalls 

that are warmer than the areas below and above them. This 

can have an impact on the fuel loading, moisture content, 

and temperature, and resulting fire behavior. 

Cross-valley winds— 

Cross-valley winds can form as a result of either differential 

heating of slopes or dynamically forced flows through and 

over the terrain. Additionally, during the breakup of the 

valley inversion, solar radiation that illuminates one side of 

Figure 2-2—Temperature and windspeed profile evolution during valley inversion breakup in Yosemite Valley, California. Down-valley 

winds are negative and up-valley winds are positive. Once inversion is destroyed, up-valley winds mix downward to the surface causing 

a rapid increase in surface windspeed and direction. Times indicated are local time. Adapted from Clements and Zhong (2004). AGL = 

above ground level.
GENERAL TECHNICAL REPORT PNW-GTR-854

16

a valley first causes a circulation to develop in the acrossvalley direction, as air within the center of the valley flows 

toward the heated sidewall and compensates for a slope 

flow and convection that develops in response to the solar 

heating (Colette et al. 2003, Whiteman et al. 2004). Rotach 

et al. (2004) found that valleys with bends can influence 

the location of the core of up-valley flow. In the Riviera 

Valley, the up-valley jet core was located closer to one 

valley sidewall because of the inertia of the flow as it came 

around a bend in the valley. This observation suggests that 

in valleys with sharp bends in the along-valley direction, the 

flow maxima can occur along one side of the valley. This 

characteristic can impact fire behavior in valleys by creating a stronger surface wind on one side of the valley. If the 

fire were to cross the valley by spotting, then the spread rate 

could potentially be much different than would be observed 

on the opposite valley sidewall.

Turbulence in mountainous regions— 

Turbulence is defined as the perturbation from the mean 

of wind velocity. Little is known on the characteristics 

of atmospheric turbulent processes in steep mountainous 

terrain (Weigel et al. 2007). The role of turbulence on fire 

behavior has been suggested as a critical driving force at the 

fire front (Taylor et al. 2004) and larger ambient scales (Sun 

et al. 2009). Both the background ambient turbulence and 

the turbulence generated by the fire itself have an impact 

on resulting fire behavior (Sun et al. 2009).

Results from the Alps (Rotach et al. 2004, Weigel et 

al. 2007) found that there is a significant spatial variability 

in surface turbulence characteristics throughout the valley 

atmosphere, which is largely determined by local topographical features such as slope. It was also found that the 

maximum in shear-induced turbulence was found to occur 

on the eastern valley sidewall (sunlight) and near the center 

of the valley at the core of the valley wind. The turbulenceproducing slope surfaces have a significant influence on the 

turbulence structure in large parts of the valley atmosphere. 

Consequently, fire behavior on slopes can be driven by a 

combination of slope effects and ambient turbulence that 

is generated by shear between the slope flow layer and the 

valley wind. As found in the Riviera Valley, turbulence 

generation can often be dominated by wind shear. Intense 

turbulence is often associated with strong wind shear generated by strong surface winds such as during foehn events 

(Sharples et al. 2010b). 

Wind Modeling Tools: WindNinja 

Determining real-time wind characteristics on an incident 

remains a challenge for sites in complex terrain. This 

need has been partially addressed by the development of 

wind modeling systems by the USDA Forest Service using 

in-house and commercially available computational fluid 

dynamics codes. The most popular modeling system is 

WindNinja (http://www.firemodels.org), which is similar 

to the more complex WindWizard model (Butler et al. 

2006). WindNinja takes a wind observation at a location 

and computes a spatially varying and high-resolution 

(100 m) wind field over the terrain, attempting to account 

for mechanical modification of the flow by the terrain. 

WindNinja is not a forecasting tool, but rather provides 

a “snapshot in time” of the wind for an area. WindNinja 

is becoming widely used on fire incidents by incident 

meteorologists and fire behavioral analysts. This is due 

to the nature of the system—it can be run on a laptop 

computer taking less than a minute to provide output. That 

is a big advantage as no forecasting system can provide this 

ease of use. The output from WindNinja is quite exciting for 

the user, but there are some major limitations of the system 

that users should be aware of. The numerics of the system 

are based on solving a rather simple set of mass continuity 

equations and optional slope flow equations. This simplicity 

is what makes WindNinja operate so fast on a laptop. These 

same simplifications are reason for caution when using it in 

complex terrain. First, the model is a mass-consistent model 

requiring air to flow around mountains rather than through 

them. The major pitfall for this type of model is the lack of 

thermodynamic fields to determine atmospheric stability, 

which would cause air to flow around or over terrain, and 

would limit its use for situations where thermally driven 

circulations dominate. The exception to this is a simple 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

17

slope flow submodel that is included in WindNinja. The 

model stability for flow computation is fixed for a neutral 

atmosphere (Butler et al. 2006), except in the initialization 

phase where WindNinja approximates lower atmosphere 

stability based on surface heat flux and subsequently uses 

a logarithmic vertical wind profile that includes adjustment 

for this stability. After the initialization phase, neutral 

stability is assumed for flow adjustment, but a method of 

relaxing this is currently being tested (Forthofer, J. 2011. 

Personal communication. Mechanical engineer, USDA 

Forest Service, Missoula Fire Sciences Laboratory, Missoula, 

MT), so the current version of WindNinja should not be 

expected to provide accurate simulations in situations where 

thermally driven flows are a dominating influence. For 

example, without the ability to run the model with specific 

stabilities such as a stable layer at crest height, the model 

may not be able to accurately model windflow during foehn 

events because the crest-level inversion is an important 

criteria for the development of downslope windstorms. Also, 

WindNinja may fail during inversion breakup or when a 

valley atmosphere is slightly stable during the day. Another 

enhancement currently being tested in WindNinja is to 

initialize with available weather model forecasts from, for 

example, the National Weather Service (Forthofer, J. 2011. 

Personal communication). WindNinja would then become 

a kind of physical downscaling of these coarse forecasts, 

which might further alleviate thermal stability issues in 

WindNinja because much of the thermal forcing might be 

included in the initialized field from the forecast model.

Kochanski et al. (2009) used multiple meteorological 

modeling systems, including WindNinja, to simulate 

flow over a simple hill. The performance and accuracy of 

WindNinja was much less than the other models, primarily 

because the version of WindNinja used did not allow for 

a user-defined vertical wind profile. Note that the other 

models used were much more sophisticated and required 

extensive computing time and processors in order to 

complete their simulations, whereas WindNinja did not. 

Forthofer (2007) simulated the same hill using a research 

version of WindNinja that did specify the measured upwind 

vertical wind profile and showed much better results on the 

upwind and top of the hill. Flow on the lee side was less 

accurate, likely owing to the crude handling of momentum/

turbulence in WindNinja, which becomes most important 

on lee slope locations.

Although there are limitations to the use of this type 

of modeling system in complex terrain, a user with an 

understanding of these limitations can use the model to get 

a general idea of the wind field over a fire area. This can be 

a benefit when there is a need to determine if the winds in 

an area are terrain forced and caused solely by topography. 

Because WindNinja provides a gridded wind field in under 

1 minute of simulation time, it is a very capable tool, but 

users should have an understanding of the issues mentioned 

above. Finally, in the summer of 2010, a major field validation experiment was conducted to provide a comprehensive 

data set for testing and improving the WindNinja application (Forthofer, J. 2010. Personal communication). It is no 

doubt going to be an improved tool in the near future. 

Summary

Atmospheric processes in complex and mountainous 

terrain result in a variety of phenomena that can affect fire 

behavior in unpredictable ways. There are two main wind 

types that should be considered for better predicting fire 

behavior in mountainous regions: large-scale dynamically 

driven winds and thermally driven winds. The most notable 

dynamically driven winds are the foehn winds that occur 

in most mountain ranges in the Western United States. 

Foehn winds are known for increasing the surface winds 

dramatically and causing very rapid warming and drying 

at the surface. The Santa Anas of southern California are 

associated with extreme windspeeds and drying and have 

also led to flow channeling in narrow canyons resulting 

in extreme fire behavior including accelerated fire spread 

down canyon. To date there are few observations of fireatmosphere interactions and resulting fire behavior during 

foehn events. More systematic observations are required to 

better understand extreme fire behavior during foehn.
GENERAL TECHNICAL REPORT PNW-GTR-854

18

Thermally driven winds in mountainous terrain occur 

regularly as they transition from up-valley/upslope during 

the daytime to down-valley/downslope at night. Thermally 

driven winds have weaker windspeeds than the dynamically 

driven winds and can be overcome by synoptic-scale winds 

aloft when atmospheric stability permits the downward 

transport of higher momentum into the valley atmosphere. 

These situations lead to a rapid increase of surface winds 

and fire spread rates. 

One of the most critical factors affecting fire behavior in valleys is inversion breakup during the morning 

transition period. During the morning transition, a stable 

atmosphere at the surface quickly mixes and becomes 

unstable owing to the development of a convective mixed 

layer over the valley floor. When this occurs, winds aloft 

above the inversion layer, that were decoupled from the 

surface, can mix down quickly bringing much stronger 

velocities to the surface and usually from a different direction. These situations can potentially lead to extreme fire 

behavior by affecting spread rates and direction. To better 

anticipate these rapid changes, vertical profiles of temperatures should be measured in real time using radiosonde 

soundings or remote-sensing temperature profilers. Realtime observations would allow fire crews to know the state 

of the atmosphere at a given instance. 

In addition to the valley inversion breakup, valley 

geometry can play a role in fire behavior. Valleys with sharp 

bends can have flow maxima along one side of the valley. 

This characteristic can potentially impact fire behavior in 

valleys by creating a stronger surface wind on one side of 

the valley. If the fire were to cross the valley by spotting, 

then the spread rate could potentially be much different than 

would be observed on the opposite valley sidewall. 

Fire behavior on slopes is often explosive in nature as 

the fire accelerates up slope. To date, most studies have used 

either wind tunnel experiments or coupled atmosphere-fire 

numerical modeling systems. Results from these studies 

indicate that rate of spread increases with increasing slope 

and the fire front shape distorts toward the slope, becoming 

more pointed. The fire front distortion also increases with 

increased slope angle. The increase in spread rate on slopes 

is caused by flame attachment to the fuel bed because the 

fuel is closer to the flame. A critical angle of 24° for flame 

attachment to occur has been found from laboratory studies. 

Observations in mountainous terrain confirm that slopes 

with angles over 25° are most prone to flame attachment and 

explains why observed eruptive fire behavior is prevalent on 

steep slopes and canyons. 

Because present knowledge of fire behavior on slopes 

is mainly a result of laboratory experiments and numerical 

modeling studies, there is still a large gap in understanding regarding the role of slope-scale winds on fire spread 

on slopes. Numerical studies have shown that the terrain 

has a more pronounced effect on fire spread on slopes than 

the ambient wind. However, there are limited field data to 

support these results. Therefore, there is an immediate need 

for well-designed field experiments over sloped terrain to 

obtain a data set for model development and validation. 

Future Needs

Most measurements of fire behavior have been limited 

to laboratory, wind-tunnel experiments and numerical 

simulations. There are few, if any, field studies of fireatmosphere interactions in real fires (Clements et al. 

2007). Therefore, to further the understanding of the 

role of complex terrain on fire behavior, a major need is 

to conduct comprehensive field experiments on slopes 

and in mountainous areas. The data collected from these 

experiments can be used to test and develop models. 

Specific experiments that are needed include: 

• Slope experiments with head fires starting on flat 

terrain and spreading upslope under various fuels 

and ambient meteorological conditions.

• Head fire experiments in chimneys and steep 

canyons.

• Experiments during inversion breakup on valley 

floors to investigate fire behavior during transition 

periods.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

19

Idealized experiments are, however, limited to smaller 

scales and do not account for true wildfire conditions. To 

overcome this, measurements can be made by incident 

meteorologists at incidents. The National Weather Service 

incident meteorologist program has begun implementing the 

use of radiosondes on incidents rather than pilot balloons. 

Having profiles of temperature, humidity, and wind at high 

temporal and spatial resolution (about 1 s, 2 m) will allow 

the incident meteorologists and fire behavioral analysts to 

determine changes in atmospheric stability on site. Additionally, the use of remote sensing technology should be 

considered a priority. These sensors include Doppler wind 

LIDAR and passive microwave temperature and humidity 

profiles. Although the cost of these technologies is high, the 

data would provide great insight into the mechanisms of 

atmospheric dynamics on fire behavior in complex terrain. 

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Introduction 

Eyewitness accounts in journals and diaries have 

documented the relationship between weather and large 

wildland fire for hundreds of years. Survivor statements 

after the 1871 Chicago, Peshtigo, and Michigan Fires, 

and the 1894 Hinckley Fire identified hot, dry, and windy 

conditions as the primary weather elements contributing to 

the destruction caused by these fires. 

In the early 1900s, technological advances in 

meteorology permitted creditable scientific research into 

weather’s influence on wildland fire, most of which was 

closely tied to the study of historical wildland fires.

Even then it was recognized that there are short 

periods of one or several days in every fire season when 

wildland fuels are unusually susceptible to large fire, and 

this was primarily dependent upon the weather. Show 

(1931) referred to these as “dangerous periods.”

However, it was not until the 1960s that critical fire 

weather patterns, producing high fire danger and large 

wildland fires, were identified for both the United States 

and Canada.

Syverson (1962) documented the first definition of 

“critical fire weather patterns” as follows:

Crisis period is defined as the critical day, week 

or month during which blow-up fires are experienced. 

Further, we might conclude that the period of critical 

fire weather is the result of that combination of weather 

patterns that have given rise to this condition and might 

further result in causing more fires or materially assist 

their spread.

Current fire behavior training courses define critical 

fire weather patterns as: the atmospheric conditions that 

encourage extreme fire behavior resulting in large and 

destructive wildland fires. 

Understanding weather’s influence on wildland fire is 

essential for safe and effective fire suppression activities. 

Fire managers and firefighters should be aware of critical 

fire weather patterns in their areas and how adverse weather 

associated with those patterns can produce extreme fire 

behavior conditions that put firefighters and the general 

public at risk. 

Weather Elements That Promote Extreme 

Fire Behavior

Early fire weather research focused on individual weather 

elements that occurred prior to and during large wildland 

fires. The culmination of these studies identified four 

critical weather elements common to wildland fires exhibiting extreme fire behavior: low relative humidity (or low 

Chapter 3: Critical Fire Weather Patterns

Paul A. Werth1

Critical Fire Weather Patterns are 

defined as the atmospheric conditions 

that encourage extreme fire behavior 

resulting in large and destructive wildland fires. 

The four critical weather elements 

common to wildland fires exhibiting 

extreme fire behavior are low relative 

humidity, strong surface wind, unstable 

air, and drought. 

atmospheric moisture), strong surface wind, unstable air, 

and drought. 

Munns (1921) found that “In months with high vapor 

pressure (high relative humidity), very few fires occurred, 

1

 Paul A. Werth, Weather Research and Consulting Services, 

LLC, Battle Ground, WA.
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while during months of low vapor pressure (low relative 

humidity) many bad fires occurred.” Separate studies by 

Hofmann (1923) in Washington and Weidman (1923) in 

Montana and Idaho concluded that relative humidity is the 

most important factor in development of dangerous forest 

fires because it significantly increases the flammability of 

forest material. In a study of southern Appalachian wildfires, McCarthy (1924) found that relative humidity was 

unusually low on high fire risk days, and that this dry air 

was advected southward by winds from the interior of the 

continent. His study was also the first to connect the occurrence of low relative humidity to specific wind directions, 

and the warming and drying of air within high pressure 

systems owing to subsidence. A study of Massachusetts forest fires by Stickel (1928) stated, “Relative humidity appears 

to be the best single indication of forest fire hazard.” He also 

indicated that “The maximum forest fire hazard occurred 

between rainy periods, when the relative humidity is 40 percent or less.” Dague (1930) identified relative humidity of 20 

percent as the point below which bad fire weather situations 

were created east of the Cascade Mountains in Washington 

and Oregon. Since that time, numerous wildland fire reports 

have substantiated the importance of unusually low relative humidity in the development of extreme fire behavior. 

Regional threshold values for low relative humidity can 

range between 10 and 40 percent, depending on fuel model.

Low relative humidity (low atmospheric moisture) 

adversely affects fire behavior by decreasing the moisture 

content of fine dead fuels, making them easier to ignite and 

carry fire. Fire line intensity (kW/m), rate of spread (m/s), 

and the probability of spotting significantly increase when 

the relative humidity is low, sometimes so rapidly that there 

is little advance warning.

The relationship between strong surface wind and 

large fires exhibiting extreme fire behavior has been well 

documented for hundreds of years. The first scientific 

research connecting the two was conducted by Beals (1914). 

He researched surface atmospheric pressure patterns and 

associated weather conditions during four large fires (1881 

Michigan, 1884 Hinckley, 1902 Columbia, and the 1910 

Great Idaho) and found that “The one striking feature of all 

large forest fires is the strong winds that prevail just before, 

during, and for a short period after the fire passes a given 

place.”

Subsequent fire weather research (Anderson 1968; 

Brotak 1979; Countryman et al. 1956; Dague 1930, 1934; 

Gisborne 1927; Goens and Andrews 1998; Hoenisch 2009; 

Hughs and Hall 2009; Jemison 1932; Joy 1923; Kauffman 1937; Krumm 1954; Schaefer 1957; Simard et al. 

1983; USDA, USDI, and USDC 1994) has documented 

strong cold front, thunderstorm, and foehn winds with the 

occurrence of extreme fire behavior conditions. (Note: For 

more information concerning foehn winds, see chapter 2.) 

Wind affects wildland fire in a number of ways. It supplies 

additional oxygen to the fire, increasing fire intensity. It 

also preheats the fuels ahead of the fire and increases rate 

of spread by carrying heat and burning embers to new fuels 

(spotting).

Until the U.S. Weather Bureau established a national 

network of radiosonde stations, fire weather research was 

limited to studying only the effects of surface weather on 

fire behavior. With the advent of radiosonde data, researchers were also able to investigate the influence of upper air 

temperature, relative humidity, and wind on wildland fire 

behavior. The concept of airmass stability was discovered 

through the analysis of vertical temperature profiles. When 

temperature decreases rapidly with height, the atmosphere 

is classified as unstable. If there is an increase, or only a 

slight decrease in temperature with height, the atmosphere 

is classified as stable. Crosby (1949) was the first to study 

the effect of atmospheric stability on fire behavior. He concluded that stable air dampened convection currents over a 

fire, whereas unstable air increased the speed and depth of 

the convection currents. Brown (1950) stated that the stability of the air at the location of a fire is as important to fire 

behavior as temperature and humidity. Byram (1954) and 

Byram and Nelson (1951) studied 17 severe fires around the 

county and identified unstable air and certain vertical wind 

profiles as being favorable for extreme fire behavior. Davis 

(1969) investigated 70 fires in the Southeastern United 
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States and found that instability increases the chance of a 

big fire more often than low relative humidity. Haines (1988) 

developed a lower atmosphere severity index based on the 

stability and moisture content of the lower atmosphere. The 

drier and more unstable the airmass becomes, the higher the 

Haines Index, and the greater the threat of large wildland 

fire and extreme fire behavior. Brotak (1992–1993) found 

that in the Eastern United States, strong surface wind in 

conjunction with low fuel moisture caused more firecontrol problems than unstable air. Werth and Ochoa 

(1990), Saltenberger and Barker (1993), and Goens and 

Andrews (1998) found good correlation between the 

Haines Index and extreme fire behavior on fires in Idaho, 

central Oregon, and Arizona. 

In summary, unstable air amplifies the vertical growth 

of the smoke plume over a fire by enhancing the strength 

of the updrafts. This increases combustion rates by supplying more oxygen to the fire. As the height and strength of 

the smoke plume increases, the potential for gusty surface 

winds, dust devils, and fire whirls also increases. Spotting 

may become profuse all around the fire as large firebrands 

are lifted in the smoke plume. (Note: For more information 

concerning the effects of atmospheric stability on extreme 

fire behavior, see chapters 5 through 7.) Unstable air also 

increases the probability of thunderstorms and strong 

downdraft winds.

Beals (1916) defined drought as “Long-continued dry 

weather, especially so long continued as to cause vegetation 

to wither.” Beals also stated that while “Drought and periods of hot weather contribute to the fire hazard, these alone 

do not necessarily portend the occurrence of a great fire, 

as without wind an incipient fire would spread slowly.” He 

recognized that drought and hot weather do not necessarily 

result in large fires, but a critical weather element, such as 

strong wind, is also needed to produce a large fire. Today 

drought is defined as a period of relatively long duration 

with substantially below-normal precipitation, usually 

occurring over a large area. Drought affects fuel availability by lowering the moisture content of both live and dead 

fuels, making them more combustible. Drought conditions 

Figure 3-1—Surface pressure map 01 September 1894 at 0800 

Central Standard Time. (Adapted from Beals 1914.)

are NOT a prerequisite for large fires, but there is a close 

relationship between drought conditions, large wildland 

fires, and extreme fire behavior when low relative humidity 

and either strong wind or unstable air are present.

Critical Fire Weather Patterns 

Critical fire weather patterns occur when atmospheric 

conditions combine to significantly increase the threat of 

destructive wildland fires that exhibit extreme fire behavior. 

Fire weather research has identified adverse atmospheric 

conditions as strong wind, unusually low relative humidity, 

and unstable air. Drought is also included as a significant 

factor, but is the result of a lack of precipitation over a 

period of weeks, months, or even years. 

Beals (1914) researched the September 1, 1894, Minnesota Hinkley Fire in which 418 people perished. He was 

a pioneer in studying synoptic weather maps depicting 

pressure, temperature, and wind patterns associated with 

large fires. On the Hinkley Fire, the weather map (fig. 3-1) 

showed a surface low pressure center in North Dakota and 

tightly packed isobars favoring strong wind in Minnesota. It 

should be noted that his map does not depict cold and warm 

fronts because frontal theory was not introduced until 1917 

by Norwegian meteorologists Vilhelm and Jacob Bjerknes.

Show (1931) was the first to document weather being 

largely responsible for dangerous fire conditions when he 
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wrote, “It was generally recognized that occasionally in 

every fire season there occurred short periods of one or 

several days when the forest cover was unusually flammable 

and at times seemed almost explosive.” He concluded, 

“Abnormal weather conditions were responsible for these 

periods.” 

The relationship between synoptic weather patterns and 

high fire danger was further advanced by Schroeder (1950). 

He noted that for the Great Lake States in May, “Nearly all 

of the critical periods were associated with an area of high 

pressure which developed near the western shore of Hudson 

Bay and subsequently moved either southward or southeastward.” 

An early definition of a critical fire weather pattern 

was provided by Syverson (1962) when he described it as a 

“crisis period.” He stated, “A crisis period is defined as: the 

critical day, week or month during which blow-up fires are 

experienced.” 

Syverson (1963) expanded his concept of a crisis period 

in an investigation of synoptic fire weather types of the 

Northern Intermountain, Northern Rockies, and the Northwestern Plains regions. He selected synoptic weather types 

(upper air 500 hPa and surface) that contributed to high fire 

potential or large forest fires. The 500 hPa upper air patterns were divided into meridional, zonal, short-wave train, 

and high-low block categories. The surface patterns were 

classified according to the origin of the surface anticyclones 

(high pressure) affecting the area. Syverson concluded, 

“The greatest danger occurs just ahead of the upper trough 

in the area of the low pressure at the surface.”

The most complete research of critical fire weather patterns was published by Schroeder et al. (1964) in Synoptic

Weather Types Associated With Critical Fire Weather. This 

study covered all the Lower 48 States and determined: 

“Periods of critical fire weather are associated with relatively few synoptic weather patterns.” They concluded that 

east of the Rocky Mountains, most critical fire weather 

patterns are associated with the periphery of high-pressure 

areas, particularly in the prefrontal and postfrontal areas. 

Along the eastern slopes of the Rocky Mountains, weather 

patterns producing Chinook winds are the most important. 

In the intermountain West, critical fire weather is associated 

with upper troughs and overhead jet streams, or surface dry 

cold front passages. Along the Pacific Coast, from Washington to California, weather patterns producing offshore flow 

or foehn wind are the most important. 

Brotak and Reifsynder (1977b) detailed the relationship 

of Central and Eastern U.S. wildland fires with surface 

frontal systems and upper level troughs and ridges. They 

found that just prior to and after passage of cold fronts (fig. 

Figure 3-2—Idealized surface map showing all fire runs. CFA = 

following cold frontal passage; CFB = preceding cold frontal 

passage; WSL = warm sector of low; and WS = warm sector of 

high. Source: Brotak and Reifsnyder 1977b. 

Figure 3-3—Idealized 500 hPa geopotential height map showing 

all fire runs. CFA = following cold frontal passage; CFB = preceding cold frontal passage; WSL = warm sector of low; and WS = 

warm sector of high. Source: Brotak and Reifsnyder 1977b.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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East of the Rocky Mountains, most 

critical fire weather patterns are 

associated with the periphery of 

high-pressure areas, particularly in 

the prefrontal and postfrontal areas. 

Along the eastern slopes of the Rocky 

Mountains, weather patterns producing Chinook winds are the most 

important. In the intermountain West, 

critical fire weather is associated 

with upper troughs and overhead jet 

streams, or surface dry cold front passages. Along the Pacific Coast, from 

Washington to California, weather 

patterns producing foehn wind are the 

most important.

3-2) were favored times for large fire growth to occur. At 

500 hPa, the favored area was between the upper ridge and 

trough axis (fig. 3-3).

Nimchuk (1983) documented the relationship between 

the breakdown of a blocking upper level ridge and severe 

fire behavior conditions in western Canada. He concluded 

that the trigger for extreme fire behavior was the breakdown 

of the upper ridge, rather than the presence of a persistent 

upper ridge. His statements concerning the fire behavior 

associated with the three stages in the life cycle of an upper 

ridge are of particular interest (fig. 3-4).

1. An establishment period characterized by warm, 

dry stable conditions, low humidity, light wind, 

rapidly decreasing fuel moisture, and low lightning risk.

2. Initial weakening of upper level disturbances, 

leading to decreased atmospheric stability and 

increased lightning activity, but little or no cooling 

or reduction in fire danger.

3. Final breakdown, accompanied by a period of 

severe burning conditions, strong winds, and 

lightning followed by cooling and a reduction in 

fire danger.

In summary, these studies indicate that most periods 

of critical fire weather occur in transition zones between 

high- and low-pressure systems, both at the surface and in 

the upper air. The surface pressure patterns of most concern 

are those associated with cold fronts and terrain-induced 

foehn winds. Cold front passages are important to firefighters because of strong, shifting winds and unstable air that 

can enhance the smoke column or produce thunderstorms. 

Foehn winds occur on the lee side of mountain ranges and 

are typically very strong, often occurring suddenly with 

drastic warming and drying. The area between the upper 

ridge and upper trough has the most critical upper air 

pattern because of unstable air and strong winds aloft that 

descend to ground level. 

Regional Critical Fire Weather Patterns

The following section will briefly describe critical fire 

weather patterns by region and season. Critical fire weather 

patterns can be separated into two primary categories:

• Those that produce strong surface wind.

• Those that produce atmospheric instability.

Figure 3-4—Life cycle stages of an upper level ridge.
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30

In both cases, an unusually dry airmass, for the region 

and season, must also occur. Strong wind with high relative humidity is not a critical fire weather situation nor is 

unstable air combined with high relative humidity. 

When critical fire weather patterns occur during 

periods of drought, the threat of extreme fire behavior significantly increases in brush and timber fuels. However, in 

grass fuels, some of the worst fire behavior has occurred in 

moist periods owing to increased fuel loadings. The key to 

identifying a critical fire weather pattern is the recognition 

that these patterns must also produce unusually low relative 

humidity for the region, along with strong surface wind or 

unstable air.

Northern Plains, Great Lakes, and the 

Northeastern United States

The fire season in this region primarily occurs before greenup in the spring and after leaf drop in the fall. The spring 

season can start as early as March in the Northern Plains 

and the Ohio River Valley and as late as April in the Great 

Lakes and Northeast States. The fall season can last through 

November. 

Critical fire weather patterns in this part of the country 

are identified by the source of surface high-pressure areas 

before or after the passage of cold fronts. That is because 

the source of these high-pressure areas determines the moisture content of the airmass and whether passing cold fronts 

will be wet or dry. There are three surface high-pressure 

types that can produce critical fire weather and extreme fire 

behavior in this region.

Pacific High— 

This high pressure originates over the Pacific Ocean 

and loses much of its moisture as it crosses the Rocky 

Mountains. It moves into the Northern Plains and Great 

Lakes States with a dry continental airmass. This is the 

most common type and shows little preference for any 

particular month.

Northwest Canadian High— 

This high pressure is normally warm and dry owing to 

its source region, subsidence warming, and southward 

movement over warmer land. Critical fire weather occurs on 

the periphery of the high, especially the north and northwest 

sides. This type occurs during the spring and fall.

Hudson Bay High— 

This is similar to the Northwest Canadian High. The most 

critical fire weather is on the northwest side of the high. 

However, dry cold fronts can produce extreme fire behavior, 

both before and after frontal passage. Schroeder (1950) 

indentified the Hudson Bay High as the principal weather 

type associated with periods of very high fire danger for the 

Great Lakes States. 

Brotak (1979) analyzed the weather and fire behavior 

conditions during the July 22, 1977, Bass River Fire in New 

Jersey. The fire claimed the lives of four firefighters when 

flames overran their position. Drought, strong wind, unusually low relative humidity, and extreme instability contributed 

to the extreme fire behavior experienced during the fire. 

The extreme fire behavior occurred after the passage of a 

cold front and in the southeast quadrant of a Hudson Bay 

high-pressure area (fig. 3-5). The 500 hPa (fig. 3-6) shows 

an upper level trough over New Jersey and a northwesterly 

flow of subsiding air in the leading edge of high pressure 

over the Great Lakes.

Figure 3-5—1400 Eastern Daylight Time surface weather map. 

Source: Brotak 1979. 
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Simard et al. (1983) researched the weather, topography, 

fuels, and fire behavior of the May 5, 1980, Mack Lake Fire 

in Michigan. They concluded that the extreme fire behavior 

observed on the Mack Lake Fire occurred as follows: 

“Ahead of the weak cold front (fig. 3-7), relative 

humidity was low at 24 percent, and the temperature 

was unseasonably high at 26.7 °C (80 °F). Windspeed (at the Mio weather station) increased 

significantly to 24 km/h (15 mi/h), gusting to 40 

km/h (25 mi/h) plus as the front approached.” This 

is a classic prefrontal critical fire weather pattern 

during the spring months for the Great Lakes States.

The August 25, 1995, Sunrise Fire on Long Island, 

New York, is another example of a fire that burned during 

a postfrontal critical fire weather pattern, with north winds 

and a relative humidity of less than 20 percent reported. 

It burned approximately 2833 ha (7,000 ac) and damaged 

numerous homes and small businesses. 

Southeastern United States

The Southeastern United States encompasses an area from 

eastern Oklahoma and eastern Texas, eastward across the 

lower Mississippi Valley and the Gulf States, to the Atlantic 

coast from North Carolina to Florida. Fire season in the 

Figure 3-6—0800 Eastern Daylight Time, 500 hPa geopotential 

height map. Source: Brotak 1979. 

Southeast is typically during the spring and fall. However, 

wildland fires do occur at other times of the year. The 

spring fire season occurs in the weeks before green-up. This 

usually begins during March near the Carolina and Georgia 

coast and the Gulf States. The fall fire season occurs in 

October and November, normally after the first frost. 

Oklahoma and Texas are typically dry in late winter, and 

large grass fires are not uncommon in February. The Florida 

season may extend through the winter and spring well into 

June, especially during periods of drought. Critical fire 

weather patterns in this region are those that produce low 

relative humidity, and either strong surface wind or unstable 

air. 

McCarthy (1923), in a study of fire weather in the 

southern Appalachian Mountains, observed, “Low vapor 

pressure (related to low dew point and low relative humidity) usually accompanies high atmospheric pressure and 

seems to be induced by prevailing wind from the west or 

northwesterly directions, while south or easterly winds tend 

to increase the humidity.”

McCarthy (1924) further stated, “Winds, coming from 

the interior of the continent and warming as they move 

southward, are usually low in humidity, a condition which 

is increased by the downward convection of cold air in 

Figure 3-7—Cold frontal positions during the Mack Lake Fire. 

Source: Simard et al. 1983.
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the high pressure zone which warms as it approaches the 

surface.” 

Williams and Smith (1962) documented the weather 

and fire behavior associated with the March 1953 Brasstown 

Fire in South Carolina. They determined that the fire’s large 

growth and extreme fire behavior occurred after the passage 

of a cold front when northwesterly winds brought dry air 

from Canada and the Great Lakes.

Early fire case studies concluded that high fire activity in the Southeast is more often associated with surface 

high-pressure systems that originate in Canada or those that 

move across the Rocky Mountains from the Pacific Ocean. 

The important characteristic of these high-pressure systems 

is the dry air that replaces the moist Gulf of Mexico or 

Atlantic Ocean airmass, which normally covers this part of 

the country.

The movement of surface high-pressure systems is 

dependent upon the upper level windflow. For that reason, 

it is difficult to discuss critical fire weather patterns without 

linking the surface features to upper level pressure patterns. 

Three upper level patterns are effective in keeping the 

Southeast under the influence of high pressure at the surface. If the antecedent conditions of below normal rainfall 

are in place, a critical fire weather pattern emerges.

Strong westerly flow— 

During the spring and fall, strong westerly winds aloft 

result in a rapid succession of Pacific fronts traversing the 

Southeast. Little, if any, moisture from the Gulf of Mexico 

is able to return to the region in advance of these cold fronts. 

Rainfall with the front is sparse and light. Exceptionally low 

relative humidity may occur the day after frontal passage, 

and little recovery can be expected before the next front 

arrives. Strong and gusty winds are a distinct possibility.

Northwesterly flow— 

Dry air, associated with Canadian high-pressure systems, 

can spread across the Southeast during the spring and fall. 

The initial Canadian cold front moves through the Southeast 

and remains stationary far south of the region until the upper 

level pattern changes. A large and stagnant high-pressure 

system settles over the region. Weak fronts from the north 

may reinforce the dry airmass. Relative humidity may not 

be quite as low as with Pacific fronts, and better humidity 

recovery can be expected at night. Strong northwest to north 

winds often occur as the surface high pressure pushes into 

the Southeast. 

Blocking ridge aloft— 

This pattern occurs when high pressure aloft persists near 

the Atlantic coast for an extended period of time, possibly 

for a few weeks. Weather systems from the west or north 

are blocked from moving through the region. Little or no 

rainfall is produced during the period that the upper level 

ridge is in place.

In addition to the upper level patterns, extreme fire 

behavior can also occur in advance of a tropical storm 

owing to subsidence-produced dry air and a strong wind 

area that extends beyond the cloud and rain shield.

Critical fire weather patterns should be carefully examined for the presence of strong low-level jets (i.e., reverse 

wind profile). Research conducted by Byram (1954) showed 

a strong connection between low-level jets and extreme 

fire behavior in the Southeast. (Note: For more information 

concerning low-level jets and adverse wind profiles, see 

chapter 5.)

The combination of extreme drought and critical fire 

weather patterns were major factors in the severe 1998 

Florida wildland fire season. Fires in the northern and 

central portions of the state experienced major fire runs on 

July 4, driven by strong westerly winds and unusually low 

relative humidity of 30 percent or less (fig. 3-8). The source 

of the dry air was the Great Plains, which was pushed into 

Florida by a northwesterly upper level windflow (fig. 3-9).

Southwestern United States 

The Southwestern region includes the states of Arizona, 

New Mexico, and west Texas. The normal fire season spans 

the months of May to October but can extend throughout 

the year in the grasslands of eastern New Mexico and 

western Texas.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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Crimmins (2005) examined the seasonal climatology 

of extreme fire weather conditions across Arizona and New 

Mexico during the period 1988–2003. He found that there 

are three key upper level patterns associated with over 80 

percent of the extreme fire-weather days identified in this 

study. These upper level patterns represent broad southwesterly flow and large geopotential height gradients and are 

very similar to the critical fire weather patterns identified by 

Schroeder et al. (1964). All three of these upper level patterns are consistent with the “breakdown of the upper level 

ridge” critical fire weather pattern defined earlier.

The major critical fire weather patterns of the Southwest are listed below.

Breakdown of Upper Ridge— 

This is the most prevalent pattern in the Southwest, as a 

mean 500 hPa ridge is frequently positioned over the area 

during the fire season. From late spring through the early 

summer, upper level troughs moving inland from the Pacific Ocean are strong enough to temporarily push the upper 

ridge east and south of the area. These upper troughs are 

Figure 3-8–Visible satellite picture 2132 Coordinated Universal 

Time 01 July 1998 shows strong, dry westerly wind pushing 

smoke columns from central Florida wildfires out over the Atlantic Ocean. Source: National Oceanic and Atmospheric Administration and University Corporation for Atmospheric Research.

Figure 3-9–500 hPa geopotential height 

map indicating northwesterly flow aloft 

over Florida. (National Oceanic and 

Atmospheric Administration National 

Centers for Environmental Prediction/

National Center for Atmospheric 

Research reanalysis.)
GENERAL TECHNICAL REPORT PNW-GTR-854

34

manifest at the surface as dry, cold fronts, which produce 

strong winds, very low relative humidity, and isolated dry 

lightning. The airmass becomes unstable as the upper level 

trough approaches, resulting in moderate to high Haines 

Index values. Strong upper level winds will frequently mix 

down to the surface, producing winds of 64 to 80 km/h (40 

to 50 mi/h). The peak fire season ends when these upper 

troughs stay well to the north and the southwest monsoon 

becomes fully developed.

Early Stage Monsoon— 

The onset of the southwest monsoon can present an opportunity for extreme fire behavior owing to the combination 

of gusty wind, low relative humidity, and dry lightninginduced fire starts. As the mean 500 hPa ridge builds north 

in June and early July, moisture begins to increase at mid 

levels while surface conditions remain hot and dry. The 

speed and strength at which the monsoon develops determines the severity of this pattern. If the monsoon starts 

slowly, there may be enough dry lightning to overwhelm 

local fire management resources. If it develops quickly, dry 

storms will rapidly become rain producers and effectively 

end the fire season. When surface dew points rise to 10 to 

15 °C (50 to 59 °F), the majority of storms will be wet.

Lee surface trough/dryline— 

This pattern occurs in eastern New Mexico and western 

Texas in advance of an approaching upper level trough. 

Well ahead of the upper trough, a north-south dryline 

develops in the surface pressure pattern that sharply divides 

moist air to the east and dry air to the west. The passage 

of a dryline is similar to that of a dry cold front. Strong, 

gusty southwest winds develop and surface dewpoint 

temperatures drop from 10 to 20 °C (50 to 69 °F) to -5 to -10 

°C (13 to 19 °F). This results in very low relative humidity 

and rapidly drying fuels. Dry, windy conditions behind a 

dryline can last for hours until the trailing cold front moves 

through with much cooler temperatures, higher relative 

humidity, and decreasing west to northwest winds. 

The May 7, 2000, Cerro Grande Fire in New Mexico 

exhibited extreme fire behavior owing to a critical fire 

weather pattern known as “the breakdown of the upper level 

ridge.” A strong upper level trough (fig. 3-10) was moving 

into Arizona and New Mexico, pushing a ridge that had 

been over the area into Texas and Oklahoma. Strong southwest surface winds (fig. 3-11) were experienced on the fire 

with gusts up to 120 km/h (75 mi/h). Drought conditions 

and extremely low relative humidity also contributed to 

the extreme fire behavior. The final size of the fire was 

20 234 ha (50,000 acres) and 235 homes were burned. 

Rocky Mountain and Intermountain Regions

These two regions cover much of the interior Western 

United States. The Rocky Mountain region includes the 

states of Montana, Wyoming, Colorado, and northern Idaho. 

The Intermountain region comprises the states of Nevada, 

Utah, and southern Idaho. The fire season ranges from May 

through October in the southern and June through October 

in the northern portions of these regions. However, in the 

grasslands of eastern Colorado, eastern Wyoming, and 

eastern Montana, it may start as early as February or March 

prior to green-up.

Figure 3-10—07 May 2000, 500 hPa geopotenital height map. 

(National Oceanic and Atmospheric Administration National 

Centers for Environmental Prediction/National Center for 

Atmospheric Research reanalysis.).
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

35

A considerable amount of fire weather research has 

been conducted in these regions, beginning with the historic 

1910 Great Idaho Fire. Beals (1914) studied this fire that 

burned over 809 372 ha (2 million acres) in Idaho and 

Montana and caused 85 fatalities. He noted, “There were 

many fires burning in northern Idaho, but they were kept 

under fair control until August 20, when a hot, high wind 

from the southwest began to blow. They burned so furiously 

that nothing could be done to stop them.”

Syverson (1962, 1963, 1964) researched and identified 

a number of critical fire weather patterns in the Northern 

Rocky and Intermountain regions as part of a “Nationwide 

Study of Synoptic Fire Weather Types” project spearheaded 

by Schroeder, Glovinsky, Hendricks, and others. He studied 

weather patterns on days when the fire danger was high on 

days of large fire activity and concluded that:

Figure 3-11—07 May 2000 visible satellite picture showing well-defined smoke plume driven by strong southwest 

winds. (National Oceanic and Atmospheric Administration, National Weather Service.)

• The area of high fire danger is almost always on the 

southwest or west side of the high-pressure cell at 

the surface.

• The greatest danger occurs just ahead of the upper 

trough in the area of the low pressure at the surface.

• The breakdown of this fire weather type (high 

pressure) comes with a strong upper air impulse of 

cooler air moving through from the Pacific.

Syverson’s conclusions agree very well with what 

occurs during the “breakdown of the upper level ridge” 

critical fire weather pattern.

Anderson (1968) examined the weather and fire environment conditions during the September 1, 1967, major 

run of the Sundance Fire in northern Idaho. He found that 

the extreme fire behavior on this fire occurred with strong 

winds and low relative humidity in the prefrontal area ahead 

of an advancing cold front. 
GENERAL TECHNICAL REPORT PNW-GTR-854

36

Werth and Ochoa (1993) documented the weather and 

fire behavior that occurred on the 1988 Willis Gulch and 

1989 Lowman Fires in central Idaho. The “breakdown 

of the upper level ridge” critical fire weather pattern was 

identified as significantly contributing to extreme fire 

behavior observed on both fires. They concluded that this 

pattern consisted of both upper level and surface pressure 

pattern components (fig. 3-12) that resulted in high Haines 

Index values. These index values correlated well with the 

rate of spread (ROS) for both fires, validating the usefulness 

of the Haines Index. The transition zone, between an upper 

level ridge and upper trough, and in an area defined by the 

surface thermal low-pressure trough, is a favored location 

for moderate and high Haines Index values.

Gibson (1996) found that the “breakdown of the upper 

ridge” critical fire weather pattern was present with major 

increases in area burned on wildland fires in the Northern 

Rocky region. Gibson stated, “The pattern develops as the 

normal upper level ridge is shoved east by an approaching 

shortwave trough.”

In a study of lightning-induced wildfires in Nevada, 

Miline (2006) found that two weather patterns account for 

all 17 of the major outbreaks over a 9-year sample period. 

Figure 3-12—Typical 500 hPa and surface pressure pattern 

associated with the “breakdown of the upper ridge” critical fire 

weather pattern. Pattern is also favorable for moderate to high 

Haines Index values. MSL= Mean sea level. Source: Werth and 

Ochoa 1993. 

The “monsoon pattern” accounted for 7 of the 17 events and 

is characterized by high pressure centered over northern 

Utah and southern Idaho. With this pattern, warm, moist air 

originating in the Gulf of California is advected northward 

into Nevada triggering thunderstorms. The second and 

more significant pattern involves a negatively tilted shortwave trough moving northeastward from the eastern Pacific 

Ocean into north-central California and through northern 

Nevada and southern Oregon.

The following is a brief summary of critical fire 

weather patterns in the Rocky Mountain and Intermountain 

regions.

Upper ridge-Surface thermal trough— 

This is the most significant pattern for these regions. It is 

characterized by a strong north-south upper ridge along 

105 to 110 degrees west longitude and a hot, dry surface 

thermal trough extending from central California to eastern 

Washington or Idaho. High fire danger results when a weak 

mid- to upper level trough moves up the west side of the 

ridge, producing dry lightning in the vicinity of the thermal 

trough. If the upper trough is strong enough, the upper ridge 

will break down and the thermal trough will shift eastward 

across the area. A dry and windy surface cold front will 

then follow the thermal trough, producing very high fire 

danger and increasing the threat of extreme fire behavior on 

ongoing wildland fires.

Early stage monsoon— 

This pattern occurs with an upper level ridge around 105 

degrees west longitude and an upper trough off the Pacific 

coast. It results in dry lightning and gusty winds over the 

southern parts of these regions.

Foehn wind/Chinook wind— 

These strong downslope winds, along the eastern slopes of 

the Rocky Mountains, are unusually warm and dry for the 

season. This pattern occurs when strong jet stream winds 

blow perpendicular to the mountains and the airmass is 

stable. They are most pronounced in the winter and spring, 

but can occur during the fall. When the upper level windflow is from the southwest, the onset of Chinook winds is 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

37

often prior to the passage of a weak cold front. When the 

flow is northwesterly, the strong wind begins after frontal 

passage.

The 1994 South Canyon Fire in western Colorado is a 

good example of a fire that burned during a “breakdown of 

the upper ridge” critical fire weather pattern. On the afternoon of July 6, the fire rapidly transitioned from a surface to 

a crown fire during the passage of a dry cold front. Tragically, 14 firefighters perished when the fire overran their 

position. The upper level pattern that afternoon (fig. 3-13) 

showed a low center in northwestern Wyoming and a trough 

southward along the Colorado/Utah border. This upper level 

system replaced an upper ridge that had been previously 

over Colorado. A surface cold front moved across the fire 

site earlier in the afternoon and at 1800 Mountain Daylight 

Time (MDT) was located in eastern Colorado (fig. 3-14). 

This weather pattern not only produced strong, gusty winds 

and unusually low relative humidity (<10 percent), but also 

very unstable air. Fuels were also especially dry owing to 

long-term drought.

The September 6–7, 1988, extreme fire behavior exhibited on the Yellowstone National Park (northwest Wyoming) 

and Canyon Creek (Montana) Fires also occurred during 

Figure 3-13—1800 Mountain Daylight Time, 06 July 1994, 500 

hPa map. Source: USDA, USDI, and USDC 1994.

Figure 3-14—1800 Mountain Daylight Time, 06 July 1994, 

surface map. Source: USDA, USDI, and USDC 1994.

South L

a “breakdown of the upper level ridge.” An upper level 

trough and a strong west-to-northwest jet stream (fig. 3-15) 

produced winds in excess of 80 km/h (50 mi/h), unusually 

low relative humidity, and major crowning on both of these 

wildland fires. The passage of two cold fronts (fig. 3-16) 

added to the severity of the weather pattern. A Chinook 

wind developed in Montana, pushing the Canyon Creek Fire 

well east of the Continental Divide. Long-term drought was 

also a major factor. 

Pacific Northwest Region

The Pacific Northwest region comprises the states of 

Washington and Oregon. The typical fire season is short 

compared to other regions and extends from June through 

early October. 

There are two critical fire weather patterns in this 

region, foehn or east winds in western Washington and 

western Oregon, and the “breakdown of the upper ridge” 

from the crest of the Cascade Mountains eastward across 

eastern Washington and eastern Oregon. 

East winds were recognized as a fire problem west of 

the Cascades from the beginning of fire weather research. 

Beals (1914) and Joy (1923) noted that large fires west of 

the Cascades were caused by strong east winds that were 
GENERAL TECHNICAL REPORT PNW-GTR-854

38

unusually hot and dry for the area. They also noted that 

these strong winds occurred when there was high pressure 

east of the Cascades and low pressure west of the Cascades.

Dague (1934) documented weather during the August 

1933 Great Tillamook Fire that burned 105 882 ha (261,640 

ac) in western Oregon. He stated, “Low relative humidity, 

fresh to strong easterly winds, and high temperatures were 

responsible for this huge fire.” Dague also observed that 

a surface low-pressure trough west of the Cascades 

contributed to the strength of these winds, and the trough 

pushed northward from the interior of California.

Saltenberger and Barker (1993) researched weather and 

extreme fire behavior conditions during the August 4–5, 

1990, Awbrey Hall Fire in central Oregon. They concluded 

Figure 3-15—1200 Coordinated Universal 

Time 07, September 1988, 500 hPa geopotential height map showing a trough over Alberta, 

Canada, and a strong northwesterly jet stream 

over Idaho, Montana, and Wyoming. (National 

Oceanic and Atmospheric Administration, 

National Weather Service.)

that the plume-dominated wildfire became severe owing to 

a combination of fuels and weather, noting, “The Haines 

Index performed well. When the index indicated moderate 

to high growth potential the fire displayed extreme behavior 

and rapid growth.”

In a study of lightning-induced wildland fires in the 

Pacific Northwest, Rorig and Ferguson (1999) discovered 

that there were distinctly different weather patterns between 

dry and wet thunderstorm days. The pattern for dry days 

showed an upper trough near the coast and a pronounced 

thermal trough at the surface in eastern Washington and 

eastern Oregon (near the Idaho border). Wet-pattern days 

show a deeper upper trough (much lower geopotential 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

39

Figure 3-16—1200 Coordinated Universal 

Time 07, September 1988, surface pressure 

map showing cold fronts over northwestern 

Wyoming along the Canadian/United States 

border. (National Weather Service 1993.)

heights) and a weak surface thermal trough in southern 

Idaho and eastern Nevada.

Critical fire weather patterns of the Pacific Northwest 

are detailed below.

Foehn wind/east wind— 

Severe east wind patterns occur when surface high pressure pushes inland behind the passage of a cold front and 

becomes centered over eastern Washington, Idaho, or western Montana. Meanwhile, the California surface thermal 

trough pushes northward along the Oregon and Washington 

coasts (fig. 3-17). This pressure pattern produces strong 

pressure differences (gradients) across western Washington 

and western Oregon, resulting in offshore flow and northeast-to-east winds of 80 to 97 km/h (50 to 60 mi/h) through 

the Columbia Gorge and the ridges and passes of the 

Cascade and coastal mountains. Subsidence also results in 

warming and drying of the airmass, and relative humidity 

can drop to 10 percent or lower. The combination of strong 

wind and unusually low relative humidity often results in 

wind-driven fires and extreme fire behavior. The upper 

level pattern (fig. 3-18) shows a strong high amplitude ridge 

off the coast between 130 and 140 degrees west longitude. 

The east wind pattern normally ends when the upper ridge 

moves inland and the surface thermal trough either dissipates or pushes east of the Cascades. This pattern typically 

occurs during September and early October and often 

represents the peak of the fire season west of the Cascades. 

Upper ridge breakdown—This is similar to the type previously described for the Rocky Mountain and Intermountain 

regions. In this case, the pattern is shifted farther west so 

the southwest flow is over Oregon and Washington. This 

pattern occurs when an upper level trough approaches the 

coast pushing the upper ridge to the east. Cooling aloft 

results in unstable air and an increased risk of lightning. If 

the airmass is dry, moderate to high Haines Index values 

and dry lightning are possible. The upper level winds will 
GENERAL TECHNICAL REPORT PNW-GTR-854

40

Figure 3-18—Typical east wind 

500 hPa geopotential height pattern 

with strong ridge off the Washington 

and Oregon coasts. (National Oceanic 

and Atnospheric Adminstration, 

National Weather Service.)

Figure 3-17–East wind surface pressure pattern with a thermal trough just 

off the coast and a high centered over 

Idaho and western Montana. Pattern 

pro-duces strong pressure gradients 

and strong winds from the crest of 

the Cascades to the Coast. (National 

Oceanic and Atmospheric Administration, National Weather Service.)
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

41

frequently mix to the surface, resulting in strong gusty 

winds. Meanwhile, the surface thermal trough will shift 

eastward across the area increasing the threat of extreme 

fire behavior on new and ongoing wildland fires.

California Region

The fire season extends from mid-May through October in 

northern California and from late March through December 

in southern California. However, during drought years, the 

season in southern California can extend throughout the 

year. 

Krumm (1954) examined the meteorological conditions 

that affected the July 9, 1953, Rattlesnake Fire in northern 

California. Fifteen firefighters were killed on this fire. 

He determined that strong downslope winds occurred on 

the fire after sunset, caused by a strong pressure gradient 

between surface high pressure along the Pacific Coast and 

a thermal trough over the Sacramento Valley. This wind 

develops and descends to the surface similar to other foehn 

winds with low relative humidity and warm temperatures.

Weather, fuels, and fire behavior of the 1956 Inaja Fire 

were researched by Countryman et al. (1956) to determine 

what caused 11 firefighter fatalities during the fire’s major 

run. They determined that the fire burned during a Santa 

Ana wind event in a very wind-prone canyon in the San 

Diego area.

Ortel (1964) studied serious fire weather conditions 

in northern and central California as part of a nationwide 

study of synoptic fire weather types. He identified five 

weather patterns of concern: an upper level high over the 

Southwestern States, an upper high over the Pacific Ocean, 

an upper trough offshore near 130 degrees west longitude, 

surface cold fronts, and easterly winds from surface highpressure systems over the Great Basin.

The following is a summary of critical fire weather 

patterns in California. 

Foehn winds/north and mono winds— 

This is the most common critical fire weather pattern in 

northern and central California. These strong, dry winds 

occur when surface high pressure builds into the Pacific 

Northwest, resulting in large pressure differences (gradients) across northern California. Dry air moves from 

Oregon southward into the Sacramento Valley with additional warming and drying. Relative humidity of 10 percent 

or less with temperatures of 43 °C (110 °F) can occur in the 

valley under these conditions. Windspeed strength depends 

on the pressure gradient, upper level windflow, and local 

topography. When the upper windflow is from the north 

or northeast, windspeed values in excess of 65 km/h (40 

mi/h) often occur. Mono winds are strong easterly winds 

that occur along the western slopes of the Sierra Nevada 

Mountains. They are similar to the above-mentioned North 

winds, but in this case, the center of the surface high pressure is located in Nevada and Utah. This is primarily a late 

summer and fall pattern, but can occur at other times during 

the year if the fuels are dry.

Foehn winds/Santa Ana and sundowner winds— 

This is the primary critical fire weather pattern for 

southern California. The pattern develops when surface 

high pressure builds over Nevada, Utah, and northern 

Arizona after the passage of an upper level trough. Meanwhile, an upper ridge of high pressure builds off the Pacific 

Northwest coast. North to northeasterly flow around the 

upper ridge results in cold air advection and strengthening 

of the surface high over the Great Basin. High pressure over 

Nevada and low pressure along the California coast result 

in strong pressure gradients over southern California. As a 

result, strong north to east winds develop from the crest of 

the mountains into the coastal areas. Air descending from 

higher to lower elevations causes compressional heating, 

which results in dramatic heating and drying of the air. 

When Santa Ana winds occur, extreme fire behavior conditions can suddenly develop as relative humidity drops to 

10 percent or less and winds increase to 80 km/h (50 mi/h) 

or more. Winds can be substantially stronger in mountain 

passes and canyons. These winds are typically strongest 

at night and during the morning hours, and diminish 

somewhat during the afternoon owing to surface heating. 

This pattern occurs most often during the fall and winter 

months. 
GENERAL TECHNICAL REPORT PNW-GTR-854

42

A sundowner wind is an offshore northerly foehn 

wind that occurs in the lee of the Santa Ynez Mountains, 

which rise directly behind Santa Barbara and the surrounding coastal area. They develop when high pressure at the 

surface is centered over the Pacific Northwest and northern 

California and pressure gradients are perpendicular to the 

east-west axis of the Santa Ynez Mountains. These winds 

often precede Santa Ana events by a day or two. The normal 

progression is for the surface high pressure to migrate into 

the Great Basin causing pressure gradients and winds to 

shift more to the northeast and east ending the sundowner 

winds. (Note: For more information on Santa Ana and 

sundowner winds, see chapter 2.) 

The October 2007 siege of wildland fires in southern 

California is a good example of a Santa Ana critical fire 

weather pattern. These massive wildfires burned hundreds 

of thousands of hectares, displaced nearly a million people, 

destroyed thousands of homes, and resulted in 10 fatalities. 

The surface and upper level pressure patterns are 

shown in (figs. 3-19 and 3-20). Strong surface high pressure 

was centered over Utah and Nevada, and an upper ridge was 

located off the California coast. A satellite picture (fig. 3-21) 

shows numerous smoke plumes being driven off the coast 

by northeast to east winds. Surface winds in excess of 80 

km/h (50 mi/h) and relative humidity of less than 10 percent 

was reported on the fires. 

Subtropical high aloft— 

This pattern occurs when the westerlies shift northward, 

causing a closed subtropical high to become centered over 

the Southwest. The upper ridge axis extends far enough off 

the coast to block subtropical moisture from the area. This 

pattern produces heat waves in California. When a weak 

Figure 3-19—1200 Coordinated Universal Time, 22 October 2007, surface 

pressure map. High centered over Utah 

with strong pressure gradients over 

southern California. (National Oceanic 

and Atmospheric Administration, 

National Weather Service.)
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

43

upper trough pushes into the western portion of the upper 

ridge, instability can result in a significant outbreak of dry 

lightning (fig. 3-22). 

Alaska

The fire season in Alaska extends from May through 

August but is most active during June and July. 

The primary critical fire weather pattern in Alaska is 

the “breakdown of the upper level ridge.” 

Breakdown of the upper ridge with southwest flow—

This pattern occurs when southeasterly winds push moist, 

unstable air into the retreating upper level ridge (fig. 3-23). 

This can bring gusty winds and dry lightning to the interior 

of Alaska. The June 1998 Carla Lake Fire burned under 

these conditions caused by wind gusts of 56 km/h (35 mi/h) 

and relative humidity of less than 25 percent.

Models and Predictive Tools

The Predictive Services Program is national in scope. It 

supports the wildland fire community and others with 

information and decision-support products. The program 

encompasses meteorologists and intelligence coordinators 

at each geographic area coordination center (GACC) and 

the National Interagency Coordination Center (NICC). 

Fire behavior or long-term analysts are detailed to GACCs 

during the fire season.

The following is a list of products produced by Predictive Services units that are useful in determining areas of 

greatest concern in relation to large fire potential and the 

possibility of extreme fire behavior.

Figure 3-20—1200 Coordinated 

Universal Time, 22 October 2007, 

50 hPa geopotential height map. 

A strong high was centered off 

the northern California coast. 

(National Oceanic and Atmospheric Administration, National 

Weather Service.)
GENERAL TECHNICAL REPORT PNW-GTR-854

44

Figure 3-22–Critical California lightning pattern with subtropical 500 hPa ridge over the Great Basin and short-wave trough 

moving inland from the Pacific Ocean. (National Oceanic and 

Atmospheric Administration National Centers for Environmental 

Prediction/National Center for Atmospheric Research reanalysis.) 

Figure 3-23—“Breakdown of the upper ridge” critical fire weather 

pattern in Alaska. Source: National Weather Service 1993.

Figure 3-21—22 October 2007 satellite picture showing smoke from numerous southern California wildfires blowing out over the Pacific Ocean. (National Oceanic and Atmospheric Administration, National Weather Service).
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

45

The National Wildland Significant Fire Potential 

Outlook

This product is prepared by NICC on the first business day 

of each month. The report consists of national maps and 

associated text that depict areas of below normal, normal, 

and above normal significant fire potential. 

GACC 7-Day Significant Fire Potential

This GACC product is produced daily during the primary 

fire season under the direction of a qualified fire weather 

meteorologist. The report contains projected fire weather, 

fuel dryness, fire danger, fire potential, and resource status 

information for the next 7-day period. A short discussion 

accompanies the report detailing weather of concern 

through the period. 

Fuel and Fire Behavior Advisories

These advisories are issued to inform fire managers and 

firefighters of safety concerns owing to existing or predicted 

fuel and fire behavior conditions.

Other GACC Products and Services

The GACC Predictive Services units provide a wide variety 

of products and services in support of wildland fire operations. These include weather/intelligence briefings, situation 

reports, and resource summaries. 

Fire Weather Watches and Red Flag Warnings

The National Weather Service (NWS) provides fire weather 

products and services in support of fire management decisions. Some of the best tools in assessing the potential for 

critical fire weather situations are the Fire Weather Watch 

and Red Flag Warning program, and Spot Weather Forecasts.

Fire Weather Watches and Red Flag Warnings are 

issued when the combination of dry fuels and weather 

conditions indicate the possibility of extreme fire danger 

or fire behavior. These conditions alert land management 

agencies to the potential for widespread new ignitions that 

could overwhelm initial attack activities, or conditions that 

could cause control problems on existing fires, etc. Any of 

these outcomes could pose a threat to life and property. Fire 

Weather Watches are issued when there is a high potential 

for the development of a Red Flag Event. Red Flag Warnings are used to warn of an impending, or occurring Red 

Flag Event. Its issuance denotes a high degree of confidence 

that weather and fuel conditions consistent with local Red 

Flag Event criteria will occur in 24 hours or less.

Spot Weather Forecasts/Digital Web Services

A spot forecast is a site-specific 24- to 36-hour forecast 

issued to fit time, topography, and weather of a specific 

location. The spot forecast can be requested for wildfires, 

prescribed burns, spray projects, and other special projects. 

Other products available include FARSITE data streams and 

point forecast matrix forecasts from the National Digital 

Forecast Database. The NWS issues thousands of spot 

forecasts per year, and there is extensive use of digital Web 

services in diagnosing fire risks resulting from critical fire 

weather patterns.

The Storm Prediction Center’s (SPC) Fire Weather program issues a daily national fire weather guidance product 

for use by the NWS, as well as other federal, state, and local 

government agencies. The product is intended to delineate 

areas of the contiguous United States where preexisting 

fuel conditions, combined with forecast weather conditions 

during the next 8 days, may result in a significant threat of 

wildfires. 

There are three types of Fire Weather Outlook areas:

• Critical Fire Weather Area for wind and relative 

humidity.

• Extremely Critical Fire Weather Area for extreme 

conditions of wind and relative humidity.

• Critical Fire Weather Area for dry thunderstorms.

The SPC Fire Weather Outlook comprises a day 1 and a day 

2 forecast, in addition to a day 3 through 8 forecast. 

Summary/Knowledge Gaps

Fire weather research has been ongoing for nearly a century, and many advances have been made during that time 
GENERAL TECHNICAL REPORT PNW-GTR-854

46

concerning weather’s effect on wildland fire behavior. 

Wind and relative humidity have been effectively incorporated into the fire behavior models. However, the effect of 

atmospheric stability on fire behavior is not modeled and 

remains subjective at best. More research is needed, beyond 

the Haines Index, to quantify the effects of atmospheric 

stability on fire behavior.

The concept of critical fire weather patterns has been 

in existence for 50 years. It has been successfully applied to 

fire case studies, but rarely has it been used in conjunction 

with weather forecast models to predict periods when large 

fires or extreme fire behavior are likely to occur.

Future research into the climatology and dynamics of 

these weather patterns and their effects on fire behavior 

would be beneficial, especially for the “breakdown of the 

upper ridge” pattern. Research concerning the fire behavior 

effects associated with the surface thermal trough also need 

to be better defined.

References

Anderson, H.E. 1968. Sundance Fire: an analysis 

of fire phenomena. Res. Pap. INT-56. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 

37 p.

Beals, E.A. 1914. The value of weather forecasts in the 

problem of protecting forests from fire. Monthly Weather 

Review. 42(2): 111–119.

Beals, E.A. 1916. Droughts and hot weather. Monthly 

Weather Review. 44: 135–138.

Brotak, E.A. 1979. The Bass River Fire: weather conditions 

associated with a fatal fire. Fire Management Notes. 

40(1): 10–13.

Brotak, E.A. 1992–1993. Low-level weather conditions 

preceding major wildfires. Fire Management Notes. 

53-54: 23–26.

Brotak, E.A.; Reifsnyder, W.E. 1977a. An investigation 

of the synoptic situations associated with major wildland 

fires. Journal of Applied Meteorology. 16(9): 867–870.

Brotak, E.A.; Reifsnyder, W.E. 1977b. Predicting major 

wildland fire occurrence. Fire Management Notes. 

38: 5–8.

Brown, A.A. 1950. Warning signs for fire fighters. Fire 

Control Notes. 11(3): 28–29. 

Byram, G.M. 1954. Atmospheric conditions related 

to blowup fires. Sta. Pap. 35. Asheville, NC: U.S. 

Department of Agriculture, Forest Service, Southeastern 

Forest Experiment Station. 34 p.

Byram, G.M.; Nelson R.M. 1951. The possible relation of 

air turbulence to erratic fire behavior in the Southeast. 

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Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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1 Mark A. Finney and Sara S. McAllister, USDA Forest Service, 

Missoula Fire Sciences Laboratory, Missoula, MT.

Introduction

Some interactions of wildland fires are experienced routinely under field conditions. Firefighters and prescribed 

fire personnel see flames tilting toward adjacent ignition 

points or fire edges, particularly as the sources advance 

closer together (Martin and Dell 1978, Rothermel 1984). In 

the extreme case, interactions occurring when large areas 

are ignited and burning simultaneously are described as 

mass fires, area fires, or “fire storms” (Countryman 1964). 

Hundreds or thousands of individual fires may interact 

over an area and exhibit some “unified” behavior. Such 

fires are generally described as having such strong indrafts 

that outward propagation is minimal. They have extremely 

tall convection columns or smoke plumes and burn for 

long durations until all the fuel within the perimeter is 

consumed. Good reviews of mass or large area fires can be 

found in Williams (1982), Pitts (1991), and Heskestad (1998). 

Mass fires were responsible for tremendous burning rates 

and tornado-strength winds (Carrier et al. 1985) witnessed 

after the fire bombings of cities in Germany and Japan 

during World War II (Hewitt 1983, Schmalz 1992) and have 

been studied mainly in relation to consequences of nuclear 

attacks (Balwin and North 1967; Chandler 1963; Countryman 1964, 1965, 1969; Eggleston 1968; German 1968; 

Hewitt 1983; Larson et al. 1982; Larson and Small 1982a, 

1982b; Lee 1969a, 1969b; Lommasson et al. 1967, 1968; 

Nielsen 1970; Nielsen et al. 1963; Parker 1967; Penner et al. 

1986; Pryor and Yuill 1966; Quintiere 1993; Sanderlin et al. 

1981; Wood et al. 1971). Many of these studies were through 

“Project Flambeau,” a joint effort between the U.S. Office of 

Civil Defense Defense Atomic Support Agency and the U.S. 

Department of Agriculture Forest Service in the mid-1960s. 

These fires were designed to mimic a suburb fire. Each 

square fuel bed was constructed with a mixture of pinyon 

pine and juniper (see “Common and Scientific Names” 

section) and was approximately the same size and fuel load 

as a typical suburban house (185.8 m2

 and about 18 000 kg 

of fuel). The spacing between fuel beds was either 7.6 m 

or 35.1 m and fire sizes were 2, 6, 12, and 20 ha. Airflow 

velocities and temperatures were measured inside and 

just outside the fire area along with thermal radiation just 

Chapter 4: Fire Interactions and Mass Fires

Mark A. Finney and Sara S. McAllister1

Figure 4-1—Prescribed fire ignition patterns designed to restrict 

or enhance fire front interactions (from Martin and Dell 1978).

Figure 4-2—Indrafting and flame response of sequential line fires 

in prescribed burning (from Rothermel 1984).
GENERAL TECHNICAL REPORT PNW-GTR-854

50

outside the fire area, oxygen and carbon dioxide concentrations inside the fire area, and the mass loss rate of the fuel 

beds (Countryman 1964, 1965, 1969).

Wildland fire interactions are intentionally manipulated 

for ignition or firing operations (see figs. 4-1 and 4-2) to 

orient spread directions (Johansen 1987), to use indrafts 

for backfire operations (Miralles et al. 2010), to increase 

the development of convection columns on prescribed fires 

through center-firing techniques (Martin and Dell 1978), 

and to limit spread and intensity with spot fire ignitions 

(Luke and McArthur 1978, Johansen 1984, Weatherspoon 

et al. 1989). Rapid increases in fire growth and energy 

release—termed “blowup”—are sometimes associated with 

fire interactions (Arnold and Buck 1954). Yet, despite the 

common usage and practical familiarity with interactions 

that fire personnel often acquire, there is very little quantitative physical understanding of these behaviors and no 

operational models that can predict them. By comparison to 

other fire behavior characteristics, such as fire spread rates, 

fire interactions at any scale have been subject to limited 

study. 

In this review, we endeavored to obtain literature from 

many sources, including wildland fire and structural fire, 

as well as combustion engineering and fluid dynamics, in 

order to cover the range of research on fire-fire interactions 

and the state of knowledge. Our search revealed that the 

topic of fire interactions overlaps considerably with other 

fire behaviors that are distinguished individually, such as 

vortices and terrain effects. These behaviors will be mentioned when appropriate, but their full discussion is beyond 

the scope of this review.

Background: Time-Dependent Fire 

Behaviors 

For a constant set of environmental conditions, fire 

behavior is known to change with time. These changes 

are not expressly considered interactions, but spread and 

intensity changes within individual fires are also affected 

during interaction among fires and may contribute to later 

development of interactions. Thus, such behaviors provide 

useful background material for discussion of fire-fire 

interactions, although studies of fire acceleration have not 

directly addressed interactions of multiple fires. Many of 

the time-dependent changes in fire behavior are associated 

with fire growth or expansion in two dimensions. Changes 

are observed in spread rates (acceleration), frontal geometry (width, curvature), and heat transfer indicated by the 

orientation and size of flames. These fire characteristics are 

interrelated with spread processes, and the literature does 

not discern the causes of observable features as distinct 

from their probable effects. 

Fire Acceleration

Fire acceleration is defined as the time-dependent changes 

in spread and intensity occurring under constant weather 

and uniform fuel conditions. The notion of acceleration 

is implicitly applied to fires that are already capable of 

spreading as compared to combinations of threshold conditions where spread only occurs above some limit. Various 

mathematical representations of acceleration (fig. 4-3) have 

been proposed from a theoretical standpoint that express 

spread rate from a point-source fire as a negative exponential function of time (Cheney 1981, Cheney and Bary 1969). 

Parameters of these equations were fit to empirical data 

Figure 4-3—Theoretical fire spread rate acceleration curves from 

point-source ignitions show asymptotic increase in spread rate 

over time toward an equilibrium (from McAlpine and Wakimoto 

1991). ROS = rate of spread.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

51

from wind tunnel experiments by McAlpine and Wakimoto 

(1991). These functions tend toward a final equilibrium rate 

and are, thus, commonly communicated in terms of the 

time to reach some fixed fraction of equilibrium (e.g., 90 

percent). A similar result was developed by Weber (1989), 

who represented acceleration of fires expanding as a circle 

from a point ignition and depended on the curvature of the 

fire front.

Studies of acceleration typically report time elapsed 

from ignition to a near-steady spread rate. Values of 20 to 

30 min for point-source ignitions in slash fuels for prescribed fire conditions (McRae 1999) and in pine litter and 

feather moss (Kucuk et al. 2007) have been reported. Winddriven grass fires in Australia (Albini 1982) showed large 

variation in acceleration times (about 6 min under slow 

wind conditions to over 45 min with faster winds) and a 

strong dependency on the width of the fire front. Wind tunnel burns of shallow (8 cm deep) pine needle and excelsior 

beds suggested time to equilibrium of only a few minutes 

(McAlpine and Wakimoto 1991) and largely independent of 

windspeed. Data from point ignitions in pine needle litter 

reported by Curry and Fons (1938) suggested windspeed 

affected acceleration rate (increased time to equilibrium) 

as well as a final spread rate. Windspeed may also affect 

acceleration times for conflagrations involving structures 

at urban densities. Chandler et al. (1963) referenced much 

longer time estimates than for wildland fuels, including 1 

hr to achieve near-steady spread rates with windspeed up 

to 6.7 m/s (15 mi/h), 2 hrs for winds to 17.9 m/s (40 mi/h) 

and possibly much longer times for stronger winds. A long 

acceleration period, exceeding the 36-min observation 

time, was described for line ignitions in heavy fuel loadings 

associated with felled eucalyptus slash (McArthur 1969a) 

(see “Common and Scientific Names” section). By contrast, 

rapid acceleration to near-steady burning after line ignition 

was reported for experimental crown fires in jack pine 

forests (Stocks 1989). Implications of a theoretical analysis 

by Albini (1982) suggests that line ignitions in surface fuels 

could accelerate very rapidly, initially overshooting the 

steady rate, but then slow and exhibit damped oscillations 

toward the steady value as the increasing vertical buoyancy 

of the combustion zone offsets horizontal wind force. From 

the existing literature, it is not clear what influences the 

various factors of fuel loading, fuel sizes, burning duration, 

and final spread rates have on acceleration time, nor more 

complicated interactions among multiple flame zones or 

heat sources.

Acceleration of fires can also occur when air inflow 

is asymmetrically restricted by surface topography, either 

in canyons (Viegas and Pita 2004), or inclined channels 

(Woodburn and Drysdale 1998) and slopes (Dold and 

Zinoviev 2009, Wu et al. 2000). Detailed treatment of these 

important fire-topographic interactions, however, is beyond 

the scope of this review of fire-fire interactions.

Length of Fire Front

Fire acceleration and final spread rate appear to be dependent on fire size. Fires accelerate slowly from point-ignition 

sources (Cheney and Gould 1995, McAlpine and Wakimoto 

1991, McRae 1999) relative to line-source ignitions (Cheney 

and Gould 1995, Johansen 1987). At the small scale of 

laboratory stick arrays, fuel bed width and proportion of 

edge on the curvature of the head fire had significant effects 

on spread rate (Fendell and Wolff 2001). In wind-driven 

grass fires, fire spread rates were found to be dependent on 

the length of the ignition for lines shorter than 50 to 75 m 

(Cheney and Gould 1995) and required longer acceleration 

times for higher winds (fig. 4-4). Experiments and modeling 

by Wotton et al. (1999) for fires in red pine litter, however, 

showed no increase in radiation from flames for ignition 

lines longer than about 2 m and no effect of line width on 

spread rate beyond about 1 m. Dold et al. (2006) offered 

an explanation for fire size effect on forward spread rate. 

As fires expand in two dimensions, the distance between 

the fire edges increases, meaning that buoyancy-induced 

inflow along segments of flaming front comes from a wider 

area. This allows ambient winds from behind the front to 

penetrate to the heading portion of the flame zone. Such 

effects on narrow combustion zones of expanding fires is 

presumably different than for mass fires or large-area 
GENERAL TECHNICAL REPORT PNW-GTR-854

52

ignitions, which create indrafts from all directions (Baum 

and McCaffrey 1989, Smith et al. 1975), and strong buoyancy-driven convection may deflect ambient airflow around 

the column (Countryman 1964).

Flame Tilt

Flame angle orientation relative to the unburned fuel is 

related to acceleration and is affected by fire size and stage 

of growth. Flames can tilt owing to wind, slope, or the 

interaction with other fires. Flames tilted away from the 

direction of spread are referred to as backing fires, and 

flames tilting toward the direction of spread are referred 

to as heading fires. Flames tilt toward the interior of the 

burned area in small fires or point-source fires, producing backing spread (Fendell and Wolff 2001, Luke and 

McArthur 1978, Tolhurst and Cheney 1999). Spread rate of 

backing fires spreading downslope has been shown to be 

only weakly diminished as slope increases (Van Wagner 

1988) and little affected by wind (Beaufait 1965, McAlpine 

and Wakimoto 1991). Backing fires have been reported to 

increase fuel consumption and residence times. As fires 

grow larger, backing fire remains only at the rear of the 

perimeter (upwind or downslope) and flames for the heading 

portion of the fire tilt toward the unburned fuel. The very 

large differences in spread rate and intensity between 

backing and heading fires (and flanking fires) can be estimated assuming elliptical fire shapes (Catchpole et al. 1982). 

Numerous studies of flame tilt angle in a wildland fuel bed 

on flat terrain in wind have consistently found a strong 

relationship to the Froude number calculated from ratios of 

windspeed to intensity or flame length (Albini 1981, 1982; 

Nelson and Adkins 1986; Weise and Bigging 1996). Similar 

experimental results were found using liquid pool fires 

(Martin et al. 1991, Pipkin and Sliepcevich 1964, Welker 

and Sliepcevich 1966, Welker et al. 1965) and explained as 

the counteraction of upward buoyant forces by crossflow, 

including flame trailing (lateral deflection of combustion 

products and flames) with high windspeeds. Recent numerical modeling (Nmira et al. 2010) has also reported Froude 

number relationships for both line-source and point-source 

simulated fires. Although slope effects were deemed significant (Weise and Bigging 1996), they are not accounted 

for in such formulations. When fires are in proximity, the 

interaction between them can change the flame tilt angle 

and rates of spread (Pitts 1991, Rios 1966, Welker et al. 

1965). In these cases, the flame tilt angles can be correlated 

Figure 4-4—Fire spread rates in 

grass fuels were found to increase 

with the width of head fires and 

depend on the final spread rates 

determined by windspeed (from 

Tolhurst and Cheney 1999).
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

53

with a modified Froude number that includes the separation 

distance of the fires (Pitts 1991, Rios 1966, Welker et al. 

1965). In the case of no wind, a modified Grashof number 

is used (Gebhart et al. 1976, Pera and Gebhart 1975) to 

describe the flame tilt purely owing to flame interaction.

Spread Thresholds

Thresholds describe a point of near-instantaneous acceleration that delineates when fire will and will not spread. 

Threshold-crossing for fire spread has been documented for 

many discontinuous fuel types including grasses (MarsdenSmedly et al. 2001), shrubs (Brown 1982, Burrows et al. 

1991, Bradstock and Gill 1993, Weise et al. 2005), and trees 

(Bruner and Klebenow 1979, Van Wagner 1977). Laboratoryscale fires reveal similar spread thresholds in arrays of 

small sticks (Beer 1995, Vogel and Williams 1970, Weber 

1990) and taller beds of excelsior (Finney et al. 2010). These 

studies reveal threshold dependencies on multiple environmental, fuel, and fire variables, such as windspeed, fuel 

moisture, slope, horizontal fuel gap dimensions, fuel bed 

depth, fuel combustion rate, and flame size. Chandler (1963) 

proposed combinations of ranges of windspeed, humidity, 

and rainfall by fuel type to define spread thresholds for significant growth of large fires. Recent studies of fire spread 

sustainability provide empirical evidence on the importance 

of fuel moisture, wind, and fuel loading (Beverly and 

Wotton 1997, Leonard 2009). As described in later sections 

of this chapter, fire interactions exert strong influences over 

many of these same environmental and fire variables and, 

thus, may elicit threshold-crossing spread for fires burning 

in discontinuous fuels.

Conditions Where Fire Interactions Occur

Interactions are possible when many separate fires grow 

together or multiple segments of a single continuous fire are 

oriented in proximity. In natural wildland fires, multiple 

fronts often occur because of spotting from a single main 

fire. Spot fires are relatively common under dry and windy 

conditions and even long-distance spotting contributes to 

fire movement (Anderson 1968). But massive deposition of 

firebrands at relatively short distances from the fire front 

(a few kilometers) can substantially increase spread rate 

and create simultaneous area ignition (Cheney and Bary 

1969). On wildfires, Cheney and Bary observed that the 

highest concentration of fire brands fell within a fan-shaped 

zone about 9 degrees in angle on either side of the primary 

wind direction and theorized that mass fire behavior could 

be achieved for certain unspecified combinations of fire 

brand density and acceleration time for individual ignitions. Johansen (1984) made similar observations for spot 

ignition patterns on prescribed burns where higher spot 

densities increased the numbers and frequencies of junction 

or merger zones. The increase in intensity at such junction 

zones have been documented empirically (Johansen 1984, 

McRae et al. 2005) and modeled (Morvan et al. 2009) 

leading to recommendations for wide separation of ignitions (Marsden-Smedley 2009, Tolhurst and Cheney 1999) 

unless area ignition is desired (Taylor et al. 1973). Mass 

ember deposition and area ignition has been documented 

by McArthur (1969b) for Tasmanian fires, which resulted in 

near-simultaneous ignition of hillsides. A similar process 

was proposed for the Air Force Bomb Range Fire (Wade 

and Ward 1973), which periodically caused area ignition 

ahead of the main front and vertical development of a 

convection column. Modeling by Weihs and Small (1986) 

showed that interactions between large mass fires can even 

cause these typically nonspreading fires to propagate toward 

one another.

How close together fires must be before flames visibly interact and subsequently merge is not clear. There 

have been many empirically derived merging criteria in 

the literature. Correlations exist for the critical parameters 

for both flame interaction (Baldwin 1968, Liu et al. 2007, 

Sugawa and Takahashi 1993) and merging (Delichatsios 

2007, Fukuda et al. 2004, Putnam and Speich 1963, Wood et 

al. 1971). These correlations take many forms—some define 

a critical ratio between the fire spacing and fire diameter 

(Sugawa and Takahashi 1993, Wood et al. 1971) or flame 

height (Baldwin 1968, Delichatsios 2007, Liu et al. 2007), 

some define a critical ratio between the flame height and 
GENERAL TECHNICAL REPORT PNW-GTR-854

54

fire diameter (Wood et al. 1971), and some define a critical 

dimensionless heat release rate (Fukuda et al. 2004, Putnam 

and Speich 1963). Upon close examination, however, it 

becomes clear that fire spacing, fire diameter, flame height, 

and dimensionless heat release rate have interdependencies, 

and, thus, these different correlations are not necessarily 

contradictory. The discussion here will focus on the relations between spacing, diameter, and flame height because 

they are the most intuitive.

Using both gas diffusion burners and pool fires, 

Sugawa and Takahashi (1993) reported that flames begin 

to interact when the ratio of the spacing distance to the 

fire diameter is less than four. In other words, flames can 

interact, here defined as visually tilting, over distances four 

times their diameter, Baldwin (1968) considered the onset 

of flame interaction in terms of flame height. Flames were 

considered to be interacting if the flame heights increased 

more than 10 percent above the independent flame height. 

Using square and round gas burners, wood cribs, and large 

timber yard fires, Baldwin (1968) (and Baldwin 1966, 

Baldwin et al. 1964, Thomas 1968) correlated experimental 

data over a wide range of scales and configurations found in 

the literature and determined that the flames would interact 

if the spacing were less than 0.22 times the flame length. 

For a characteristic dimension D and height L, this correlation holds for 1 < L/D < 300. Liu et al. (2007) also found 

the same dependency but with a slightly different constant 

of proportionality for merging of round pool fires. In their 

experiments, flame merging was likely to occur when 

closer than 0.29 to 0.34 times the merged flame length. 

Delichatsios (2007) also found that flames began to merge 

at spacing less than 0.33 times the actual flame length for 

gaseous burners. The discrepancy in these constants may 

be due to different definitions of flame interaction (tilting 

versus change in flame height) and flame merging (using 

completely merged flame height versus actual flame height), 

different fuels, and possibly uncertainty of measuring flame 

dimensions. In comparing the results of the Project Flambeau fires to those using a sand-filled pan burner, Wood et 

al. (1971) reported that flames merged if the flame height 

was at least half of the fire diameter. Heskestad (1998) 

clarified that this occurs when the nondimensional group 

N ~Q2/D5 is near 10-5 (Q is the heat release rate and D is 

the fire diameter). Clearly there is no definitive criterion for 

when flames begin to interact and merge, and these relations 

will remain qualitative guidelines until there is some sort of 

unifying theory.

An opposing effect may occur with area fires over 

large homogenous fuel beds (small flame height compared 

to fire diameter). For a sufficiently large fuel bed, it may be 

impossible for a continuous flame to exist over the entire 

bed. Instead of one continuous flame, the fire may break 

up into many distributed flamelets (Countryman 1969, 

Heskestad 1991, Wood et al. 1971). Heskestad (1991) showed 

that the breakup of continuous flames occurs when the 

nondimensional group N ~Q2/D5 is near 10-6. The convection column for these cases has been described as having 

two modes: Bénard cell convection near the surface, which 

then merges and transitions to a more organized convective 

plume (Fosberg 1967).

Specific Effects of Fire Interaction

Studies of fire interactions involve specific types of behavior of the combustion and observable fire characteristics. 

Much of the research on these behaviors comes from laboratory experiments with artificial fuel sources and attempts to 

isolate the particular response of interest.

Burning Rate

When fire fronts are close enough to interact and merge, 

such as in a mass fire, the mass of fuel burned as a function 

of time, or burning rate, of the fire can change dramatically. 

Much of the research on fire interactions has been done 

using gas burners with a fixed burning rate, but there has 

been some work on the interaction of flames over liquid 

pool fires and wood crib fires. Although the geometry and 

heat transfer mechanisms inside the fuel bed are different, 

liquid pool fires are much like fires burning over solid 

fuel in that the heat transfer from the fire back to the fuel 

controls the burning rate. In contrast, the burning rate of a 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

55

gas burner is controlled by using a fixed fuel supply rate. 

Results from pool and crib fire experiments can often be 

extended to larger fuel beds using appropriate scaling laws 

(Emori and Saito 1983). 

The experiments by Huffman et al. (1969) clearly reveal 

the effect of spacing on the burning rate of pool fires. In 

this work, the burning rate of an array of liquid pools was 

measured while keeping a constant fuel depth and varying 

the number of pools, pool diameter, fuel, and pool separation distance. In general, the burning rate of each individual 

pool burner increases as the burners are brought closer 

together and the flames began to interact. In particular, 

the pools in the middle of the array show a very dramatic 

increase. For example, figure 4-5 shows that the burning 

rate of 4-inch (10-cm)-diameter pools of cyclohexane 

experienced over a 400 percent increase in burning rate 

when the separation distance was halved. At the onset of 

flame merging, the burning rate is at its maximum. As the 

flames merge, the burning rate decreases as the separation 

distance continues to decrease. In the limit of zero separation distance, however, the burning rate of the individual 

fires is still larger than if they were burning independently 

with no interaction effects. These trends were also seen by 

Grumer and Strasser (1965) with solid fuel beds.

Kamikawa et al. (2005) studied the effect of flame 

merging on heat release rates (heat released per time). Heat 

release rate is calculated by multiplying the burning rate 

(mass of fuel burned per time) by the heat of reaction (heat 

released per mass of fuel burned). However, the heat of 

reaction is dependent on the fuel and the mixture ratio of 

fuel to air. In large fire arrays, the inner regions of the array 

typically experience a shortage of air. Without sufficient air, 

the fuel cannot completely react and release the full potential heat, i.e., the combustion efficiency is low and less heat 

is released per mass of fuel. Not surprisingly, Kamikawa et 

al. saw the same trend with heat release rates as Huffman 

et al. (1969) with burning rates. When the flames are merged, 

the heat release rate increases with separation distance. 

Figure 4-5—Burning rate as a function of separation distance for 10.1-cm-diameter cyclohexane 

burners (from Huffman et al. 1969).
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As the burners are moved farther apart, more air can penetrate into the inner regions of the array. More air entrainment means greater combustion efficiency and greater heat 

release. This, in turn, heats up and evaporates the unburned 

fuel more quickly, increasing the burning rate. 

Liu et al. (2009) explained the mechanisms behind 

these trends in burning and heat release rate with separation 

distance. The non-monotonic behavior seen in figure 4-5 

is the result of two competing mechanisms: heat feedback 

enhancement and air entrainment restriction. As the burners 

are moved closer, the view factor between neighboring fires 

increases. In other words, the fires can “see” each other 

better, increasing the radiative heat transfer in addition to 

the convective heat transfer (Grumer and Strasser 1965). 

Because the burning rate is dictated by the heat feedback 

from the flame, this increased radiative heat seen by the fuel 

will evaporate the fuel more quickly and increase the burning rate. Conversely, as the fires get sufficiently close, there 

is less room to entrain air inside the array and the flames 

become “choked.” When the flames are merely interacting, 

the heat feedback mechanism is more important than the air 

restriction and the burning rate increases. When the flames 

have merged, the air restriction is the dominant mechanism 

and the burning rate decreases.

Because the experiments by Kamikawa et al. (2005) 

used wood crib fires, they were also able to examine the 

release rate as a function of time for merged flames. As 

with most wildland fires, the heat release rate (and burning rate) of wood crib fires increases as the fire builds, 

reaches a maximum, then begins to decrease as the fuel is 

depleted. Kamikawa et al. (2005) made the observation that 

as the number of fires increases, the peak heat release rate 

increases above that expected by multiplying the independent fire heat release rate by the number of fires. This 

discrepancy grows as the number of fires increases. So the 

burning and heat release rates of interacting and merging 

fires not only are dependent on the spacing of the fires, but 

also on the total number of fires (see also Liu et al. 2009).

Fire interactions can increase burning rates by another 

mode as well. If the fires interact such that vorticity is generated, fire whirls can form. Although not discussed further 

here, it has been shown that fire whirls have dramatically 

increased burning rates in comparison to an equivalent, 

nonrotating fire (see e.g., Emmons 1965, Grishin et al. 2004).

Flame Dimensions

Flame height trends for a non-premixed flame, such as those 

in a wildfire, are usually discussed in terms of two dimensionless parameters: the dimensionless flame height and the 

dimensionless heat release rate. The dimensionless flame 

height is usually defined as the flame height divided by 

the characteristic burning area diameter (D). The characteristic burning area diameter is a dimensioned parameter 

frequently introduced in fire arrays and is usually some 

function of the number of fires, fire diameter, and the fire 

arrangement (separation distance). The dimensionless heat 

release rate (Q*) is usually defined as the total heat release 

rate of the group divided by the characteristic burning 

area diameter to the five-halves power (material property 

constants are used to make the ratio dimensionless: Q* ~

Q tot /D5/2). The dimensionless heat release rate for natural 

fires tends to fall between 0.05 and 5 (McCaffrey 1995).

Much of the research on flame height has been performed using gas burners. However, two regimes of flow 

from a gas burner can be identified. When the flow velocity 

is low or the burner diameter is large, the momentum of 

the gaseous fuel is due primarily to its buoyancy. When the 

flow velocity is high or the burner diameter is small, the 

flow is like a jet. Putnam and Speich (1963) have a method 

for determining whether the flow from a gas burner is a 

high-momentum jet or buoyancy controlled. The discussion 

here will be limited to turbulent, buoyancy-driven flames, 

as this situation better describes what occurs during a 

wildfire. 

In general, the flame height increases as the fires are 

moved closer. When the flames begin to merge, the flame 

height will dramatically increase with further decreases 

in separation distance. However, once the flames are fully 

merged, further decreases in separation distance will have 

little effect (Chigier and Apak 1975, Fukuda et al. 2004, 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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Putnam and Speich 1963). The dimensionless flame height 

has successfully been correlated to the dimensionless heat 

release rate raised to some power, a. Because the dimensionless heat release rate can range over at least seven orders 

of magnitude, this power “a” can take on three different 

values depending on the range of the dimensionless heat 

release rate. As shown in figure 4-6 (Quintiere and Grove 

1998), the dimensionless flame height increases with the 

dimensionless heat release rate. These correlations were 

originally developed for the flame height of a single independent burner where the characteristic dimension is the 

burner diameter, and hold for buoyancy-driven gas burners, 

liquid pool fires, and wood crib fires. However, there is an 

indication that these correlations also apply to interacting 

flames when the characteristic burning area dimension is 

given as discussed above. For example, for the interaction of 

relatively tall flames compared to the actual burner diameter 

(Lf /D > 1, or high values of Q*), Putnam and Speich (1963) 

and Sugawa and Takahashi (1993) showed that the dimensionless flame height correlates well with the dimensionless 

heat release rate to the two-fifths power (Lf /D ~ Q*2/5). 

Delichatsios (2007) successfully correlated the dimension 

2.5 two thirds power (Lf /D ~ Q*2/3) for Q* between 0.1 and 

1. On the other hand, Weng et al. (2004) and Kamikawa et 

al. (2005) showed that the data for merged flame height is 

better correlated with the exponent “a” varying with the 

number of burners. 

With all else remaining constant, these correlations 

suggest that an increase in either the number of fires or the 

individual fire heat release rate will increase the interacting 

or merged flame height. Increases in the separation distance 

or the fire diameter will result in a decrease in the interacting or merged flame height. An interesting caveat to these 

correlations is that the burning rate for individual pool or 

crib fires is not constant, but is a function of the separation 

distance as discussed above. This trend is not necessarily 

captured in figure 4-6 or by Putnam and Speich (1963) (gas 

burners), Kamikawa et al. (2005), Fukuda et al. (2004), or 

Figure 4-6—Dimensionless flame height (Lf /D) correlations with dimensionless heat release 

rate (Q*) (Quintiere and Grove 1998).

Lf /D~Q*2 Lf /D~Q*(2/3) Lf /D~Q*(2/5)
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Delichatsios (2007) (all fully merged flames). Also, vorticity 

can greatly increase flame height as well (Emmons 1965).

This literature suggests that in a mass fire situation, as 

the flames grow closer together, the heat release rate and 

characteristic “burner” diameter should increase. The net 

effect is most likely an increase in the flame height. If more 

spot fires were ignited in the burning area, for example, 

the flame height would increase further. This is consistent 

with the observations of spot ignitions on prescribed burns 

(Johansen 1984) and mass spotting in wildfires (Cheney 

and Bary 1969). However, for a sufficiently large area or 

mass fire, when the nondimensional group N ~Q2

/D5 is near 

10-6, the fire is not expected to burn as a continuous flame 

but will break up into many distributed flamelets (Countryman 1969, Heskestad 1991, Wood et al. 1971). In this case, 

the flame height will be less than that predicted for a fully 

merged, continuous flame but larger than that of isolated 

flames (Thomas 1963). 

Flame Temperatures and Pollutants

As discussed in relation to flame height, as fires are moved 

closer together, air entrainment is blocked and the gaseous 

fuel must travel higher to find sufficient air for combustion. 

Experiments by Chigier and Apak (1975) indicated that a 

fuel particle on its journey from the base to the tip of an 

interacting turbulent flame would experience delayed combustion compared to an independent flame (see fig. 4-7a). 

The delay means that the maximum temperature of the 

interacting flames would occur further from the flame base. 

With limited mixing of fresh air into the flame to provide 

cooling, the temperatures inside an interacting flame decay 

more slowly with height so the flame is hot over a greater 

portion. In addition, limited mixing of air into the flames 

causes the formation of more carbon monoxide inside the 

flame zone. This prompted Countryman (1969) to speculate 

that the lack of oxygen in conjunction with elevated carbon 

monoxide could be fatal to ground personnel trapped inside 

the burning area.

Figure 4-7a—Effect of nearby burners on flame temperature (from Chigier and Apak 1975). 

DT is throat diameter, DE is exit diameter, a is separation distance, Tm is merged flame 

temperature, and Ts

 is single flame temperature.

a
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Figure 4-7b—Temperature compared to independent flame for varying axial distance along flame, 

number of burners, burner arrangement, and burner spacing (from Chigier and Apak 1975).

Chigier and Apak (1975) also showed that the maximum temperature achieved by interacting turbulent flames 

is also a function of the separation distance and the number 

of burners (see fig. 4-7b). When the flames are close enough 

to interact, they lose less heat from radiation (the surroundings are at the same temperature) and by mixing with cool, 

fresh air. The maximum temperatures inside interacting 

flames therefore increase as the number of fires increases 

and as the burners get closer together. These increased temperatures could produce more of the smog-forming nitrogen 

oxide emissions (Tarr and Allen 1998).

Indraft Velocity

In typical fire situations where the flame height is relatively 

tall compared to the fire diameter, standard correlations 

exist to predict the mass of air entrained by the fire and its 

plume owing to the velocity difference between the plume 

gases and the ambient air. This air entrainment causes an 

inflow into the fire and is generally responsible for the 

bending of two flames in relative proximity. However, 

the standard correlations of plume theory are valid only 

above the flame. Although several plume theories exist 

in the literature (see review in Heskestad 2008), there is 

general agreement that the total mass of air entrained can be 

estimated as proportional to the convective heat release rate 

(heat release rate minus radiative and other losses) raised to 

the one-third power and to the height above the fire source 

to the five-thirds power. Fires with greater heat release 

rate entrain more air, and the total amount of air entrained 

increases with height above the plume. Note, however, that 

the velocity of the flow inside the plume decreases with 

height, so at some point near the top of the plume no further 

air is entrained (no velocity difference). Current research 

on the indraft caused by entrainment as related to fire 

interactions is focused mainly on providing better quantitative predictions with computational fluid dynamics (CFD) 

modeling (Morvan et al. 2009, Roxburgh and Rein 2008).

However, plumes from wildfires can interact with local 

meteorology (Weber and Dold 2006) such as wind and 

atmospheric conditions. Additionally, classic plume theory 

b
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for entrainment rates may not hold for small ratios of the 

flame height to fire diameter (Lf /D). Although the exact 

threshold is not known, Heskestad (2008) contends that 

the standard plume theory falls apart for Lf /D somewhere 

between 0.14 to 0.9. The perimeter of the plume where 

entrainment occurs becomes too small in relation to the 

volume of air inside and the slow moving entrained air will 

not have much effect on the momentum of the entire plume. 

Mass fires by definition fall into the range of flame height 

to fire diameter ratios where classic plume theory does not 

hold. The results of the Project Flambeau burns confirm that 

there is little entrainment into the plume core (Palmer 1981). 

Many authors (e.g., Adams et al. 1973, Small et al. 1983, 

Smith et al. 1975) also argue that the entrainment of plume 

theory does not account for the reported high-velocity winds 

associated with mass fires. As discussed earlier, mass fires 

are characterized by such strong indrafts that the fire does 

little outward propagation. In their review of the range of 

possible indraft velocities, Trelles and Pagni (1997) showed 

that indraft velocities of large fires can range from about 

2 to 40 m/s. In the Project Flambeau burns, Countryman 

(1964, 1965, 1969) also reported complicated airflow patterns and strong downdrafts that cannot be accounted for 

with simple plume theory.

There seem to be two main theories in the literature 

as to what causes the high-velocity inflows. One theory, 

advanced by Baum and McCaffrey (1989) and Carrier et al. 

(1985) is that large-scale vorticity in conjunction with heat 

release is responsible. These models contend that the entire 

fire plume slowly rotates. Note, however, that Church et 

al. (1980) and McRae and Flannigan (1990) characterized 

this type of motion as one type of fire whirl. In Baum and 

McCaffrey’s model (also used by Trelles and Pagni 1997 

and Ohlemiller and Corley 1994), this rotation is caused 

by density gradients from the high heat release, and not 

necessarily by any imposed swirling caused by the ambient 

environment. The slow rotation of such a large mass of air 

above the ground translates to high-velocity, purely horizontal, and nonrotating flow at the ground. One unique feature 

of the Baum and McCaffrey model is that it treats the large 

area fire as an ensemble of randomly distributed individual 

fires of varying strengths. Because of the method chosen 

to represent the fire, the model is only valid for heights 

above the fuel bed where the plumes of the individual fires 

have not merged. The model of Carrier et al. (1985) was 

intended to determine how long it would take to spin up the 

convective column and under what conditions this occurred. 

Based on the fact that the fire in Hamburg, Germany, took 2 

hours to develop, they concluded that the growth of swirl, at 

least in this case, was most likely due to the intensification 

of a preexisting vortex from earlier fires and bombings. 

Although this contradicts the Baum and McCaffrey model, 

the experiments and discussion by Church et al. (1980) 

support this argument. The spatial orientation of individual 

fires may cause a swirling flow owing to the interaction of 

the indrafts to each fire (Soma and Saito 1991). Carrier et 

al. (1985) found that large-diameter plumes spin up faster, 

and proposed a set of four criteria that must be met for a 

“firestorm” to develop: heat release of 106 MW over a localized area for 2 to 3 hours, a preexisting weak vortex, low 

ambient winds, and a nearly dry-adiabatic lapse rate over 

the first few kilometers of the atmosphere. 

Because it seems unlikely that all the criteria for 

spin-up of a convective column will be met, another theory, 

advanced by Smith et al. (1975) and Small et al. (1983) is 

proposed. These authors claimed that buoyancy-induced 

pressure gradients are responsible for the large indrafts. 

Smith et al. (1975) used a simple two-dimensional model of 

a convective column over a hot area to effectively show that 

near the fire, a dynamic pressure gradient can cause highvelocity inflow. This dynamic pressure gradient is caused 

by a balance between hydrostatic pressure and buoyancy. 

Buoyancy pushes the hot gases up while atmospheric 

pressure pushes fresh air at the ground in toward the fire 

horizontally to fill the gap left by the rising gases. Smith 

et al. (1975) also suggested that the traditional “weakly 

buoyant” plume theories described above may be valid for a 

small range of plume heights sufficiently far away from the 

fire and any inversion layer above. Small et al. (1983) used 

a similar model to that of Smith et al. (1975) but included 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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Figure 4-8—Model results for flow-field streamlines for three fires in proximity (from Weihs and Small 1986).

point source. Interestingly, both the Baum and McCaffrey 

(1989) and Small et al. (1983) models reasonably replicate 

what little experimental data are available. However, the 

theories differ slightly in their predictions of the distance 

away from the fire that these indrafts extend (Pitts 1991). 

The model of Baum and McCaffrey (1989) predicts that the 

high-velocity indrafts will extend much farther from the fire 

compared to the model of Small et al. (1983). Without more 

detailed experimental data, it is impossible to say which 

model more accurately portrays the physics. 

Pulsation

Although not an effect of flame interactions, flame pulsation 

(or puffing) is an interesting phenomenon that can occur in 

stationary fires, such as a mass fire. This pulsation typically 

occurs in circular or axisymmetric fires in weak ambient 

wind and is periodic in nature. Flame pulsation is important 

to many researchers because it can have a large influence 

on air entrainment rates and therefore heat release rates and 

pollution formation (Ghoniem et al. 1996). Observations of 

this phenomenon reveal the expansion of the flame near the 

a volume heat addition and large density and temperature 

gradients. Small et al. (1983) also numerically matched 

their model results of the area near the fire to the results of 

traditional plume theory for the region far from the fire. In 

both the Smith et al. (1975) and Small et al. (1983) models, 

the fire is treated as a single large heat source (fig. 4-8). 

Small et al. (1983) used their model to demonstrate how the 

maximum indraft velocity varies with fire radius, burning 

rate, and fire height (fig. 4-8). They showed that the maximum indraft velocity at first increases but eventually levels 

off (to approximately 40 m/s) with increases in both the fire 

radius and the burning rate. On the other hand, the maximum indraft velocity appears to be linear with fire height.

A third, not yet well-explored explanation was proposed by Carrier et al. (1984). In this work, they used classic 

plume theory, but assumed that the fire does not burn as a 

single fire, but a collection of individual fires. They hypothesized that the high indraft velocities are then due to the 

increased fire perimeter from this “multicellular burning 

zone.” This hypothesis was not further developed, and in 

later works, these authors treated the fire as a subterranean 
GENERAL TECHNICAL REPORT PNW-GTR-854

62

base of the fire as a toroidal vortex, about the size of the fire 

diameter. As this vortex is shed and propagates upward, the 

flame necks inward giving the appearance of a “mushroom” 

shape. Figure 4-9 illustrates the process with time sequence 

of photos. Not all circular flames pulsate, however. Using 

dimensional analysis, Byram and Nelson (1970) attempted 

to describe what type of fires will pulsate. They defined a 

dimensionless “buoyancy” number, π2 = Qc /[(gD)

0.5

pcpT], 

where Qc is the rate of convective heat release per area, g is 

the acceleration due to gravity, D is the fire diameter, and ρ, 

cp, and T are the density, specific heat, and temperature of 

the ambient air. Although no quantitative values were given, 

they argued that a fire will not pulsate if π2 is either too 

small (low heat release rate relative to large fire diameter) 

or too large (large heat release rate relative to small fire 

diameter).

Because this puffing occurs in nonreacting helium 

plumes, it is actually not caused by a combustion instability, but instead is produced by a fluid dynamic instability 

(Cetegen and Ahmed 1993). There is disagreement about the 

actual cause of the instability (Tieszen 2001), but the vortex 

is generally thought to be formed because of the interaction 

between gravity and the density gradient between the flame 

and ambient air temperatures (Ghoniem et al. 1996).

Most of what has been learned about the characteristics 

of pulsation has been learned through experiments. Cetegen 

and Ahmed (1993) showed that the toroidal vortex forms 

within one fire diameter above the flame base and that the 

frequency of the puffing is insensitive to the fuel or the heat 

release rate. By plotting the available data in the literature, 

Cetegen and Ahmed, and later Malalasekera et al. (1996), 

showed that the pulsation frequency is proportional to the 

fire diameter raised to the negative one-half power (f ~D-1/2) 

so that large fires pulsate at a much lower frequency than 

small fires. Though this correlation was developed using 

data from fires ranging from 0.1 to 100 m in diameter (four 

orders of magnitude) using gaseous, liquid, and solid fuels, 

Baum and McCaffrey (1989) suggested that it may well hold 

for much larger fires as well. For a large fire with a diameter 

on the order of 20 km, Larson et al. (1982) estimated that 

the pulsation will occur every 20 min. Although it is not 

accounted for in the above correlation, Malalasekera et al. 

(1996) showed that increasing fuel flow rates also result in a 

small increase in puffing frequency, especially for small fire 

sizes. Because of this, Malalasekera et al. (1996) correlated 

the puffing frequency in a slightly different manner using 

the dimensionless Strouhal number (ratio of oscillation 

frequency to 1 over the characteristic time of convection) 

Figure 4-9–Thermal images of flame pulsation (from Malalasekera 

et al. 1996).
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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and Froude number (ratio of inertia force to gravitational 

force), which retains the same dependency on fire diameter 

but allows for a correction owing to changes in fuel flow 

velocity.

Convection Column 

Mass fires are also described as having very tall convection columns, or smoke plumes with large cloud structures 

because of the moisture release from combustion (Small 

and Heikes 1988). As discussed in the section on indraft 

velocities, the entrainment of cold, ambient air slows the 

rise of the hot gases by cooling them. Additionally, the 

density of the ambient air itself decreases with elevation. As 

the hot gases rise and cool, the density difference driving 

their upward motion disappears. It follows then that the top 

of the smoke plume corresponds to the height where the 

combustion products stop rising. As the fire diameter grows, 

however, the entrainment predicted by classic plume theory 

becomes less effective. Entrainment occurs at the perimeter 

of the plume, and with large fire sources there is such a 

large core of hot gases that entrainment is less effective at 

slowing the rise of the combustion products (Palmer 1981). 

Thus, it takes longer to entrain enough cold air to slow 

the combustion products, and therefore the smoke plume 

becomes taller. For example, a lack of entrainment to the 

convection column was noted and discussed by Taylor et 

al. (1973) on a large prescribed burn. In fact, the plume 

from a sufficiently large mass fire may be almost as wide 

as it is tall, so Brode and Small (1986) and Palmer (1981) 

contended that air entrainment is not likely to be a major 

influence on plume height and that it is the structure of the 

atmosphere itself that is the limiting factor. The plume of 

large mass fires is therefore more sensitive to atmospheric 

gradients, inversion heights, and upper atmosphere crosswinds (see also Penner et. al 1986). Brode and Small (1986) 

showed that the tropopause/stratosphere transition may be 

what actually caps the smoke plume. Note, these theories 

contradict the suggestion of Smith et al. (1975) that the 

traditional plume theory holds at some intermediate height 

above the ground. Perhaps the scale of the fires modeled by 

Smith et al. (1975) was not large enough to see this effect.

Palmer (1981) described the interesting structure of 

the convection columns that formed during the Project 

Flambeau tests. In the first few minutes of these large-scale 

burns, the majority of the gaseous combustion products 

were contained in a “bubble” near the fire. Once the 

“bubble” got sufficiently hot, the associated buoyancy was 

enough to overcome the surface drag forces and the bubble 

rose. As the bubble rose, a vortex ring would form in a 

similar manner described above with respect to flame pulsations. Regardless of the atmospheric stability, this vortex 

ring would rise until it encountered a region of vertical wind 

shear. The vertical wind shear weakened the vortex enough 

for the plume to then follow the prevailing horizontal winds. 

Palmer (1981) also noted that the “exterior form of the 

convection column at a particular altitude was determined 

by the initial vortex bubble as it passed that altitude.” Most 

of the plumes in these fires began to rotate as a single vertical vortex, as suggested by the Baum and McCaffrey (1989) 

model. This rotation further inhibits entrainment, which 

would also prevent the use of classic plume models for mass 

fires (Banta et al. 1992). 

Summary of Interaction Effects

As the individual spot fires grow together, they will begin 

to interact. This interaction will increase the burning 

rates, heat release rates, and flame height until the distance 

between them reaches a critical level. At the critical separation distance, the flames will begin to merge together and 

burn with the maximum rate and flame height. As these 

spot fires continue to grow together, the burning and heat 

release rates will finally start to decrease but remain at a 

much elevated level compared to the independent spot fire. 

The flame height is not expected to change significantly. 

The more spot fires, the bigger the increase in burning rate 

and flame height.

Needs for Further Research and 

Application

The characteristics of many fire interactions have been 

examined and reported in the research literature, leaving 
GENERAL TECHNICAL REPORT PNW-GTR-854

64

little doubt that local spread and behavior experienced by 

wildland fire personnel can be greatly influenced by fire 

configurations at larger scales. The ignition patterns and 

“suppression fire” tactics used in firefighting (Castellnou 

et al. 2010, Miralles et al. 2010) depend on understanding 

these interactions. However, questions remain about how 

to extend the findings of fundamental research to the field 

scale for wildland fires and mass fires. In particular, there 

is no clear method to determine the minimum separation 

distance between two fires for interaction and merging to 

occur. The influence of ambient winds or topography on 

interactions is directly relevant to wildfire management 

activities and tactics but has not been explored. Large-area 

fires were discussed as an extreme case of fire interactions 

and often behave quite differently than propagating line 

fires. Just how much area must be ignited to display “mass 

fire” characteristics is unknown. In the Project Flambeau 

experiments, Countryman (1964) argued that even these 

large fires were not large enough to be considered mass 

fires. Both Byram (1966) and Thomas et al. (1968) developed scaling laws in an attempt to answer this question, but 

many potentially limiting assumptions were made in the 

development and the laws were not validated. Baldwin and 

North (1967) attempted to quantify the minimum area for 

urban applications based on city layout and historical fires, 

but their estimations are admittedly crude. As discussed, 

there is no consensus in the literature about the convection 

column dynamics of mass fires and what mechanism is 

responsible for the reported strong indrafts. These suggestions are merely a starting point, as the subjects of fire 

interactions and mass fires clearly involve a great deal of 

physics and require the union of many fields of study.

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Introduction

“Plume dynamics” refers to the airflow related to a fire’s 

updraft and the way that updraft changes over time. In 

terms of extreme fire behavior, plume dynamics matter 

because they can bring the wind, moisture, and temperature 

conditions above the ground down to the ground where the 

fire is. These aboveground conditions may or may not be 

the same as the conditions at the ground, and the differences can produce unexpected changes in fire behavior. 

The updraft is part of a fire-driven circulation that includes 

downward air motion in other areas. The circulation also 

modifies the horizontal winds near the fire. The updraft 

also lifts burning embers that can subsequently lead to spot 

fires. Any of these can lead to a nonsteady state, or for the 

purposes of this review, extreme fire behavior. This section 

examines several concepts and tools related to plume 

dynamics that are well known in the fire management community, noting their foundations, strengths, weaknesses, 

and limitations. Spot fires are addressed in more detail in 

chapter 6.

Before considering specific concepts, it is important to 

understand how the plume, or updraft, relates to other air 

currents associated with a fire. These currents are of greater 

concern to the firefighter than the updraft, most of the time. 

Observations of wildland fires (Banta et al. 1992; Clements 

et al. 2006, 2007; Countryman 1969; Goens and Andrews 

1998; Reid and Vines 1972; Schroeder and Buck 1970; 

Taylor et al. 1968, 1971, 1973) reveal many of these air 

currents. In addition, studies of the air currents in and 

around thunderstorms, in the absence of fire (Browning et 

al. 1976, Foote and Frank 1983, Houze et al. 1989, Klemp 

et al. 1981) provide additional information about the threedimensional airflow associated with an updraft or plume. 

This model is a simplification and does not include the 

rapidly changing features of a real plume, but it is a 

useful foundation for discussing how plume dynamics contribute to extreme fire behavior. The rapidly changing features not included in this simple model include fire whirls 

(vortices), waves in the vertical and horizontal airflow, and 

turbulent eddies on a wide range of scales. Because they 

are not steady-state features, these are all closely related 

to what is commonly recognized as extreme fire behavior 

(fig. 5-1). The most consistently observed, best recognized 

characteristics of a fire plume are the updraft column, eddy 

vortices (whirls) along the perimeter at the fire’s head, and 

the winds blowing into the rear and sides of the fire at the 

ground. The updraft is the most obvious part of the plume 

as it is where the smoke is. It may consist of a sequence 

of puffs or turrets (each an updraft, itself) that separate 

from the fire front and move downwind. Near the ground, 

the updraft accelerates as it rises. The rate of acceleration 

depends on the stability and wind profiles of the atmosphere 

around the plume. The rear inflow, less often observed but 

often implicitly recognized, descends toward the back of the 

fire’s head. The acceleration of the updraft and the detailed 

path of the rear inflow are difficult to observe, so details 

Chapter 5: Column/Plume Dynamics

Brian E. Potter1

Figure 5-1—Conceptual model of the airflow in and around a 

fire’s plume.

1 Brian E. Potter, U.S. Department of Agriculture, Forest Service, 

Pacific Wildland Fire Sciences Laboratory, Seattle, WA.

DESCENDING REAR INFLOW

REAR SURFACE INFLOW

LATERAL SUFACE INFLOW

ACCER 

erating UPFLOW
GENERAL TECHNICAL REPORT PNW-GTR-854

74

of their character are very poorly known. The descending 

rear inflow, a sustained feature of moderate speed, is not 

the same as a downburst, which is a fast, short-lived and 

localized current.

Plume-Dominated and Wind-Driven Fires

The most recognized connection between extreme fire 

behavior and plume dynamics is the concept of a plumedominated or wind-driven fire. Generally, wind-driven 

fires are more predictable because the fire spreads with the 

wind. When someone labels a fire “plume dominated” it is 

because the smoke plume is standing near vertically and 

there is little visible influence of horizontal wind on it. In 

common usage, the implication is that the fire’s behavior 

may change rapidly and that the fire’s direction of spread 

could change unexpectedly. The distinction between these 

two broad categories comes from Byram (1959), who introduced the power of the fire and power of the wind (Pf

 and 

Pw ) and identified them as the energy-criterion equations. 

The equations and energies are intended to help identify 

when a fire is dominated by what Byram called “its own 

energy” as opposed to being dominated by the energy of the 

wind field. Byram originally provided the equations without 

any derivation and eventually Nelson (1993) presented derivations based on notes he had from Byram. Both authors 

emphasized the ratio of the power of the fire to the power of 

the wind, which they call the convection number, Nc, rather 

than the power expressions. Nelson (1993) provided a brief 

discussion explaining how turbulent mixing and a stable or 

unstable temperature profile may affect the equations, and 

Nelson (2003) expanded on these two points.

Byram (1959) provided one example of the power equations applied to an actual fire, using Rhode Island’s Wood 

River Valley Fire of May 1951. He stated:

...studies have shown that extreme fire behavior and 

blowup characteristics occur when Pf > Pw for a 

considerable height above the fire–usually at least 

1000 feet and more, often greater than 3000 or 4000 

feet. Possibly one of the most erratic conditions is in 

the transition zone where Pf and Pw are nearly equal.

Three fire case studies (Aronovitch 1989, Simard et al. 

1983, Wade and Ward 1973) have included Byram’s powers or convection number. Rothermel (1991) incorporated 

Byram’s equations into his model for predicting behavior 

and size of crown fires in the Northern Rockies. In the case 

studies, authors had to make numerous assumptions about 

the weather: neutral stability, winds constant over time, 

winds constant with height, and some calculation of fire 

intensity, and none of the studies reaches any substantial or 

verifiable conclusions about the fire’s behavior based on the 

power of the fire, power of the wind, or convection number. 

In describing the use of the crown fire nomograms, Rothermel (1991) assumed that windspeed is constant at the 6-m 

(20-ft) speed and does not adjust for the fire’s rate of spread, 

even though he stated, “this can produce errors and should 

not always be assumed.”

Byram’s equations, or Nelson’s more general equations, 

represent the energy produced by combustion and that 

contained in the wind field. Application of the equations 

for case studies has a very low precision, and obtaining the 

necessary data to apply the equations is nearly impossible 

for an ongoing fire. Although the National Wildfire Coordinating Group (NWCG) S-490 course objectives discuss 

estimation of power of the fire and power of the wind, there 

is no quantitative exercise or explicit discussion of how to 

estimate them, and it is not listed in the instructor handbook 

among the topics to be tested.

Adverse Wind Profiles and Low-Level Jets

Closely related to the power of the wind is Byram’s other 

well-known contribution to the science of fire behavior, 

the idea of the “adverse wind profile” (Byram 1954). This 

is often simply referred to as the “low-level jet.” Byram’s 

adverse profile/low-level jet concept is taught in all of the 

NWCG fire behavior classes. The paper discusses turbulence, instability, and the location of the jet stream in the 

upper troposphere in addition to wind profiles, but it is the 

windspeed profiles that remain widely known and remembered.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

75

Byram examined the wind profiles measured near 17 

fires. He identified nine profile types from the 17 fires and 

noted that six of the profiles were associated with extreme 

fire behavior. Furthermore, he noted that there were differences in fire behavior within any profile type and associated 

those differences with the height of the low-level jet above 

the ground. Notably, his type 1-a profile, “one of the most 

dangerous types that can exist from the standpoint of 

personnel safety and erratic and unpredictable fire behavior” has no jet in it. In addition to discussing the windspeed 

profile, Byram stated, “The direction profile is an extremely 

important part of the complete wind profile,” but did not 

provide an explanation of the directional profiles. The 

figures show a variety of directional profiles, as well as 

changes in those profiles during the fires.

Subsequent discussions of Byram’s profiles appear in 

Steiner (1976) and Brotak (1976). Steiner’s discussion is in 

general terms and does not reach any particular conclusions. 

Brotak (1976) considered 62 “large and extremely serious” 

wildfires in the Eastern United States and found that only 

one-third of the fires considered displayed such a jet.

The variety of wind profiles, both in terms of speed 

and direction, are so great that one could not definitively or 

authoritatively associate any observed wind profile with just 

one of Byram’s adverse wind profiles. Rather than trying, 

one could note the general structure Byram described as 

related to blow-up fires: 

• Windspeed of 18 mi/h or more measured in the “free 

air” at the elevation of, or slightly above, the fire’s 

elevation.

• Wind decreasing with height for several thousand feet 

above the fire with the possible exception of the first 

few hundred feet.

Although the general concept of the low-level jet or the 

adverse wind profile has moved into the realm of common 

usage, no one has explained or demonstrated why or how it 

influences a fire’s behavior. Byram (1959) did not do so, he 

solely described how he identified it in conjunction with the 

fires he examined. It is possible to hypothesize on ways the 

profile interacts with the plume updraft or descending rear 

inflow, and there are studies of thunderstorm dynamics that 

can lend insight, but there is no work at present that clearly 

shows why or how Byram’s wind profiles would lead to 

extreme fire behavior or blow-up fires. 

The earlier discussion of Byram’s power of the fire and 

power of the wind equations noted the case studies that used 

them assumed constant winds with height. The low-level jet 

seen in Byram’s most dangerous profiles directly contradicts this assumption. Using constant winds underestimates 

the power of the wind and therefore also underestimates 

the value of the power of the fire necessary to yield a 

convection number, Nc, greater than 1. The only operational 

application of the Byram wind profiles is subjective assessment of observed profiles. 

Stability and Instability

Few fire managers or fire researchers doubt that there is 

a connection between atmospheric stability and extreme 

fire behavior. The idea first appeared in Foley (1947), 

followed by Crosby (1949), and Byram and Nelson (1951). 

Davis (1969) examined stability accompanying 70 fires in 

Arkansas, Alabama, Louisiana, Mississippi, and Tennessee 

using broad stability and fire size classes. Later, Brotak and 

Reifsnyder (1977) found that unstable air above a fire was 

present for a number of large fires. Haines (1988b) used 

these observations as the basis of his Lower Atmosphere 

Severity Index, now known widely as the Haines Index. 

Potter (2002) discussed atmospheric stability influences 

on the circulation created by a fire and showed that greater 

instability leads to stronger updrafts and surface winds, 

and that the influence is greater when the instability is 

closer to the ground. In terms of basic physics, an unstable 

atmosphere provides less resistance than a stable atmosphere to the ascent of hot air in the fire’s plume and is more 

conducive to general mixing of air between the ground and 

regions higher up. 

Instability itself cannot directly influence the combustion process—it must be converted into wind. Nor can 

instability directly start a fire—it may enable thunderstorms 

that produce lightning, but that is an indirect connection 
GENERAL TECHNICAL REPORT PNW-GTR-854

76

and is not related to extreme fire behavior. In addition to the 

circulation that a fire in an unstable environment creates, 

instability can influence a fire by allowing existing windy 

or dry air from upper levels to more easily reach the ground 

where it can interact with the fuels and combustion. Foley 

(1947), Crosby (1949), and Byram and Nelson (1951) all 

connected instability to fire behavior through the turbulence 

and high winds it can produce and transport downward to 

the fire. Brotak and Reifsnyder (1977), Haines (1988b) and 

Potter (2002) all included some measure of moisture aloft 

in their discussions of instability. It was Brotak and Reifsnyder’s (1977) correlation between low moisture aloft and 

large fire occurrence at the ground that led to its inclusion in 

the Haines Index.

Davis (1969), Brotak and Reifsnyder (1977) and Haines 

(1988b) did not examine fire behavior qualities: intensity, 

rate of spread, or flame length. Rather, Davis considered 

stability at the time of fires over 120 ha (300 ac) provided to 

him by state fire control staff. Brotak and Reifsnyder looked 

at atmospheric properties present at the time of “large” 

fires, defined as larger than 2000 ha (5,000 ac). Haines’ fire 

data included 74 fires reported by wildland fire management 

units as “their worst situations over 20 [years].” How to 

translate these to the concept of extreme fire behavior, if it 

is even possible to do so, is not clear. The Haines Index is 

the only quantitative measure of stability used in wildland 

fire management to indicate the regional potential for fires 

to become large or display erratic behavior. This application is consistent with its origins and derivation. There are 

other stability indices used for thunderstorms (such as the 

K, Lifted, Showalter, SWEAT, and the Total Totals indices) 

or smoke dispersion (such as the Lavdas Atmospheric 

Dispersion Index), but none of these have been scientifically 

evaluated for use in predicting extreme fire behavior, or fire 

behavior of any kind.

Much remains unknown about instability’s influence on 

fire behavior. Is it possible to separate instability’s influence 

on plume strength (and subsequently ground-level inflow 

winds) from the relationship between instability and turbulence? Heilman and Bian (2010) showed that multiplying 

the Haines Index by the surface turbulent kinetic energy 

(TKE) differentiates fires larger than 400 ha (1,000 ac) 

from smaller fires better than the Haines Index does alone, 

suggesting that it is the turbulence generated by the instability that matters for fire size, but does not rule out plume 

strength as an additional contributing factor. In addition, the 

questions noted above regarding moisture interactions with 

instability and which measures of fire behavior are influenced by instability are subjects needing further research. 

Although answers to these questions may prove useful, 

there is sufficient evidence of instability’s ties to extreme 

fire behavior to justify great caution by fire management 

when unstable conditions exist. 

Downbursts and Plume Collapse

In meteorology, a downburst is both a broad description of 

a family of phenomena and a specific member of that family. The fire behavior community generally uses the broader 

definition: “An area of strong, often damaging winds 

produced by one or more convective downdrafts over an 

area from less than 1 to 400 km in horizontal dimensions.” 

(AMS 2000) The physical processes driving a downburst 

rely critically on moisture in the plume and, to an extent, on 

the vertical wind profile. Details of the downburst process 

can be found in Houze (1993) and other books describing 

the dynamics of severe storms. 

The related term “plume collapse” (sometimes called 

column collapse) evokes vivid images of towering smoke 

plumes rising upward and then falling back toward the 

ground. The origins of the term in the fire community are 

unclear. There is no official definition in the fire community 

for plume collapse, nor does there appear to be any generally agreed upon standard. The idea of plume collapse is 

taught in S-290 and in more detail in S-390, but there is no 

single stated definition in those curricula, nor is there a 

definition in the NWCG Glossary of Wildland Fire Terminology. The S-390 precourse work still references the Fire

Weather handbook (Schroeder and Buck 1970) that includes 

a discussion of the air mass thunderstorm concept—a highly 

idealized simplification—suggesting that this is the 

intended use of the term for wildland fire management.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

77

For the present discussion, plume collapse is a special 

case of a downburst. In plume collapse, the energy source 

driving the updraft is cut off or ceases, and the updraft 

decays or reverses its motion, producing a downburst. A 

downburst, more generally, is a strong, short-lived downdraft that occurs near the continuing updraft, not necessarily including the loss of the driving energy source or the 

cessation of the main updraft. This is compatible with the 

equivalence of plume collapse and dissipating convection in 

the current S-390 course.

At the ground, the symptoms of a downburst are a 

rapid but brief period where the wind dies down, followed 

by a sudden gust of winds radiating out from the center of 

the sinking air. There may be precipitation at the ground 

or signs of precipitation aloft, such as virga. The area 

underneath the precipitation is the most likely candidate 

for the center of the sinking air. Air temperatures may drop 

suddenly at the ground, but this may be difficult to detect 

near a fire and will occur at the same time as the arrival of 

the wind gust. 

Downbursts associated with fire behavior appear in the 

fire literature going back to Cramer (1954), although that 

paper calls them thundersqualls. There and in Schroeder 

and Buck (1970) and Haines (1988a), discussion focused on 

downbursts generated by nearby thunderstorms, not by the 

fire column itself. Haines (1988a) noted that downbursts 

with heavy precipitation are more common in the Eastern 

United States, labeling them “wet downbursts.” Dry downbursts, in contrast, are more common in the arid regions 

of the United States where the precipitation may evaporate 

before reaching the ground. 

Goens and Andrews (1998) hypothesized that the fatalities on the 1990 Dude Fire resulted from a fire-generated 

downburst driving the fire on the heels of the fleeing fire 

crew. They presented fire behavior observations and meteorological observations consistent with the development of 

such a downburst. The observations included light precipitation at the ground, a strong convection column, and a calm 

just before the downburst. The downburst, when it came, 

brought winds of 18 to 27 m/s (40 to 60 mi/h) and lasted 

only a few minutes. In this instance, topography added 

to the danger of the downburst. The air in a downburst is 

denser than the air around it, so it will flow downhill. If that 

flow runs into the fire, it will carry the fire downhill with it 

at speeds more typical of an uphill run.

The only reference to plume (column) collapse in 

the scientific literature on wildland fires is Fromm and 

Servranckx (2003). They referred to the Chisolm Fire in 

2001, and the use of the term “convective collapse” is not 

clarified; it appears to mean that the plume top, which had 

been well above the tropopause, sank down to be closer to 

the tropopause. Because the reported surface winds at this 

time were between 30 and 50 km/h (20 and 30 mi/h), the top 

of the convective plume would have been well downwind of 

the fire when this occurred, and the event does not qualify 

as plume collapse under the definition stated above. There 

is no clear evidence that collapse near the tropopause led to 

fire behavior changes at the ground.

It is clear that the processes involved in plume collapse are poorly understood, but that does not negate the 

importance of the characteristics frequently attributed 

to plume collapse. Firsthand observations of showers of 

embers, increasing smoke, or sudden changes of wind 

and fire spread are not in question, and many people have 

observed these. What is questionable or unknown is what 

caused these things to happen, whether it in any way relates 

to the idea of plume collapse as defined here, or what factors 

control the timing and location of these processes. Haines 

(1988a) listed several fires where thunderstorm downbursts 

were considered responsible for firefighter fatalities and 

extreme fire behavior. The Dude Fire study by Goens and 

Andrews (1998) appears to be the only case study specifically documenting a downburst created within the fire’s 

plume. There is no doubt that downbursts can cause extreme 

fire behavior.

The useful questions about downbursts center on 

understanding when the temperature, wind, and moisture 

profiles at a fire favor the occurrence of downbursts and 

whether those conditions can be predicted with sufficient 

lead time to allow any action. The wind profile interacts 
GENERAL TECHNICAL REPORT PNW-GTR-854

78

with temperature and moisture in complex ways, influencing when downdrafts occur and where they occur relative 

to the updraft. The question of precisely when or where a 

downburst will occur relative to the fire is much more difficult to answer and of limited value for operational purposes. 

If the possible location of the downburst and its influence 

on the fire’s direction or rate of spread change more rapidly 

than resources or fire crews can adapt, then simply knowing 

it can occur is more useful information. 

Although there is no scientific study of plume collapse 

(as defined here) in wildland fires, management anecdotes 

and physics both support it as a sound explanation for 

some situations, notably the stage in slash burns when the 

fire’s energy output ceases or drops off rapidly. The stated 

significance of plume collapse in the NWCG fire behavior 

courses indicates the potential value in scientific study of 

just what conditions can yield plume collapse. The ambiguity and imagery inherent in the phrase “plume collapse” 

remain problematic, however. Eliminating the term “plume 

collapse” in the context of fire behavior and just discussing 

“downbursts” could reduce confusion.

Summary

Although scientific studies provide a consistent qualitative 

picture of the fire’s plume, the quantitative tools associated 

with plume structure are less robust. Operationally, there 

are two significant limitations on applying even qualitative 

relationships of plume dynamics to extreme fire behavior. 

The first is the fact that all of the plume aspects discussed 

here require information on the three-dimensional structure of the atmosphere before they can serve as a basis for 

action. Practitioners must either have experience and intuition that allow them to accurately understand and predict 

the changing nature of the atmosphere in three dimensions, 

or else they must have access to numerical model data that 

tell them the current and future structure of the atmosphere. 

The second limitation is that although the plume dynamics and indices discussed here have to some degree been 

documented accompanying extreme fire behavior, there 

is no scientific study showing that they are absent during 

nonextreme fire behavior events. For example, it is entirely 

possible that one or more of Byram’s adverse wind profiles 

occurs on every fire, no matter how small. If an index or 

process is just as common during nonextreme fire behavior 

as it is during extreme fire behavior, then the potential false 

alarm rate for that property is quite high and its value to the 

management community proportionally diminished.

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Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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Introduction

Spotting is specifically cited in the National Wildfire 

Coordinating Group (NWCG) definition of extreme fire 

behavior. It also qualifies as extreme fire behavior under the 

working definition used for this synthesis because its irregularity and unpredictable nature are inherently not steady 

state. Spotting and spot fires are dangers to fire management 

because of the ember showers they create near the main fire 

front and their potential to cross substantial barriers.

The following discussion of spotting addresses what is 

known of the process itself, the tools available to managers, 

and the primary areas for further research and tool development. Specific papers are cited when appropriate, but there 

are many more studies in the scientific literature that are 

related to spotting but not cited here. Readers looking for a 

more detailed discussion of the science should consult Koo 

et al. (2010) and Ellis (2000), both of which are themselves 

valuable resources, but also because they contain many 

relevant references. 

The Spotting Process

To understand the factors that influence spotting, consider 

the life cycle of an ember or fire brand. The ember starts as 

a leaf, twig, seed, nut, or pine cone, piece of bark, or small 

fragment of a larger piece of fuel that was partially consumed. It may originate on the ground, in the understory, or 

in the canopy. The air currents associated with the fire must 

lift the ember up into the fire’s plume, until the air currents 

or gravity throw it out of the updraft. (See chapter 5 for a 

more detailed description of the air currents associated with 

the fire’s updraft.) As the ember falls to earth, the winds 

continue to push it horizontally—and perhaps vertically, 

if it gets caught in an eddy. All through this journey, the 

ember continues to burn, losing mass and getting smaller. 

(If it stops burning, there is no longer a spotting hazard.) 

Chapter 6: Spot Fires

Brian E. Potter1

1 Brian E. Potter, U.S. Department of Agriculture, Forest Service, 

Pacific Wildland Fire Sciences Laboratory, Seattle, WA.

As the ember shrinks, it is more easily pushed and 

carried by the wind, and it settles toward the ground more 

slowly. Eventually, however, the ember reaches the ground 

or perhaps it comes to rest in a tree canopy or understory 

vegetation. If it lands in flammable fuel, it may ignite that 

fuel if the ember has enough energy to dry and heat the fuel 

to the combustion point. That drying and heating may take 

some time, resulting in an ignition delay. At this point, the 

ember has started a spot fire.

Now consider not just the one ember, but all of the 

embers generated by a fire. At any time there can be many 

embers in the air around a fire. The size, shape, and number 

of embers depend on the fuel type and the intensity of the 

fire. Some fuels, such as eucalypts and chaparral, produce 

embers in abundance. Eucalypts also produce some of the 

most aerodynamic embers, strips of bark capable of sailing 

long distances (McArthur 1967). Grasses, owing to their 

fineness and short consumption time, produce fewer embers 

that survive to return to the ground.

Some embers, especially large ones, land relatively 

close to the main fire front. The closer they land to the 

front and the longer they take to ignite the recipient fuels, 

the more likely it is that the main fire front will overrun 

them and the less likely that they will cause the fire front 

to “hop” forward. The zone where overrunning occurs has 

no definite size—it depends on the fire’s rate of spread, the 

fuel moisture, and, again, the fuel type. One can estimate 

the width of the overrunning zone by multiplying the spread 

rate of the main fire front times an estimate of the time it 

takes embers landing in the fuel to establish themselves as 

new fires. Even in the overrunning zone, however, heavy 

showers of embers can gradually dry and heat the fuels 

so that the fire spreads more rapidly when the main front 

arrives. Fire spread models based on measurements of 

spread in actual forest fires will implicitly reflect any such 

effects, if spotting occurred on the fires used to develop the 

model. Spread models based on laboratory measurements, 
GENERAL TECHNICAL REPORT PNW-GTR-854

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however, will not include any preconditioning effects from 

spotting.

Just beyond this zone, there is a chance for embers to 

ignite a new fire and for that fire to establish itself before the 

main front arrives. This could be considered the establishment zone, and it extends as far as any embers can travel 

and still ignite fuels when they alight (fig. 6-1). If there are 

few embers landing here, they will create spot fires that 

grow independently of one another and largely independently of the main fire front. Numerous spot fires, however, 

may form close enough to one another that their heat and 

air currents will influence one another (see chapter 4). A 

collection of nearby spot fires that coalesce will grow more 

rapidly than a fire lit by a single ember (the same principle 

that lies behind aerial ignition of multiple spot fires.) Just 

how many embers actually ignite spot fires, or how quickly 

those fires can grow and merge, depends strongly on the 

fuel where the embers land. The chance of an ember igniting and growing into a new fire increases as the fine fuel 

load increases, as the windspeed increases, and as the fuel 

moisture decreases. It also depends on the fuel species.

Multiple, nearby spot fires may coalesce into one or 

more larger fires, and depending on where they are relative 

to the main fire front, they may remain a separate fire for 

many days or indefinitely. For example, fires that coalesce 

across a fire break, across a ridge or valley, or on the flank 

of the main fire can grow with little chance of merging with 

the main fire. Spot fires that coalesce ahead of the main fire 

front with no barrier between, can effectively increase the 

fire’s forward rate of spread. This effect was described in 

detail in the Wade and Ward (1973) study of the Air Force 

Bomb Range Fire.

Embers that reach these distances require strong winds 

and enough vertical lofting that they take a long time to 

come back to the ground. They must be big enough at the 

outset to still be burning when they land, yet small enough 

for the winds to carry them the necessary distance. Embers 

with low trajectories, primarily carried by horizontal winds, 

will typically fly out directly ahead of the fire, driven by 

those same winds. Embers lofted high may experience 

winds aloft very different from those driving the fire front. 

Wind direction can vary significantly in the lowest 600 to 

900 m (2000 to 3000 ft) of the atmosphere. In the Northern 

Hemisphere, the wind typically turns to the right as height 

increases, whereas in the Southern Hemisphere, it tends to 

turn to the left. There is a recognized tendency for spot 

Figure 6-1—Conceptual diagram (not to scale) of the spotting process, illustrating the influence of ember size and shape, lofting height, 

and ignition. Thin arrows are ember trajectories, those ending in dashed lines indicate the ember burned completely and never reached 

the ground. Round embers represent woody embers, and the stick-shaped ember leaving the top of the plume represents aerodynamic 

embers (typically bark) or large embers lofted extremely high by fire whirls or intense surges in the fire.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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fires to develop on these respective flanks. Complex terrain 

and strong weather fronts can also modify wind direction 

at different levels, although less predictably. 

There may be some embers that are big or aerodynamic 

enough to keep burning as they travel great distances, 

McArthur (1967) cited spot fires up to 29 km (18 mi) for the 

March 1965 fires in Victoria, Australia. For these embers 

to get up into the air in the first place, requires substantial 

vertical wind. Early studies of ember lofting and transport 

(e.g., Lee and Hellman 1969, Muraszew 1974, Muraszew et 

al. 1975) concluded that the necessary updrafts had to be 

of such magnitude and spatial extent that they were physically improbable. For example, Muraszew (1974) calculated 

that an updraft of 30 m/s, 1 km wide at the ground, only 

transported embers 7 km. The only way to lift these embers 

would be in a fire whirl or very intense flareup. (This region 

is labeled the “whirl/sailing zone” in fig. 6-1.) The embers 

have to be highly aerodynamic, as well, so that they could 

sail efficiently on the winds. They would be lofted to such 

heights that they would almost certainly encounter winds of 

variable speed and direction, making their ultimate resting 

place even more difficult to predict.

Management Tools

There are five tools available that translate the complex 

science of spotting into an operational context. Like any 

model or tool based on so many interacting processes, 

these require many assumptions and simplifications. Frank 

Albini of the USDA Forest Service developed three of the 

tools, and each addresses a specific type of ember source. 

The assumptions, simplifications, and limitations of each 

model are clearly stated and acknowledged in each case. All 

three provide the same output, an estimate of the maximum 

spotting distance. One of the other two tools deals with the 

probability spotting will span a fire break on a grass fire, 

and the last tool provides an estimate of the minimum spotting distance at which fires can become established before 

they are overrun.

The first tool is for embers and spotting from single 

torching trees or groups of up to 30 trees (Albini 1979.) The 

calculations estimate the maximum likely spotting distance 

for such embers, under very specific conditions. The model 

does not apply to “running crown fires, fires in heavy slash 

or chaparral under extreme winds, or fires in which fire 

whirls loft” the embers. It assumes the ember comes from 

near the top of the torching tree(s). It assumes a specific 

vertical profile for the wind, one with constant direction 

and speed increasing from canopy top to a constant value 

several hundred feet above. Furthermore, it cannot predict 

the effects of wind eddies caused by terrain, such as lee 

waves or rotors, or flow in canyons. It assumes the ember is 

a cylinder of wood, or similar shape. (Laboratory studies by 

Tarifa et al. (1967) showed that shape is not a major factor 

most of the time. Cylinders, spheres, and plates of a given 

density travel roughly the same distance.) The model does 

not predict the probability of ignition when an ember lands, 

nor does it predict the number of embers reaching the estimated maximum distance. The model allows lofting up to 

305 m (1,000 ft) above ground and predicts distances up to 

3.2 km (2 mi) over flat terrain with uniform fuels. Multipliers for noflat terrain can increase spot distance to as much 

as 5.6 km (3.5 mi), if the source is atop a 1200-m (4,000-ft) 

ridge and the distance to the valley bottom exceeds 3.2 km 

(2 mi).

Albini (1981) modified the torching tool so that it 

could be used for isolated, more sustained sources such as 

slash piles or fuel “jackpots.” Although the earlier model 

assumes a short-lived heat source and a brief surge in the 

fire’s plume, the newer model allows a more sustained heat 

source and plume lofting the ember. This model also allows 

estimation of maximum spotting distance when there is not 

uniform-height forest along the ember’s flight path. The 

estimate assumes a neutral or stable atmosphere and still 

assumes constant wind direction. The strongest plumes 

and most intense fires, however, tend to develop under 

conditions with unstable air near the ground: how much this 

would affect spot distance estimates is not clear.

The third tool/model is from Albini (1983) and extends 

the earlier models for use with wind-driven surface fires 

without timber cover. The model assumes the fire is linear, 
GENERAL TECHNICAL REPORT PNW-GTR-854

84

perpendicular to the wind, and much longer than the frontto-back depth of the flaming front. It also assumes that any 

embers lofted by a surface fire rise in short surges in fire 

intensity. This aspect of the model relies on a theoretical 

model of the duration and strength of the surges, and Albini 

said outright “this hypothesis, crucial in the model’s development, is unlikely to ever be tested directly.” The results 

of this extension only predict the lofting height of embers, 

which then go into the Albini (1979) model to provide the 

estimated maximum spotting distance. Albini (1983) did not 

state whether the surface-fire model still assumes, cylindrical wood embers; if so, it may not be appropriate for grassy 

fuels without any woody component.

The next tool is a pair of graphs (fig. 6-2) from Wilson 

(1988) that indicate the probability that a grass fire will spot 

across a firebreak of given width. Note that this is based 

on, and only strictly appropriate for, grass fires with very 

few trees. The only input required is an estimate of the 

fire’s intensity and whether or not there are trees in the fire 

perimeter within 20 m of the fire break. The figures (and the 

equation from which they are derived) indicate the probability the fire will spot across a break of given width.

The last tool is also a graph, and it is for use during 

active crown fires. Specifically, it is calibrated for use on 

crown fires in open canopy coniferous fuel types (Forestry 

Canada Fire Danger Group 1992). Alexander and Cruz 

Figure 6-2—Probabilities of breaching firebreaks of specified 

width as a function of fireline intensity. Based on equations in 

Wilson 1988.

Figure 6-3—Mimimum distance at which a spot fire will not be 

overrun by the main fire. Distance depends on ignition delay and 

is shown for delays of 0.2 minutes (solid line); 1.5 minutes (dotted 

line), 5 minutes (dashed line), and 10 minutes (dash-dot line). 

Following Alexander and Cruz 2006.
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(2006) provided an estimate of the minimum distance at 

which a newly ignited spot fire will not be overrun by the 

main fire front. This is essentially the depth of the overrun 

zone mentioned previously. The graph (fig. 6-3) indicates 

the distance/depth based on rate of spread and ignition delay 

(ID). It assumes that once an ember ignites a fire, it will 

accelerate and achieve 90 percent of its steady-state spread 

rate in 20 min. Fires that require longer to reach a steadystate rate of spread, including those in a more closed canopy 

environment, will result in increased separation distances or 

overrun zone depths.

Knowledge Gaps

There are several opportunities for improvement in both the 

tools and the underlying science of spot fires. Foremost of 

these is one basic element: validation or evaluation of how 

the Albini tools currently perform. Albini stated repeatedly that the models were not tested in the field, and that 

“assumptions, approximations, and inadequately supported 

empirical relationships are sprinkled throughout.” There 

is no published study scientifically evaluating any of the 

Albini submodels or models in their whole. Two publications (Norum 1982 and Rothermel 1983) cite the use of the 

models in the field and view them favorably, but neither of 

these constitutes a rigorous model evaluation.

The science already exists to estimate trajectories in 

a complex wind field. Smoke and air pollution models 

regularly and quickly compute trajectories in wind fields 

that vary in the vertical, horizontal, and time. These could 

be combined with the lofting and burnout models of Albini 

(the components most strongly supported by research). The 

existing tools could also be modified to include more aerodynamic fuels like eucalyptus bark, or lofting by short-lived 

fire whirls. 

These adaptations would still only address maximum 

distance and direction for single embers, however. The 

number, size, and spotting density of embers at shorter distances are important for the accurate estimation of spread 

rate of the main fire front. Before any tools can provide 

this information, however, basic studies must measure 

these properties for various fuel types and fire intensities. 

Manzello et al. (2007) examined embers produced by single 

Douglas-fir trees (see “Common and Scientific Names” 

section), but it is the only study of this nature. Related to 

this is the need for research on the effect of embers and 

spotting on preconditioning fuels. Incorporation of this into 

fire behavior tools would provide the ability to estimate 

any acceleration of the fire spread in the overrun zone and 

“hopping” potential in the establishment zone.

There are several studies that examine the probability 

of an ember igniting fuels after it lands (e.g., Ganteaume et 

al. 2009; Manzello et al. 2006a, 2006b), and a few consider 

the time interval between the ember landing and fuels 

igniting (Alexander and Cruz 2006, Ganteaume et al. 2009). 

No tool, however, gathers this information and uses it to 

indicate the delay or probability of ignition for recipient 

fuels. Such a tool would admittedly require many assumptions about ember size, burning rate, and fuel characteristics, but those assumptions are not necessarily any more of 

a limitation than the assumptions Albini had to make for 

the spotting distance models. Overall, there are two basic 

management implications of the state of knowledge on spot 

fires. First, the existing tools have clearly stated limitations 

with respect to when they apply, what they predict, and 

how certain those predictions are. Users should know the 

limitations when applying the tools. Taken as a whole, the 

limitations mean the tools can provide general guidance 

regarding how far from the main front spot fires can be 

expected, but they do not claim to be, nor should they be 

used as, substitutes for lookouts and constant vigilance.

Second, in the absence of scientific information on a 

particular fuel’s tendency to produce embers or ignite when 

embers land, local knowledge, familiarity, and observations 

are essential. The number and size of embers or spot fires, 

and the observed distance and direction of ember travel are 

all important pieces of information. If spot fires are close 

enough together to merge before the main front arrives, or 

are observed on the flank of the fire, they could produce a 

new front or a rapid jump in the main front or a flank that 

could endanger crews.
GENERAL TECHNICAL REPORT PNW-GTR-854

86

Literature Cited

Albini, F.A. 1979. Spot fire distance from burning trees

—a predictive model. Gen. Tech. Rep. INT-56. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 

25 p.

Albini, F.A. 1981. Spot fire distance from isolated 

sources—extensions of a predictive model. Res. Note 

INT-309. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Forest and Range 

Experiment Station. 9 p.

Albini, F.A. 1983. Potential spotting distance from 

wind-driven surface fires. Res. Pap. INT-309. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 

27 p.

Alexander, M.E.; Cruz, M.G. 2006. Evaluating a model 

for predicting active crown fire rate of spread using 

wildfire observations. Canadian Journal of Forest 

Research. 36: 3015–3028.

American Meteorological Society [AMS]. 2000. Glossary 

of meteorlogy. http//amsglossary.allenpress.com/

glossary. (June 16, 2011).

Ellis, P.F. 2000. The aerodynamic and combustion 

characteristics of eucalypt bark—a firebrand study. 

Canberra, ACT, Australia: Australian National 

University. Ph.D. thesis.

Forestry Canada Fire Danger Group. 1992. Development 

and structure of the Canadian Forest Fire Behavior 

Prediction System. Information Report ST-X-3. Ottawa, 

Ontario, Canada: Forestry Canada, Science and 

Sustainable Development Directorate. 63 p.

Ganteaume, A.; Lampin-Maillet, C.; Guijarro, M.; 

Hernando, C.; Jappiot, M.; Fonturbel, T.; PérezGorostiaga, P.; Vega, J.A. 2009. Spot fires: fuel bed 

flammability and capability of firebrands to ignite fuel 

beds. International Journal of Wildland Fire. 18: 951–969.

Koo, E.; Pagni, P.J.; Weise, D.F.; Woycheese, J.P. 2010. 

Firebrands and spotting ignition in large-scale fires. 

International Journal of Wildland Fire. 19: 818–843.

Lee, S.L.; Hellman, J.M. 1969. Study of firebrand 

trajectories in a turbulent swirling natural convection 

plume. Combustion and Flame. 15: 645–655.

Manzello, S.L.; Cleary, T.G.; Shields, J.R.; Yang, J.C. 

2006a. Ignition of mulch and grasses by firebrands in 

wildland-urban interface fires. International Journal of 

Wildland Fire. 15: 427–431.

Manzello, S.L.; Cleary, T.G.; Shields, J.R.; Yang, J.C. 

2006b. On the ignition of fuel beds by firebrands. Fire 

and Materials. 30: 77–87.

Manzello, S.L.; Maranghides, A.; Mell, W.E. 2007. 

Firebrand generation from burning vegetation. 

International Journal of Wildland Fire. 16: 458–462.

McArthur, A.G. 1967. Fire behaviour in eucalypt forests. 

Leaflet 100. Canberra, Australian Capital Territory, 

Australia: Commonwealth of Australia, Forestry and 

Timber Bureau, Forest Research Institute. 

Muraszew, A. 1974. Firebrand phenomena. Aerospace 

Report ATR-74(8165-01)-1. El Segundo, CA: The 

Aerospace Corporation. 109 p.

Muraszew, A.; Fedele, J.B.; Kuby, W.C. 1975. Firebrand 

investigation. Aerospace Report ATR-74(7470)-1. El 

Segundo, CA: The Aerospace Corporation. 104 p.

Norum, R.A. 1982. Predicting wildfire behavior in black 

spruce forests in Alaska. Res. Note PNW-401. Portland, 

OR: U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Forest and Range Experiment Station. 

10 p.

Rothermel, R.C. 1983. How to predict the spread and 

intensity of forest and range fires. Gen. Tech. Rep. INT143. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Forest and Range Experiment 

Station. 161 p.
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Tarifa, C.S.; del Notario, P.P.; Moreno, F.G.; Villa, A.R. 

1967. Transport and combustion of firebrands. Final 

report of grants FG-SP-114 and FG-SP-146. Madrid, 

Spain: Instituto Nacional de Tecnica Aeroespacial. 90 p. 

Vol. II.

Wade, D.D.; Ward, D.E. 1973. An analysis of the Air 

Force Bomb Range Fire. Res. Pap. SE-105. Asheville, 

NC: U.S. Department of Agriculture, Forest Service, 

Southeast Forest Experiment Station. 38 p.

Wilson, A.A.G. 1988. Width of a firebreak that is necessary 

to stop grass fires: some field experiments. Canadian 

Journal of Forest Research. 18: 682–687.
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Introduction

Large fire whirls are often one of the more spectacular 

aspects of fire behavior. Flames flow across the ground 

like water feeding into the base of the vortex, the lowest 

thousand feet of which often takes on an orange glow from 

combusting gases rising within the vortex core. Burning 

debris lofted within the vortex can lead to a scattering of 

spot fires some distance from the main fire. With their 

sudden formation, erratic movement, and often sudden 

dissipation, fire whirls are a good example of extreme fire 

behavior. However, other forms of vortices are actually 

quite common on wildland fires and receive less attention 

despite their potential to dramatically alter fire behavior.

This chapter is designed to provide a better understanding of vortices associated with wildland fires, both 

fire whirls and horizontal roll vortices. A key point will be 

providing a basic understanding of what aspects of the fire 

environment contribute to the development and growth of 

these vortices. The next section of the chapter supplies a 

brief introduction to vorticity, a measure of the atmosphere's 

tendency to spin or rotate about some axis. With this basic 

understanding of vorticity, we will examine the common 

vortex forms described in the fire behavior literature, fire 

whirls and horizontal roll vortices.

Vorticity Basics

Simply stated, vorticity is the measure of spin about an axis. 

That axis can be vertical, as in the case of a fire whirl, or 

horizontal for a roll vortex, or somewhere in between. Figure 7-1 is an idealized illustration of a cross section through 

a fire with no ambient horizontal wind. The vertical winds 

near the ground can be characterized by a strong updraft 

over the fire and descending air outside of the fire area. 

The change in the vertical velocity across the fire imparts 

rotation to the flow field about an axis perpendicular to the 

page. Horizontal vortices form at both edges of the fire area 

and rise along the edge of the plume. While rising, these 

vortices can grow and will transfer energy to other vortices, 

which is one way that atmosphere dissipates energy. 

Chapter 7: Vortices and Wildland Fire

Jason A. Forthofer and Scott L. Goodrick1

1 Jason A. Forthofer, USDA Forest Service, Missoula Fire Sciences 

Laboratory, Missoula, MT.

Scott L. Goodrick, U.S. Department of Agriculture, Forest Service, 

Southern Research Station, Athens, GA.

Figure 7-1—Cross section through idealized fire illustrating occurrence of vortices owing to horizontal 

gradient of vertical motion produced by buoyancy from the fire.
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The change of vorticity can be described by the following equation where the terms on the righthand side of 

the equation can be grouped as either modifying terms or 

producing terms. 

Vorticity = (Transport + Tilting + Stretching)modifying + 

 (Baroclinic + Shear + Body)producing (1)

The first group of terms in equation 1 can modify 

vorticity that is already present in the atmosphere, but 

cannot create new areas of rotation. Transport is the process 

by which the mean wind can move vorticity from one area 

to another. Tilting involves changing the orientation of 

the axis of rotation (e.g., a horizontal vortex can be tilted 

into a vertical orientation). As will be shown later, this is 

an important process for wildland fires. The third term, 

stretching, can modify a vortex by changing the magnitude 

of the vorticity (how fast it rotates). The first two terms, 

transport and tilting, are only capable of moving or reorienting a vortex, but not strengthening or weakening one. A 

converging airflow, such as indraft feeding a fire, acts to 

strengthen a vortex by concentrating its circulation about a 

smaller radius, much like ice skaters spinning faster when 

pulling their arms in.

The producing terms on the righthand side of equation 

1 describe processes that create areas of rotation within the 

atmosphere. The baroclinic term generates vorticity in cases 

where the gradients in pressure and density are not parallel. 

In the case of a fire, rapid heating develops a horizontal 

temperature gradient that is not aligned with the vertical 

static pressure gradient. This misalignment of the vertical 

pressure gradient and horizontal thermal gradient leads to 

rotational motions to mix warm and cold fluid in an attempt 

to restore balance. The shear term describes the generation 

of vorticity from viscous shear stress. Wind shear induced 

by surface drag is a source of vorticity; therefore, if the 

wind is blowing at the earth's surface, horizontal vorticity 

is being generated. The final producing term in equation 

1 represents changes in vorticity from body forces such as 

gravity acting on the fluid. 

In summary, the vorticity at any location changes 

owing to the transport of vorticity from one place to 

another, the tilting of vorticity from one axis to another, 

the stretching and intensifying of vortices by convergence, 

or by the generation of vorticity through buoyancy or wind 

shear.

Fire Whirls

Fire whirls are vertically oriented, intensely rotating 

columns of gas found in or near fires. They have been 

observed in wildland, urban, and oil spill fires and volcanic 

eruptions. Dynamically they are closely related to other 

swirling atmospheric phenomena such as dust devils, waterspouts, and tornadoes (Emmons and Ying 1967). Fire whirls 

have also been called fire devils, fire tornadoes, and even 

fire-nados. They are usually visually observable because 

of the presence of flame, smoke, ash, or other debris. The 

definition of a fire whirl used here includes those whirls 

caused by the buoyancy of a fire but with no inner core of 

flame. Fire whirls range in size from less than 1 m in diameter and velocities less than 10 m/s up to possibly 3 km in 

diameter and winds greater than 50 m/s (Goens 1978). The 

smaller fire whirls are fairly common, whereas the larger 

whirls are less common. All fire whirls, especially the larger 

ones, represent a considerable safety hazard to firefighters 

through increased fire intensity, spotting, erratic spread rate 

and direction, and wind damage (Emori and Saito 1982, 

Moore 2008, U.S. Bureau of Land Management 2006).

Several extremely large fire whirls have been reported 

in urban fires that illustrate their potentially destructive 

nature. In 1871, the Great Chicago Fire generated whirlwinds that lifted and transported burning planks 600 m 

ahead of the main fire, which contributed greatly to the 

spread and destruction of the fire (Musham 1941). On the 

same day, a fire in Peshtigo, Wisconsin, generated a whirl 

that was strong enough to lift a house off its foundations 

(Gess and Lutz 2002). Hissong (1926) also reported a whirl 

strong enough to move a house. This whirl was one of many 

that formed during a large oil storage facility fire. The whirl 

separated from the fire and moved 1000 m downwind, lifted 

a small house, and moved it 45 m killing the two residents 

inside. A much more devastating whirl formed in 1921 when 
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a magnitude 7.9 earthquake hit the Tokyo, Japan, area causing a mass urban fire. This fire spawned an extremely large 

fire whirl that killed an estimated 38,000 people in less than 

15 min (Soma and Saito 1988). The victims had gathered 

in an area of sparse fuel 0.16 km2 in size, and the whirl 

moved over the area. Last, the World War II city bombings 

of Hamburg, Dresden, and Hiroshima were reported to have 

caused very large and destructive fire whirls. The Hamburg 

whirl was estimated at 2.4 to 3 km in diameter and 5 km tall 

(Ebert 1963).

Large and intense fire whirls also occur on wildland 

fires. Graham (1952, 1955, 1957) described several large 

whirls that were able to lift large logs and other debris and 

break off large standing trees. He indicated that many form 

on lee slope locations. Pirsko et al. (1965) reported on a 

very intense fire whirl that moved out of the fire area in the 

downwind direction and destroyed two homes, a barn, three 

automobiles, toppled almost 100 avocado trees, and injured 

four people. They also believed that the terrain and lee slope 

location contributed to the formation of the whirl. Additionally they cited moderate winds, an unstable atmosphere, and 

a large heat source as contributors. King (1964) analyzed 

video of a fire whirl and found that maximum vertical 

velocities in the whirl core were up to 91 m/s. Large fire 

whirls have also been documented on flat ground. Haines 

and Updike (1971) described several medium to large fire 

whirls that occurred during prescribed fires on flat ground. 

They cited a super-adiabatic lapse rate in the lower atmosphere as an important factor. Umscheid et al. (2006) also 

reported on a large fire whirl that occurred on flat ground 

and gave convincing arguments that a major contributor to 

the whirl was vorticity associated with passage of a cold 

front. Billing and Rawson (1982) also reported on a large 

whirl that may have been influenced by a cold front passage. McRae and Flannigan (1990) described many large 

whirls that occurred on prescribed fires. One of the largest 

and most intense whirls was 400 m in diameter and ripped 

standing trees out of the ground and lifted them upward. 

This whirl occurred on a cloudy day with a temperature 

lapse rate of -6 °C/1000 m in the first 1000 m above the 

ground. They concluded that the influence of the environmental lapse rate on fire whirl formation is unclear and that 

whirls can form under lapse rates other than dry or super 

adiabatic.

Fire whirls have severely injured firefighters in the past. 

Emori and Saito (1982) described a wildland fire in Japan 

that may have spawned a fire whirl that injured firefighters. 

The 2001 Fish Fire in Nevada generated a fire whirl that 

caused firefighters to deploy their fire shelters (U.S. Bureau 

of Land Management 2001). Another whirl in 2006 in 

Nevada injured six firefighters (U.S. Bureau of Land Management 2006). Finally, a very large whirl formed on the 

2008 Indians Fire in California that injured four firefighters 

(Moore 2008).

Fire Whirl Physics

Over the past few decades, a significant body of information 

has accumulated on fire whirl structure and influencing 

factors. The different techniques used to investigate fire 

whirls include field and laboratory-scale experiments, as 

well as analytical, physical, and numerical modeling. This 

work has revealed some of the main features of fire whirls. 

For example, it is commonly accepted that the formation 

of fire whirls requires a source of ambient vorticity and a 

concentrating mechanism (Emmons and Ying 1967, Goens 

1978, Meroney 2003a, Zhou and Wu 2007). Ambient 

vorticity in the atmosphere can be generated at the ground 

through wind shear, horizontal density gradients, and from 

the Earth’s rotation. The concentrating mechanisms in fires 

are produced by the buoyant flow. It reorients horizontal 

vorticity into the vertical direction and provides vortex 

stretching.

Vorticity Sources

In the wildland fire context, there are many possible 

sources of ambient vorticity that could contribute to fire 

whirls. Morton (1966) discussed some of these sources. One 

important source may be the shear layer that develops when 

ambient wind flows over the ground surface, producing horizontally oriented vorticity. This type of vorticity generation 
GENERAL TECHNICAL REPORT PNW-GTR-854

92

corresponds to the shear term on the righthand side of 

equation 1. As shown in figure 7-2, this horizontal vorticity 

can then be reoriented, or tilted, by the fire’s buoyant flow 

into the vertical (Church et al. 1980, Cunningham et al. 

2005, Jenkins et al. 2001) and may be a major contributor 

to many fire whirls. Similarly, it is likely that the indrafting 

to a buoyant plume develops a shear layer near the ground 

that also generates horizontally oriented vorticity that can 

be tilted to the vertical. This source of vorticity could be 

present even in zero ambient wind situations. Complex 

terrain can also generate vorticity through channeling and 

shear of ambient and fire-induced winds (Pirsko et al. 1965). 

Turbulent wake regions behind terrain features such as hills 

and mountains are thought to produce favorable vorticity for 

fire whirls (Countryman 1964, 1971; Goens 1978; Graham 

1957). Another source of ambient vorticity for some whirls 

may be vorticity present along frontal boundaries (Billing 

and Rawson 1982, Umscheid et al. 2006). This may be similar to the meteorological setting for many non-mesocyclone 

tornadoes (Umscheid et al. 2006).

Another possible source of vorticity in fire whirls is 

the baroclinic term in equation 1. At this time, it is unclear 

how important this source of vorticity is to fire whirls. 

McDonough and Loh (2003) provided an initial examination using numerical modeling. They mainly examined 

grid resolution requirements and were not able to make any 

strong conclusions about the significance of baroclinically 

generated vorticity, other than that it warrants further study.

Vortex Stretching

The primary vorticity-concentrating mechanism in fire 

whirls appears to be vortex stretching owing to vertically 

accelerating flow in the whirl core (Snegirev et al. 2004). 

The vertical acceleration is due to buoyant forces from hot 

gases in the core of the fire whirl. This acceleration causes 

a reduction in the diameter of a horizontal area enclosed 

by a chain of fluid particles (horizontal convergence), 

thereby increasing nonzero vorticity at any location on the 

horizontal area (Jenkins et al. 2001). This is analogous to a 

reduction in the moment of inertia of a rotating solid, causing increased rotation rate to conserve angular momentum. 

Figure 7-2—A schematic 

showing how shear-generated 

horizontal vorticity present 

in the atmosphere near the 

ground can be reoriented to 

the vertical by a fire (from 

Church et al. 1980).
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Snegirev et al. (2004) indicated that the whirl core radius 

is not dependent on the initial or imposed circulation, but 

that it is probably dependent on vortex stretching owing to 

vertical acceleration.

This same mechanism may also contribute to reduction in whirl vorticity (Snegirev et al. 2004) high up in the 

vortex where the vertical velocity decreases with height. 

This could occur when the core’s buoyancy is reduced 

from ambient air entrainment or encountering a stable 

atmospheric lapse rate aloft. The vertical deceleration 

would reduce the vorticity.

Increased Combustion Rates

A number of researchers have noted significant increases in 

burning rates of laboratory fire whirls (Byram and Martin 

1962, Chuah et al. 2009, Emmons and Ying 1967, Martin et 

al. 1976). In all of these studies, the burning rate is defined 

as the mass loss rate of the fuel source (solid or liquid). 

Byram and Martin (1962) found a threefold increase in 

alcohol burning rate when a whirl formed. Emmons and 

Ying (1967) found that the burning rate of their acetone pool 

fires was a function of the externally imposed circulation, 

with increases of up to seven times the nonwhirl conditions. 

Martin et al. (1976) measured burning rates in fires fueled 

by cross-piled wood sticks of varying sizes that were 1.4 to 

4.2 times the nonwhirl fire rate.

Scaling Fire Whirls

Much of what is known about fire whirls comes from 

small-scale laboratory experiments. Full-scale experiments are usually not practical because of safety concerns, 

economic aspects, and difficulties of controlling boundary 

conditions (Emori and Saito 1982). Because of this, scaling 

laws are very important to consider when attempting to 

apply information gained from small-scale experiments to 

full-scale fire whirls. Several authors have examined scaling 

related to fire whirls.

Kuwana et al. (2007, 2008) examined several experimental and full-scale whirls and concluded that a critical 

crossflow wind velocity exists where fire whirls are most 

likely to occur. This critical velocity was found to be 

proportional to the vertical buoyant velocity, which depends 

on the burning rate and length scale of the burning area.

Fire Whirls in the Real World: Common 

Features

Many factors appear to influence the development of fire 

whirls on wildland fires. These factors interact in complex 

ways, and it is doubtful that firefighters will ever have 

very accurate predictive tools to foresee whirl formation, 

especially in a timely manner to make real-time decisions. 

The hope at this point is to identify situations that are 

more likely to form whirls. The following are some likely 

scenarios where fire whirls have been known to form. It is 

probable that some of these types of fire whirl scenarios 

could combine to possibly make whirl formation more 

likely or more intense.

Whirl Shedding on the Lee Side of a Plume 

This type of whirl forms when a plume is subjected to 

a crossflow wind. The whirl forms on the lee side of the 

plume. It separates from the plume and advects in the 

downwind direction. It is similar in appearance to Von 

Karman vortex shedding behind an obstruction in a flow. 

Often, as the whirl moves away from the fire, it contains 

no flaming combustion. Wind in the whirl can be strong 

enough to cause damage to trees, structures, vehicles, etc., 

and the whirl may stay intact for several minutes and travel 

for distances of possibly 1.6 km (1 mi). Its ability to stay 

intact even though most of its vortex stretching mechanism 

(buoyancy) is lost is probably due to the strong reduction 

in turbulent diffusion of the core. Examples of this type of 

whirl have been reported by many authors (Church et al. 

1980; Clements et al. 2008; Dessens 1962; Hissong 1926; 

Pirsko et al. 1965; Soma and Saito 1988, 1991) and video 

and images of others are on file at the U.S. Forest Service 

Missoula Fire Sciences Laboratory.

It is probable that a critical crossflow wind velocity is 

very important to this type of fire whirl, as discussed in the 

section on scaling firewhirls. Its main source of vorticity 
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may come from the tilting of horizontally oriented, sheargenerated vorticity in the ambient crossflow. The significance of other sources of vorticity is currently unknown. 

Others (Fric and Roshko 1994, McMahon et al. 1971, Moussa 

et al. 1977) have shown that the same shedding whirls are 

present in an isothermal vertical jet in crossflow, giving 

some credibility to the notion that the main source of vorticity comes from the shear-induced ambient vorticity

L-Shaped Heat Source in Crossflow

Soma and Saito (1988, 1991) first investigated this type of 

fire whirl as an explanation for a historic and catastrophic 

fire whirl that occurred in 1923 in Tokyo. Unlike the 

shedding whirl, this type of whirl seems to be mostly 

stationary. It occurs when a roughly L-shaped heat source 

is subjected to a crossflow wind as shown in figure 7-3. The 

whirl forms in the inside bend of the L-shaped heat source. 

As in the shedding whirl, a critical crossflow windspeed is 

thought to be important (Soma and Saito 1988, 1991). If the 

wind is above or below this speed, whirls are less likely to 

form. This type of whirl is probably very much related to 

the shedding whirl type, including the important vorticity 

source from the ambient shear flow.

Vorticity Associated With Cold Fronts

This type of whirl forms when ambient vertical vorticity 

from cold fronts interacts with a fire plume. Billing and 

Rawson (1982) and Umscheid et al. (2006) discussed cases 

where this type of whirl formed over flat terrain. The key 

feature of these two examples is that they occurred almost 

exactly when a cold front passed over the fire area. Umscheild et al. (2006) discussed the associated ambient vertical 

vorticity present along a cold front boundary and identified 

some similarities between this type of fire whirl and the 

formation mechanisms of non-mesocyclone tornadoes. At 

this time, it is not clear why fire whirls form under some 

cold front passage conditions, but not others. Perhaps nonmesocyclone tornado genesis research can help identify 

why these whirls form.

Multiple Interacting Plumes

This type of fire whirl occurs from the interaction of 

multiple plumes with no ambient crossflow wind. Entrainment into each plume is affected by the nearby plumes, 

and under the correct configuration and buoyant plume 

strengths, a whirl can form. Figure 7-4 shows a schematic 

of how five fires could be oriented to cause a fire whirl. 

Lee and Otto (1975) observed whirl formation owing to 

plume interaction in their experiment using two asymmetric burning wood piles. Zhou and Wu (2007) examined 

the multiple interacting plume whirl in more detail using 

experimental fires, numerical simulation, and some scaling 

analysis. They discussed configurations under which whirls 

would and would not form. They also showed that whirls 

can form under randomly oriented plume locations (fig. 7-5). 

This has implications for wildland fire under mass ignition 

conditions. Occurrence of fire whirls under such conditions 

might be very likely, so long as the multiple plumes are 

drafting a significant amount of air and are properly spaced 

and organized.

Figure 7-3—Schematic of fire whirl formation for an L-shaped 

heat source in a crossflow wind. View is from above.
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Lee Side of a Hill/Mountain

These fire whirls occur when a fire plume exists on the 

lee side of a terrain obstruction such as a hill or mountain. 

The plume uses vorticity existing in the wake region of the 

obstruction to form the whirl. Countryman (1971) stated 

that this is the most favorable situation for generation of 

fire whirls. During investigations of full-scale mass fires, 

Countryman (1964) intentionally burned a fire on a lee 

slope under moderate wind to investigate this type of whirl. 

Several whirls formed during the burn, with the largest 

occurring near the end. Pirsko et al. (1965) described a 

whirl that formed on the lee side of a terrain obstruction and 

then shed from the plume in the downwind direction. The 

whirl caused significant wind damage to several houses, 

trees, and vehicles. Windspeed at the time was 9.4 m/s (21 

mi/h) with gusts to 13 m/s (29 mi/h). 

Horizontal Vortices

Horizontal vortices are quite common in the atmosphere 

and have been extensively studied (see Brown [1980] and 

Etling and Brown [1993] for reviews). In the absence of 

wind, when the ground is heated, the warm air near the 

ground will eventually begin to rise in circulation cells, a 

process known as Rayleigh-Bernard convection (Fernando 

and Smith 2001). In the presence of vertical wind shear, 

these cells begin to transition from disorganized and transient to an organized state: hexagonal lattice of convective 

cells. Fair-weather cumulus clouds often mark the tops of 

updrafts of these cells. As the wind shear increases, the 

convective cells further organize into horizontal convective rolls that are perpendicular to the mean wind; further 

increases in the vertical wind shear change the balance 

between buoyancy-driven vorticity and shear-driven vorticity and lead to the convective rolls being oriented parallel to 

the mean wind (Küettner 1971). These longitudinal convective rolls are easily seen in satellite images as parallel 

bands of cumulus clouds known as cloud streets. Figure 

7-6 illustrates the structure of these cloud streets. Although 

such horizontal convective rolls are a common feature of the 

atmosphere in the planetary boundary layer, the presence of 

Figure 7-4—Schematic showing one configuration that can cause 

a multiple interacting plume type fire whirl (from Zhou and Wu 

2007).

Figure 7-5—A computer simulation showing how randomly 

oriented heat sources could cause a fire whirl through plume 

interactions (from Zhou and Wu 2007).
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a fire adds a complicating factor in the form of a horizontal 

temperature gradient that can locally alter the convective 

organization of the boundary layer.

Horizontal vortices associated with wildland fires have 

received less attention than their vertical counterparts, fire 

whirls. Haines and Smith (1987) provided descriptions 

of three distinct types of horizontal vortices observed on 

wildland fires: the transverse vortex, which is perpendicular 

to the flow direction; a single longitudinal (flow parallel) 

vortex; and a counter-rotating longitudinal vortex pair.

Transverse Vortices

Transverse vortices are described by Haines and Smith 

(1987) as a series of vortices “climbing” the upstream side 

of the convective column under conditions of low ambient 

windspeeds and intense burning. The mechanism Haines 

and Smith (1987) proposed for the development of such 

vortices involves the development of buoyancy-forced ring 

vortices rising through the smoke column. Haines and 

Smith (1987) further hypothesized that only the upwind 

portion of the ring is clearly visible, as turbulent mixing 

is thought to render the downwind section of the ring less 

distinct. Transverse vortices on wildland fires have received 

little attention, but extensive literature is available on ring 

vortices associated with pool fires. 

The buoyancy-forced ring vortex is a common feature 

of fluid flows associated with heat sources ranging in 

scales from candles to pool fires up to large mass fires; 

however, they are most clearly visible under conditions 

of weak mean horizontal flow. For these ring vortices, the 

vorticity is generated through the baroclinic term from 

equation (1). Because the thickness of the density layer 

controls the magnitude of the baroclinically forced vorticity, the strongest vortices have scales similar to that of the 

flame surface (Cheung and Yeoh 2009). As buoyant forces 

cause these vortices to rise, a process often referred to as 

“amalgamation” takes place as the rising vortices merge and 

grow and manifest themselves in the oscillatory necking 

and bulging of the fire that results from the Rayleigh-Taylor 

instability. The same basic process can be observed at the 

scale of the smoke plume, leading to the development of the 

transverse vortices described by Haines and Smith (1987). 

The oscillatory nature of the development of these vortices 

has been extensively studied for pool fires (Cetegen and 

Ahmed 1993); however, little has been done at the scale of 

wildland fire events.

Although descriptions of vortex rings are quite common in the literature, little is mentioned about transverse 

vortices outside of Haines and Smith (1987). These vortices 

manifest themselves on the upwind side of the plume and 

add a boiling appearance to the plume. Although the vortices themselves are not a source of erratic fire behavior, their 

presence is an indicator of a potential increase in the rate of 

combustion and an associated change in fire behavior.

Longitudinal Vortices

Single longitudinal vortex— 

Longitudinal vortices differ from their transverse counterparts in that their axis of rotation is oriented parallel to 

the mean flow. The first class of longitudinal vortices from 

Haines and Smith (1987) is the single longitudinal vortex, 

of which only one case is presented, the Dudley Lake Fire 

as described by Schaefer (1957). The vortex was oriented 

in the direction of the mean flow, which was quite strong 

that day as surface winds were between 16 and 22 m/s. The 

Figure 7-6—Illustration of the role of horizontal vortices in the 

development of cloud streets (based on Etling and Brown 1993).
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diameter of the vortex was estimated at 1800 m. Smoke 

entrained within the vortex delineated the corkscrewlike 

nature of the vortex and allowed the vortex to be observable 

500 km downwind. The scale of this vortex is similar to 

those of the convective boundary layer rolls responsible for 

cloud streets and shows strong similarities to roll vortices 

associated with other crown fires (Haines 1982) with the 

main exception being that this was only a single vortex.

A possible answer to the question of why only a single 

vortex was observed may be answered through the numerical modeling work of Heilman and Fast (1992). In this study, 

a computer model of the atmospheric boundary layer was 

initialized with multiple heat sources some distance apart 

to examine how circulations induced by each heat source 

interacted and how the collection of these flows responded 

to the introduction of a transverse wind component (wind 

blowing perpendicular to the axis of the roll vortices). The 

introduction of the transverse wind component tended to 

destabilize the longitudinal vortices, and, in some cases, 

eliminated the upwind vortex entirely. Haines and Smith 

(1992) similarly found in their wind tunnel studies that a 

slight transverse component to the flow destabilized the 

vortex pair, causing the collapse of the downwind (relative to the transverse wind component) vortex, which on a 

wildland fire would cause the vortex to fall outward across 

the flank of the fire, providing an additional mechanism for 

lateral fire spread and a threat to firefighter safety. On the 

Dudley Lake Fire, Schaefer (1957) observed that at regular 

intervals, the outward/downward moving segments of the 

vortex would mark lateral surges in the fire growth, indicating the possible presence of some slight shifts in the wind 

that may have inhibited the presence of the other vortex. 

This vortex type differs from the other two types 

described by Haines and Smith (1987) in that the fire is not 

necessarily an integral forcing term in the development of 

the vortex. Conditions in the atmosphere may already favor 

the development of the convective rolls, and the fire may 

simply act to enhance the vortex through additional thermal 

instability. Although the transverse vortices are most pronounced at low windspeeds, the Dudley Lake vortex was 

accompanied by surface winds of 16 to 22 m/s (the mean 

windspeed for the 12 crown fire cases in Haines 1982 was 

5.5 m/s).

Counter-rotating, longitudinal vortex pair— 

Of the three types of horizontal vortices described by Haines 

and Smith (1987), the counter-rotating, longitudinal vortex 

pair is the best documented, although early work (Scorer 

1968, Turner 1960) focused on vortex pairs associated with 

smokestack emissions rather than wildland fires. The key 

feature of this vortex type is obviously the paired nature of 

the vortices rotating in opposite directions. These vortices 

often occur along the flanks of the fire and can also be 

observed in the main plume at the head of the fire; this is 

often referred to as a bifurcating smoke column. Figure 7-7 

shows a numerical simulation of a bifurcated smoke plume 

as viewed from behind the fire. Cunningham et al. (2005) 

showed that the degree to which the smoke plume splits is 

related to the depth of the surface shear layer.

Figure 7-7—Bifurcated smoke plume from a computer simulation. 

View is from upwind of the fire (from Cunningham et al. 2005).
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The New Miner Fire in central Wisconsin in 1976 is 

one example of a bifurcated smoke column provided by 

Haines and Smith (1987). This fire burned under very low 

relative humidity conditions for the region (minimum of 

23 percent) with light winds averaging around 2 m/s. The 

bifurcated column consisted of a pair of vortices approximately 30 m in diameter that rotated fairly slowly compared 

to other atmospheric whirls like tornadoes. These columns 

would intermittently collapse and spill over the fire's flanks, 

bringing hot gases and embers into contact with unburned 

fuels and providing for rapid lateral spread. Obviously, 

such behavior is a threat to fire crews that often focus their 

suppression efforts along the flanks of the fire. A key difference between these vortex pairs and the single vortex is the 

scale; the bifurcated columns were approximately 30 m in 

diameter, whereas the vortex on the Dudley Lake Fire was 

over 1 km.

As part of a 1979 study conducted at the Centre de 

Recherches Atmosphériques Henri Dessens in France, 

Church et al. (1980) studied the vortices produced by the 

Météotron, an array of 105 oil burners with a total heat 

output of 1000 MW. Three types of vortices were observed: 

(1) a columnar vortex that had the entire smoke column 

rotating, (2) small dust-devil-like vortices just downwind 

of the burner array, and (3) a large, counter-rotating vortex 

pair within the plume that started as vertical vortices at the 

burn site, but became horizontal and oriented parallel to the 

wind as the plume rose and moved downwind. The first two 

vortex types are vertical vortices as described in the section 

on fire whirls. 

The last type resembles the bifurcating column 

described for the New Miner Fire. At a height of 40 to 50 m, 

the smoke column of the Météotron experiment bifurcated 

into a pair of counter-rotating vortices with initial diameters 

of 30 to 60 m (Church et al. 1980). The dominant motion 

associated with these vortices was rotation about their axis 

with little noticeable motion along the axis, a stark contrast 

to the strong axial flow observed in many fire whirls.

The forcing of the counter-rotating vortex pair is 

complex and has parallels with the forcing of similar vortex 

pairs by nonbuoyant jets in a crossflow (see Margason 1993 

for a review). The split plume develops through the interaction of the ambient vorticity in the flow owing to vertical 

wind shear with the jet shear layer (or plume shear layer in 

the case of wildland fires). The presence of buoyancy adds 

additional complexity to the forcing of the split plume compared to the nonbuoyant jet. Church et al. (1980) put forth a 

pair of physical processes capable of describing the development of the bifurcating smoke column. The first process 

focuses on the reorientation and stretching of the horizontal 

vorticity in the ambient flow. Initially, the ambient vorticity can be thought of as a collection of horizontal tubes 

oriented perpendicular to the wind with upward motion 

along the upwind side of the tube and downward motion 

along the downwind side. As these vortex tubes encounter 

the rising air at the fire, the portion of the tube over the fire 

is lifted, which acts to tilt the vortex tube at the edge of 

the fire into a vertical orientation, producing a hairpin-like 

shape. As the lifted portion of the vortex tube continues to 

rise in the plume, it encounters stronger horizontal winds 

that transport this portion of the tube downwind faster than 

the surface parts, stretching the arms of the hairpin vortex. 

Eventually the combined processes of the lifting and faster 

downwind transport leads to the majority of the hairpin 

vortex being oriented horizontal and parallel to the mean 

flow. This is illustrated in figure 7-2.

The second process proposed by Church et al. (1980) 

deals with the generation of vorticity through the combined 

effects of buoyancy and surface drag forces. This process is 

actually a variation on the buoyant rings discussed earlier. 

The variation is the impact of the crossflow on the rising 

ring vortex. On the upwind edge of the ring, the crossflow 

enhances entrainment of ambient air on that side of the 

plume, which decreases the vertical velocity of that part of 

the plume. This causes the downwind section of the ring to 

rise faster than the upwind side, tilting some of the vorticity into a vertical orientation. The downwind section also 

encounters the stronger winds aloft before the upwind side, 

which leads to a stretching/intensifying of the streamwise 

sections of the ring. Experiments by Tsang (1970, 1971) 
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support the viability of this method in generating the 

counter-rotating vortex pair.

Although both physical processes are plausible explanations for the development of the counter-rotating vortex 

pair, both are not equally supported by the observations. 

Many of the observed fire plumes exhibited significant nearsurface vertical vorticity, which is best supported by the 

first process, which relies upon the reorientation of ambient 

vorticity (Cunningham et al. 2005). Wind tunnel studies 

of the longitudinal vortex pair offer further support for 

the ambient vorticity process, as Smith et al. (1986) found 

the vorticity in the streamwise vortex pair to agree quite 

well with the vorticity of the ambient flow as it approached 

the heat source. This is not to suggest that the buoyancy 

generated from the fire has no impact, just that it is not the 

dominant forcing for the development of the vortex pair.

Numerical modeling studies of the longitudinal vortex 

pair have largely been two dimensional (Heilman and Fast 

1992; Luti 1980, 1981; Porteire et al. 1999) or quasi-three 

dimensional (streamwise flow component assumed constant) 

where the governing equations are solved for a number 

of planes perpendicular to the streamwise flow (McGratten et al. 1996, Trelles et al. 1999, Zhang and Ghoniem 

1993). Cunningham et al. (2005) conducted the first fully 

three-dimensional simulation of fire plumes to focus on 

the development of vortical structures. Their simulations 

revealed the relationship among the depth of the shear 

layer, fire intensity, and the behavior of the vortex pair. The 

basics of this relationship centered around how long it took 

a buoyant air parcel to traverse the shear layer. Keeping the 

mean crossflow constant, a deeper shear layer would lead 

to a wider split of the smoke column. If the fire intensity 

is increased, the air parcels travel through the shear layer 

faster, which leads to a decrease in the width of the plume 

split. One interesting observation is that for a given fire 

intensity, the plume rise is not affected by the width of the 

smoke column's bifurcation, although its horizontal spread 

and deviation from a Gaussian distribution is strongly 

affected.

Another aspect of the counter-rotating vortex pair 

described by the numerical simulations of Cunningham et 

al. (2005) is the potential for oscillations, with each branch 

periodically exhibiting dominance. These oscillations 

were linked with localized regions of vertical vorticity of 

alternating signs being shed from either side of the plume in 

a manner similar to wake vortices observed for fluid flowing 

around a cylinder. These results were limited to a narrow 

range of flow parameters, but the simulations indicate that 

the counter-rotating vortex pair are not necessarily stable. 

Wind tunnel studies using a heated wire to mimic the flank 

of a crown fire have shown that perturbations in the flow 

component perpendicular to the mean flow can cause the 

vortex pair to collapse (Haines and Smith 1992). These flow 

perturbations could be caused by upstream topographic 

features, possibly groups of trees, or even natural shifts in 

the ambient wind.

In the previous discussion, the wind profile reflected 

typical conditions where windspeed increased with height. 

Byram (1954) noted that a number of major fire runs 

occurred when the windspeed decreased with height near 

the surface, a condition known as an adverse wind profile. 

Clark et al. (1996) examined the potential impact of an 

adverse wind profile on fire spread through the use of a 

three-dimensional coupled fire-atmosphere model. In their 

simulations, a counter-rotating vortex formed through the 

reorientation of the ambient boundary layer vorticity as 

described above; however, this time, the rotation was in 

the opposite direction (see figure 7-2 of Clark et al. 1996), 

which leads to narrow regions of hot, high-speed air shooting out of the fire front. This dynamic fingering occurred at 

scales of the order of tens of meters and has the potential to 

augment fire spread.

Tree Crown Streets

Some fires exhibit complex patterns of alternating strips of 

burned and unburned fuel, often referred to as tree crown 

streets. One possible explanation for the development of 

tree crown streets involves horizontal roll vortices (Haines 

1982). It is hypothesized that on one side of the vortex, 
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100

descending air cools the fuels and causes surface winds to 

diverge, thus inhibiting crown fire spread. On the other side 

of the vortex, upward motion is enhancing the convective 

column owing to the associated surface wind convergence, 

which can in turn enhance a spreading crown fire. Tree 

crown streets are cited as evidence for the presence of 

horizontal roll vortices on the Mack Lake Fire (Simard et al. 

1983). Wade and Ward (1973) observed complex patterns of 

intermittent strips of unburned fuel in the Air Force Bomb 

Range Fire and suggested some potential hypotheses for 

these patterns including brief fluctuations of windspeed 

or direction, or pulsations of long-range spotting linked to 

an erratic convective column. Although often considered a 

fingerprint for the presence of horizontal roll vortices, the 

exact cause of tree crown streets is not known.

Summary

Vorticity describes the degree of rotation in the atmosphere 

about some axis. Two factors that induce rotation in the 

atmosphere are wind shear and sharp horizontal gradients 

in temperature. Once one of these factors has generated 

vorticity, that vorticity can be transported by the mean wind 

to other locations, reoriented from one axis to another (a 

horizontal vortex can be tilted to become a vertical vortex), 

or enhanced by flow convergence that stretches the vortex. 

It is rare for the atmosphere to be completely devoid of 

vorticity. If the wind is blowing at all, there is vorticity 

produced near the ground by surface drag. Terrain features 

provide flow obstacles whose drag produces wind shear, 

thus generating vorticity. Different ground surfaces heat at 

different rates, which also generates vorticity. Vortices are 

present across a broad spectrum of spatial scales, continuously transferring energy between scales, mostly from large 

scales to smaller scales. A fire not only interacts with and 

modifies this ambient vorticity, but also generates additional 

vorticity. 

For convenience in our discussion of wildland fire 

vortices, we split our discussion into vertical and horizontal 

vortices. Vertical vortices, often referred to as fire whirls, 

are often the most dramatic and often-described type of 

vortex. Fire whirls, especially the larger ones, represent a 

considerable safety hazard to firefighters, as these vortices 

can result in sudden increases in fire intensity, spotting, 

erratic spread rate and direction, and damaging winds. 

Most often, the source of vorticity for a fire whirl is not 

the fire itself; rather, the vorticity is present in the ambient atmosphere. This ambient vorticity may be generated 

by wind shear, vortex shedding in the wake of a plume or 

topographic obstruction, or an approaching cold front. The 

fire plays a much more important role in modifying the 

ambient vorticity field by tilting horizontal vortices toward 

the vertical, and increasing the vorticity magnitude through 

the stretching term as surface flow converges at the fire to 

feed the strong updraft. 

Similarly, two of the three horizontal vortex types 

described by Haines and Smith (1987) rely on ambient 

vorticity. The counter-rotating vortex pair builds upon the 

tilting and stretching vortex modifications that enable a 

fire to transform horizontal vorticity generated by wind 

shear into a vertically oriented fire whirl. The key addition is stronger winds above the surface that sweep the 

upper part of the hairpin vortex described in figure 7-2 

downwind, bending the vortices back toward a horizontal 

orientation. For the single longitudinal vortex described for 

the Dudley Lake Fire, the fire is interacting with vorticity 

on a much larger scale, a boundary layer roll whose depth 

can occupy the entire mixed layer. Again the fire's role is 

one of modifying the vortex, which can in turn modify the 

fire environment by changing windflow patterns near the 

fire and creating a positive feedback loop leading to fire 

intensification.

Vortices are common features of the atmosphere occurring across a broad range of spatial scales. Our understanding of how wildland fires interact with this broad spectrum 

of atmospheric vortices is still very much in development. 

Table 7-1 summarizes the various vortices described in the 

text along with their causes and potential threats. Although 

the occurrence of these vortices is currently impossible to 

predict with precision, having a basic understanding of the 

importance of ambient atmospheric vorticity for vortex 
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development provides some guidance on situations that 

require awareness. Examining surrounding topography 

relative to the expected wind direction, can reveal features 

that may block or channel the flow. Wind profiles, when 

available, can provide information on wind shear as can 

watching direction and speed of cloud movements and their 

organization (are the clouds forming in lines?). Things 

to observe include (1) the behavior of the fire and smoke 

plume. Vortices are almost always present along the flaming 

front at some scale. (2) Vortices that grow or persist. (3) 

Signs of rotation or splitting in the smoke plume. This 

information is not sufficient for predicting the occurrence 

of intense vortices on wildland fires, but it can help identify 

potentially hazardous conditions.

Table 7-1—Summary of vortices, their causes, and potential threats

Phenomenon Causal factor(s) Potential danger

Fire whirl formation Shear-generated vorticity near the ground Increased energy release rate, spread rate, 

 on the lee side of a is concentrated and reoriented to the and spotting. The whirl could travel downwind 

 plume vertical on the lee side of the plume. from the fire and overtake firefighters.

Fire whirl formation Shear-generated vorticity near the Increased energy release rate, spread rate, and 

 near an L-shaped ground is concentrated and reoriented spotting. The whirl could suddenly form in the

 fire in a crossflow of the vertical on the lee side of the L, as “interior” area of L.

 wind shown in figure 7-3.

Fire whirl formation Vorticity along the frontal boundary is Increased energy release rate, spread rate,

 near a cold front concentrated in to a fire whirl. and spotting.

Fire whirl formation The indrafting and blocking effects of Increased energy release rate, spread rate, and

 owing to multiple of multiple interacting fire plumes spotting. Whirl could build into a fire storm.

 interacting fire concentrates vorticity that was likely 

 plumes shear generated near the ground.

Fire whirl formation Vorticity associated with the wake region Increased energy release rate, spread rate, 

 on the lee side of a of a terrain obstruction such as a hill or and spotting. The fire could quickly switch from

 hill/mountain mountain is concentrated into a fire a sheltered, backing fire with low fire behavior 

 whirl. to more extreme fire behavior. The whirl could 

 travel downwind from the fire and overtake 

 firefighters.

Transverse vortex Horizontal vorticity is produced Not a source of erratic fire behavior, but rather 

 on upwind side of through buoyancy. an indicator of a potential increase in the rate 

 smoke column of combustion and an associated change in

 fire behavior.

Single longitudinal Unstable atmosphere and strong winds Slight variations in wind direction can 

 vortex generate horizontal vortices with axis destabilize the vortex, causing the vortex to fall 

 parallel to the wind direction. Vortex outward across the flank of the fire, providing a 

 formation is not tied to the fire. mechanism for lateral bursts in fire spread. 

Counter-rotating Transverse ambient vorticity from Can produce concentrated wind bursts at the head 

 longitudinal vortex surface wind shear is altered by the fire of the fire front. The vortices are not always 

 pair as it is tilted into the vertical and stable as variations in wind direction can cause 

 reoriented to the longitudinal direction. one of the vortices to collapse and bring hot 

 Evident as a bifurcated smoke plume. gases and fire brands into contact with the 

 unburned fuel.
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102

Knowledge Gaps

Vortices in the atmosphere occur across a broad range of 

scales and may impact wildland fires in numerous ways. 

Vortices on wildland fires can develop rapidly, and their 

behavior is quite erratic and difficult to predict. As with 

tornadoes, understanding the environmental conditions that 

favor vortex formation is the first step to understanding the 

phenomenon. However, in the case of fire-related vortices, 

an important aspect not shared with tornadoes is the 

coupling between the fire and the ambient vorticity field. As 

highlighted in the fire whirl scaling section, a relationship 

exists—between the vigor of the vertical motions induced 

by the fire and the ambient crossflow—that describes a 

range of windspeeds that would support whirl development. Further investigation of this relationship and similar 

relationships that exist for horizontal vortices would provide 

a sound basis to develop management tools for assessing the 

potential for vortex development on a fire.

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As for big fires in the early history of the Forest Service,

a young ranger made himself famous by answering the

big question on an exam, “What would you do to control

a crown fire?” with the one-liner, “Get out of the way and

pray like hell for rain.”—Norman Maclean (1992)

Introduction

Three broad types of fire are commonly recognized in 

conifer-dominated forests on the basis of the fuel layer(s) 

through which they are spreading:

• Ground or subsurface fire

• Surface fire

• Crown fire

Ground or subsurface fires spread very slowly and with no 

visible flame. Heading surface fires can spread with the 

wind or upslope, and backing surface fires burn into the 

wind (fig. 8-1 A) or downslope. A crown fire is dependent 

on a surface fire for both its initial emergence and continued 

existence. Thus, a crown fire advances through both the 

surface and tree canopy fuel layers with the surface and 

crown fire phases more or less linked together as a unit (fig. 

8-1 B and C). The term “crowning,” therefore, refers to both 

the ascension into the crowns of trees and the spread from 

crown to crown.

From the perspective of containing or controlling 

wildfires or unplanned ignitions, the development and 

subsequent movement of a crown fire represents a highly 

significant event as a result of the sudden escalation in 

the rate of advance and the dramatic increase in flame 

size and thermal radiation as well as convective activity, 

including fire-induced vortices and, in turn, both short- to 

long-range spotting potential. As a consequence, crown 

fires are dangerous for firefighters to try to control directly 

by conventional means. Suppression actions and options 

Chapter 8: Crown Fire Dynamics in Conifer Forests

Martin E. Alexander and Miguel G. Cruz1

Figure 8-1—Variations in fire behavior within the jack pine/black spruce fuel complex found at the International Crown 

Fire Modeling Experiment study area near Fort Providence, Northwest Territories, Canada: (A) surface fire, (B) passive 

crown fire, and (C) active crown fire. For additional photography carried out on experimental basis, see Alexander and De 

Groot (1988), Alexander and Lanoville (1989), Stocks and Hartley (1995), and Hirsch et al. (2000).

1 Martin E. Alexander, Department of Renewable Resources and 

Alberta School of Forest Science and Management, University of 

Albert, Edmonton, Alberta, Canada. Miguel G. Cruz, Bush Fire 

Dynamics and Applications, CSIRO Ecosystems Sciences–Climate 

Adaptation Flagship, Canberra, ACT 2601, Australia.
GENERAL TECHNICAL REPORT PNW-GTR-854

108

at the head of the fire tend to be severely restricted until 

there is a major change in the prevailing fuel, weather, or 

topographic conditions (e.g., a drop in windspeed, a major 

fuel discontinuity). As a result, crown fires are capable of 

burning large tracts of forested landscape, thereby posing a 

threat to public safety and properties, potentially adversely 

impacting other values-at-risk, and increasing suppression 

expenditures.

Prolific crowning is an element or characteristic of 

extreme fire behavior in conifer-dominated forest cover 

types. This chapter constitutes a state-of-knowledge summary prepared for operational fire management personnel 

in the United States concerning our current understanding 

of the characteristics and prediction of crown fire behavior in such fuel complexes. Information on crown fire 

phenomenology is drawn upon from a number of sources, 

including relevant observations and data from Canada and 

Australia. The dynamics of crown fires in tall brush fields 

(e.g., chaparral) and other forest types (e.g., eucalypt) will 

not specifically be dealt with here. For present purposes, it 

is assumed that there is a distinct separation between the 

canopy fuel layer and the ground and surface fuels by an 

open trunk space in which ladder or bridge fuels may be 

present (fig. 8-2). Certain aspects of crown fire behavior are 

not addressed here but can be found in other chapters of this 

synthesis document (e.g., horizontal roll vortices, plume- or 

convection-dominated crown fires, influences of atmospheric conditions aloft, fire-atmosphere interactions).

Types of Crown Fires

Van Wagner (1977a) proposed that crown fires in conifer 

forests could be classified according to their degree of 

dependence on the surface fire phase and the criteria could 

be described by several semi-mathematical statements 

(fig. 8-3). He recognized three types of crown fires (box 1). 

According to Van Wagner (1977a), the type of crown fire to 

be expected in a conifer forest on any given day depends on 

three simple properties of the canopy fuel layer (box 2) and 

two basic fire behavior characteristics:

• Initial surface fire intensity

• Foliar moisture content

• Canopy base height

• Canopy bulk density

• Rate of fire spread after the onset of crown 

combustion

Figure 8-2—Cross section of a stylized, wind-driven surface head 

fire spreading behind a conifer forest canopy on level terrain.

Figure 8-3—Type of fire classification scheme based on Van 

Wagner’s (1977a) theories regarding the start and spread of crown 

fires in conifer forests.
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Box 1:

Crown Fire Classification

For the longest time, only two kinds of 

crown fire were generally recognized, 

namely “dependent” and “running” 

(Brown and Davis 1973). Van Wagner 

(1977a) recognized three classes or 

types of crown fires:

Passive Crown Fire

Passive or dependent crown fires can 

involve a portion or all of the canopy 

fuel layer in combustion, but the overall rate of spread is largely determined 

by the surface phase. Passive crown 

fires cover a range in fire behavior 

from moderately vigorous surface 

fires with frequent crown ignition 

occurring behind the surface flame 

front up to high-intensity surface 

fires spreading with an almost solid 

flame front occupying the canopy and 

subcanopy or trunk space that have 

nearly achieved the critical minimum 

spread rate for active crowning. Passive crown fires can occur under two 

broad situations. First, the canopy 

base height and canopy bulk density 

are considered optimum, but fuel 

moisture and wind conditions are not 

quite severe enough to induce fullfledged crowning (fig. 8-1 B). Second, 

the canopy base height and canopy 

bulk density are, respectively, above 

and below the thresholds generally 

Box 1: continued 

considered necessary for crowning 

(e.g., tall or open-forest stand types), 

so that even under severe burning 

conditions (i.e., critically dry fuels and 

strong surface winds), active crown 

fire spread is not possible, although 

vigorous, high-intensity fire behavior 

can occur.

Active Crown Fire

Active or running crown fires are 

characterized by the steady advancement of a tall and deep coherent flame 

front extending from the ground 

surface to above the top of the canopy 

fuel layer (fig. 8-1 C). The surface 

and crown phases are intimately 

linked, but fire propagation is largely 

determined by the crown phase. The 

spread of active crown fires requires 

(1) relatively dry and plentiful surface 

fuels that allow for the development 

of a substantial surface fire (2) low to 

moderately high canopy base height, 

and (3) a fairly continuous crown layer 

of moderate to high canopy bulk density (>0.1 kg/m3

) and low to normal 

foliar moisture content. 

Independent Crown Fire

An independent crown fire no longer 

depends in any way on the surface 

phase, spreading ahead of the surface 

phase in the crown fuel layer entirely 
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110

Box 1: continued 

on its own. Stand conditions favoring an independent crown fire are 

a continuous crown layer of low to 

moderate canopy bulk density and an 

abnormally low foliar moisture content. For a truly independent crown fire 

to develop on flat topography would 

require very strong, sustained winds. 

In mountainous terrain, slope steepness would no doubt compensate for a 

lesser velocity.

The vast majority of crowning 

forest fires spread either as passive 

or active crown fires, each controlled 

by a different set of processes. Van 

Wagner (1993) acknowledged that the 

concept of a truly independent crown 

fire as a stable phenomenon on level 

terrain is dubious but that it “may still 

have value in rough or steep terrain 

and as a short-term fluctuation under 

the most extreme conditions.” Indeed, 

there are reports of the flames in the 

crown extending 50 to 150 m ahead 

of the surface burning in momentary 

bursts and of crown fires spreading up 

steep, partially snow-covered slopes 

in the spring (Mottus and Pengelly 

2004). These incidents might possibly 

give the appearance of being evidence 

for independent crown fires. However, 

there is no steady-state propagation 

Box 1: continued 

as seen with passive and active crown 

fires. 

It is worthwhile noting that the 

concept of passive crowning implies 

an element of forward movement or 

propagation of the flame front. The 

incidental ignition of an isolated tree 

or clump of trees, with the flames 

spreading vertically from the ground 

surface through the crown(s) without 

any form of forward spread following, 

does not constitute passive crowning. 

Flame defoliation of conifer trees by 

what amounts to stationary torching 

or “crowning out,” especially common 

during the postfrontal combustion 

stage following passage of the surface 

fire, generally does not generate any 

kind of horizontal spread.

Scott and Reinhardt (2001) claimed 

that the possibility exists for a stand 

to support an active crown fire that 

would otherwise not initiate a crown 

fire. They referred to this situation as 

a “conditional surface fire.” Later on 

Scott (2006) termed this a “conditional 

crown fire.” To our knowledge, no 

empirical proof has been produced to 

date to substantiate the possible existence of such a situation, at least as a 

steady-state phenomenon. 
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The first three quantities determine whether a surface 

fire will ignite coniferous foliage. The last two determine 

whether or not a continuous flame front can be sustained 

in the canopy fuel layer. The initial surface fire intensity 

and rate of fire spread after the onset of crown combustion 

would, in turn, include the effects of windspeed, slope 

steepness, fuel dryness, air temperature, relative humidity, 

and fuel complex characteristics. Examples of how canopy 

base height and canopy bulk density CBD vary with tree 

and stand characteristics is presented here for ponderosa 

pine in figures 8-4 and 8-5.

Albini and Stocks (1986) considered the factors 

included in Wagner’s (1977a) proposed criteria for the start 

and spread of a crown fire as “heuristically valid.” Subsequent experience and analysis has shown both the strengths 

and limitations of his approach (Cruz et al. 2003c, 2004, 

2006a).

Crown Fire Initiation 

For a crown fire to start, an intense surface fire is generally 

required. The questions then become: How do we define fire 

Figure 8-5—Canopy bulk density (A) and canopy fuel load (B) for ponderosa pine stands as a function of stand density and basal area 

according to Cruz et al. (2003a).

Figure 8-4—Canopy base height for ponderosa pine stands as a 

function of average stand height and basal area according to Cruz 

et al. (2003a). The regression equation used to produce this graph 

is not valid for tree heights of less than 1.0 m. 
GENERAL TECHNICAL REPORT PNW-GTR-854

112

Box 2: 

Canopy Fuel Characteristics in 

Van Wagner’s (1977a) Crown Fire 

Initiation and Propagation Models

Canopy Base Height

Canopy base height (CBH) represents 

the mean height from the ground surface to the lower live crown base of the 

conifer trees in a forest stand (fig. 8-2). 

Canopy base height is dependent on the 

mean tree height and live stem density 

(fig. 8-4). Ladder or bridge fuels (e.g., 

loose bark and dead bole branches on 

tree boles, lichens, shrubs, and small 

conifers) in the space between the 

ground surface and the canopy “must 

presumably be present in sufficient 

quantity to intensify the surface fire 

appreciably as well as to extend the 

flame height” (Van Wagner 1977a). 

Unfortunately, our ability to assess ladder or bridge fuel effects on crown fire 

initiation remains qualitative (Menning 

and Stephens 2007).

Canopy Bulk Density

Canopy bulk density (CBD) represents 

the amount of available crown fuel 

within a unit volume of the canopy. 

Canopy bulk density is computed by 

dividing the canopy fuel load (CFL) 

by the canopy depth (fig. 8-2), which 

represents the average tree height of 

the stand minus the CBH. The CFL 

Box 2: continued

represents the quantity of crown fuel 

typically consumed in a crown fire, 

principally needle foliage. Both the 

CBD and CFL are, in turn, functions 

of stand structure characteristics (fig. 

8-5). 

Foliar Moisture Content

Foliar moisture content (FMC) represents a weighted average or composite 

moisture content for the various needle 

ages found within the canopy fuel layer. 

Needles decrease in moisture content 

with age following their initial flushing 

(Keyes 2006).

Some researchers such as Scott and 

Reinhardt (2001) have applied different 

criteria to the CBH, CFL, and CBD 

inputs in their use of Van Wagner’s 

(1977a) models (Cruz and Alexander 

2010, in press). However, strictly speaking, such ad hoc adjustments or modifications are not compatible with the 

use of these models. Still others have 

in some cases recommended or applied 

potentially unrealistically low values 

of FMC (Cruz and Alexander 2010). 

Varner and Keyes (2009) have outlined 

other faulty assumptions and common errors regarding modeling inputs 

involved in simulating fire behavior 

potential. 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

113

intensity? and How intense is intense enough with respect 

to the convective and radiative energy transferred upward 

to the canopy fuels necessary to initiate crowning? The 

distance the canopy fuel layer (fig. 8-4) is from the heat 

source at the ground surface will dictate how much energy 

is dissipated before reaching the fuels at the base of the 

canopy. Furthermore, if the moisture content of the canopy 

fuels is high, greater amounts of energy are required to raise 

the tree foliage to ignition temperature.

Byram (1959a) defined fireline intensity (I, kW/m) as 

the rate of heat released from a linear segment of the fire 

perimeter as calculated by the following equation:

 I = H × w × r (1)

where H is regarded as the net low heat of combustion (kJ/

kg), w is amount of fuel consumed in the active flaming 

front (kg/m2

), and r is the rate of fire spread (m/s) (Alexander 1982). If we assume H = 18 000 kJ/kg, then the equation 

for calculating fireline intensity can also be expressed as 

follows:

 I = 300 × w × ROS (2) 

where ROS is the rate of fire spread given in m/min. A 

graphical representation of this relation is presented in 

figure 8-6. Wendel et al. (1962) concluded that the probability of blowup fires decreased rapidly when available fuel 

loads were less than 1.35 kg/m2

.

Using physical reasoning and empirical observation, 

Van Wagner (1977a) proposed that vertical fire spread 

could occur in a conifer forest stand when the surface fire 

intensity (SFI) attains or exceeds a certain critical surface 

intensity for combustion (SFIcritical, kW/m) as dictated by 

the foliar moisture content (FMC, %) and the canopy base 

height (CBH, m) according to the following equation which 

is graphically presented in figure 8-7:

 SFIcritical = (0.01 × CBH × (460 + 25.9 × FMC))1.5 (3)

Figure 8-6—Head fire rate of spread and fuel consumed in relation 

to the type of fire and six distinct levels of Byram’s (1959a) fireline 

intensity, assuming a net heat of combustion of 18 000 kJ/kg. 

Figure 8-7—Critical surface fire intensity for crown combustion in 

a conifer forest stand as a function of canopy base height and foliar 

moisture content according to Van Wagner (1977a).
GENERAL TECHNICAL REPORT PNW-GTR-854

114

Thus, according to Van Wagner’s (1977a) theory of 

crown fire initiation, if SFI > SFIcritical, some form of 

crowning is presumed to be possible, but if SFI < SFIcritical, 

a surface fire is expected to prevail (fig. 8-3). In applying 

this criterion, it is assumed that a conifer forest stand possesses a minimum CBD that will allow flames to propagate 

vertically through the canopy fuel layer.

One of the appealing aspects of eq. (3) is its simplicity, 

but with this comes a major underlying assumption. According to Van Wagner (1977a), the 0.01 value given in eq. (3) is 

an empirical constant of “complex dimensions.” He derived 

this value from an outdoor experimental fire in a red pine 

plantation stand (see “Common and Scientific Names” 

section) with CBH of 6.0 m and a FMC of 100 percent and 

the SFI was about 2500 kW/m just prior to crowning (Van 

Wagner 1968). This widely used relation represented by eq. 

(3) therefore incorporates a fixed set of burning conditions, 

fuel characteristics, and surface fire behavior (e.g., in-stand 

wind, ladder fuels, fuel consumed, flame depth, and 

spread rate). Subsequent research has shown this empirical 

constant to be a variable quantity dependent on these factors 

(Alexander 1998, Cruz et al. 2006a).

From figure 8-7, it should be clear that the higher the 

CBH and/or FMC, the more intense a surface fire must 

be to cause crowning. It is worth noting that the flames 

of a surface fire don’t necessarily have to reach or extend 

into the lower tree crowns to initiate crowning (Alexander 

1988). The experimental fire used by Van Wagner (1977a) to 

Table 8-1—Fire suppression interpretations of flame length and fireline intensity

Flame Fireline

lengtha intensity Fire suppression interpretations

Meters kW/m

<1.2 <346 Fire can generally be attacked at the head or flanks by persons using hand tools. Handline 

should hold the fire.

1.2 to 2.4 346 to 1730 Fires are too intense for direct attack on the head by persons using hand tools. Handline can 

not be relied on to hold fire. Equipment such as plows, dozers, pumpers, and retardant 

aircraft can be effective.

2.4 to 3.4 1730 to 3459 Fires may present serious control problems: torching out, crowning, and spotting. Control 

efforts at the fire head will probably be ineffective. 

>3.4 >3459 Crowning, spotting, and major fire runs are probable. Control efforts at head of fire are 

ineffective.

a

 Based on Byram’s (1959a) flame length (L, m)-fireline intensity (I, kW/m) relation: L = 

0.0775 × (I)0.46.

Adapted from Burgan 1979.

Figure 8-8—Critical minimum spread rate for active crowning in a 

conifer forest stand as a function of canopy bulk density according 

to Van Wagner (1977a).
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

115

parameterize his crown fire initiation model represented by 

eq. (3) would, for example, have had a flame length of 2.6 m 

according to Byram’s (1959a) formula linking flame length 

to fireline intensity (table 8-1). 

Crown Fire Propagation

Assuming a surface fire is of sufficient intensity to initiate 

and sustain crown combustion from below, the question 

now becomes, Can a solid flame front develop and maintain 

itself within the canopy fuel layer in order for horizontal 

crown fire spread to occur? Van Wagner (1977a) theorized 

that a minimum flow of fuel into the flaming zone of a 

crown fire is required for combustion of the canopy fuel 

layer to continue. In this conceptual formulation, the flame 

front is viewed as stationary with the fuel moving into it.

Van Wagner (1977a) proposed that a critical minimum 

spread rate needed to preserve continuous crowning (ROScritical, m/min) could be estimated on the basis of the stand’s 

canopy bulk density (CBD, kg/m3) using the following 

simplistic equation:

 ROScritical = 3.0 ÷ CBD (4)

According to eq. (4), ROScritical increases as the CBD

decreases (fig. 8-8). High CBD levels are associated with 

dense stands and low values with open stands (fig. 8-5). 

Active crowning is presumably not possible if a fire does 

not spread rapidly enough following initial crown combustion. Albini (1993) viewed this criterion for active crowning as a “lean flammability limit.” Thus, if a fire’s actual 

ROS after the initial onset of crowning, which is in turn a 

function largely of the prevailing windspeed or slope, is less 

than ROScritical, a passive crown fire is expected to occur 

(fig. 8-3).

The 3.0 empirical constant given in eq. (4) was derived 

largely on the basis of a single experimental crown fire in 

a red pine plantation stand exhibiting a CBD of 0.23 kg/

m

3

 (Van Wagner 1964). However, the robustness of this 

value has since been confirmed on the basis of an analysis 

of a relatively large data set of experimental crown fires 

in several different conifer forest fuel complexes (Cruz 

et al. 2005) and a detailed wildfire behavior case study 

(Alexander 1998). These analyses also support Agee’s 

(1996) assertion that a CBD of about 0.1 kg/m3 constitutes 

a significant threshold for active crown fires. Furthermore, 

it appears from the function represented by eq. (4) and the 

available empirical evidence (fig. 8-8), that active crown 

fires are unlikely to occur at CBD levels below about 0.05 

kg/m3, although this is not to suggest that a very vigorous, 

high-intensity passive crowning is not possible.

Crown Fire Rate of Spread

Surface fires spreading beneath conifer forest canopies 

seldom exceed 5 to 10 m/min without the onset of crowning 

in some form or another (fig. 8-6). The exceptions would 

involve open stands with a low CBD (say less than 0.05 

kg/m3) or closed-canopied stands exhibiting a very high 

canopy base height (perhaps 12 to 15 m or greater), in which 

case, spread rates might reach as high as 25 m/min with 

associated fireline intensities of 10 000 kW/m.

General observations of wildfires and documentation of 

experimental crown fires indicate that a rather abrupt transition between surface and crown fire spread regimes is far 

more commonplace than a gradual transition (Van Wagner 

1964). With the onset of crowning, at a minimum, a fire 

typically doubles or even triples its spread rate in comparison to its previous state on the ground surface (McArthur 

1965). This sudden jump in the fire’s rate of spread occurs 

as a result of (i) the enhanced radiant heating owing to the 

taller and deeper flame fronts, (ii) the fact that the windspeeds just above the tree canopy are two and one-half to 

six times that of the winds experienced near ground level 

inside the stand, (iii) increased efficiency of heat transfer 

into a tall and porous fuel layer, and (iv) an increase in spotting density and distance just beyond the fire’s leading edge.

Once crowning has commenced, a fire’s forward rate 

of spread on level terrain is influenced largely by wind 

velocity and to a lesser extent by physical fuel properties 

and dryness. Continuous crowning generally takes place 

at spread rates between about 15 and 45 m/min. Crowning 

wildfires have been known to make major runs of 30 to 

65 km over flat and rolling to gently undulating ground 
GENERAL TECHNICAL REPORT PNW-GTR-854

116

during a single burning period or over multiple days, as so 

vividly demonstrated, for example, on the Rodeo-Chediski 

Fire in northern Arizona in June 2002 (Paxton 2007). A 

wildfire crowning through sand pine forests in north-central 

Florida on March 12, 1935, travelled nearly 32 km in about 

6 hours with intervening spread rates of 135 to 150 m/min 

(Folweiler 1937). During the major run of the Mack Lake 

Fire in Michigan that occurred on May 5, 1980, the crown 

fire rate of spread in jack pine forests peaked at nearly 190 

m/min during a 15-min interval (Simard et al. 1983). Grass 

fires have been reported to spread at twice these rates on 

level ground and are thus capable of spreading the same 

distance as crowning forest fires in half the time (Cheney 

and Sullivan 2008).

In some conifer forest fuel types exhibiting discontinuous or very low quantities of surface fuels, surface fire 

spread is nearly nonexistent, even under moderately strong 

winds. However, once a certain windspeed threshold is 

reached with respect to a given level of fuel dryness, a 

dramatic change to crown fire spread suddenly occurs. 

This type of fire behavior has been observed in pinyonjuniper woodlands of the Western United States (Bruner 

and Klebenow 1979) and in the sand pine forests of Florida 

(Hough 1973), for example. The same phenomenon has been 

observed in certain grassland and shrubland fuel complexes 

(Cheney and Sullivan 2008, Lindenmuth and Davis 1973).

Slope dramatically increases the rate of spread and 

intensity of wildland fires by exposing the fuel ahead of 

the advancing flame front to additional convective and 

radiant heat. Fires advancing upslope are thus capable of 

making exceedingly fast runs compared to level topography. 

For example, one would expect a crown fire burning on a 

35-percent slope to spread about 2.5 times as fast as one 

on level terrain for the same fuel and weather fuel conditions (Van Wagner 1977b). As slope steepness increases, 

the flames tend to lean more and more toward the slope 

surface, gradually becoming attached, the result being a 

sheet of flame moving roughly parallel to the slope. Rothermel (1985) has stated that although there is no definitive 

research on the subject of flame attachment, “it appears 

from lab work and discussions with users that the flames 

become attached near 50 percent slope with no prevailing 

wind.” The critical value will actually differ depending on 

the prevailing wind strength (Cheney and Sullivan 2008) 

as well as on the fuel type characteristics. The time-lapse 

photography taken of the rapid upslope runs on the 1979 

Ship Island Fire in central Idaho as shown in the video Look

Up, Look Down, Look Around (National Wildfire Coordinating Group 1993) constitutes a good example of such fire 

behavior.

With the exception of very long slopes such as found, 

for example, in the Salmon River country of central Idaho, 

the rate of advance of wind-driven crown fires in mountainous terrain tends, over the duration of their run, to be well 

below what would be expected on flat ground, even under 

critical fire weather conditions. This is a result of the degree 

of terrain exposure to the prevailing winds, which limits 

the full effectiveness of windspeed on fire spread, as well as 

differences in fuel moisture owing to aspect (Schroeder and 

Buck 1970). However, when the advancing crown fire front 

encounters a situation where wind and topography result 

in a favourable alignment, spread rates of ~100 m/min are 

quite easily possible for a brief period over short distances 

with only moderately strong winds (e.g., Rothermel and 

Mutch 1986). Fire spread rates in grassland and shrubland 

fuel types at even twice this level can easily occur (Butler et 

al. 1998, Rothermel 1993).

It is worth highlighting the fact that crown fire runs in 

mountainous terrain are not limited to just upslope situations. Cases of crown fires burning downslope or crossslope under the influence of strong winds have occurred in 

the past (Byram 1954, McAlpine et al. 1991). The major run 

of the Dude Fire in northern Arizona on June 26, 1990, that 

led to the deaths of six firefighters involved downslope and 

cross-slope spread as a result of the strong downdraft winds 

caused by the fire’s collapsing convection column (Goens 

and Andrews 1998).

Crown Fire Intensity and Flame Zone 

Characteristics

When a conifer forest stand crowns, additional fuel is 

consumed primarily in the form of needle foliage but also 
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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mosses and lichens, bark flakes, and small woody twigs. 

The additional fuel consumed by a crown fire owing to the 

canopy fuel involvement generally amounts to 0.5 to 2.0 

kg/m2 depending on stand characteristics (i.e., an increase 

in fuel consumption with respect to fireline intensity of 

one-quarter to a doubling in the amount). Combined with 

the increase in rate of fire spread after crowning, fireline 

intensities can easily quadruple in value within a few 

seconds (e.g., from 3000 kW/m to 12 000 kW/m). In such 

cases, is there any wonder why some fires seem to literally 

“blow up”?

A fire’s flame zone characteristics (i.e., depth, angle, 

height, and length) are a reflection of its heat or energy 

release rate. As the fireline intensity or rate of energy 

released per unit area of the flame front increases because 

of a faster rate of spread and a larger quantity of fuel being 

volatilized in the flaming front, flame size or volume 

increases. Fireline intensities of wind-driven crown fires 

can exceed 100 000 kW/m for significant periods (Anderson 

1968, Kiil and Grigel 1969).

The flame depth (D, m) of a spreading wildland fire (fig. 

8-2) is a product of its ROS and the flame front residence 

time (tr, min) which represents the duration that a moving 

band or zone of continuous flaming combustion persists at 

or resides over a given location: 

 D = ROS × t

r

 (5)

Residence times are dictated largely by the particle 

size(s) distribution, load, and compactness of the fuelbed. 

Residence times for conifer forest fuel types at the ground 

surface are commonly 30 sec to 1 min compared to 5 to 

10 sec in fully cured grass fuels. Assuming tr = 0.75 min 

(i.e., 45 sec), a surface fire in a conifer forest spreading at 

4.0 m/min would thus have flame depth of around 3.0 m 

according to eq. (5). Crown fires are capable of producing 

very deep flame fronts. The depth of the burning zone in the 

surface fuels of a crown fire spreading at 60 m/min would, 

for example, be around 45 m. The flame depth of a grass 

fire advancing at this rate would in contrast be only about a 

tenth of this value. Residence times within the canopy fuel 

layer of a crown fire are approximately one-half to one-third 

those experienced at ground level (Anderson 1968, Taylor 

et al. 2004). This is reflected in the gradual convergence of 

the flaming zone depth with height ending in the flame tip 

above the tree crowns.

The flame front of a crown fire on level ground appears 

to be roughly vertical or nearly so. This appearance has 

led to the popular phase “wall of flame” when it comes 

to describing crown fire behavior. Typically though, tilt 

angles are 5 to 20 degrees from the vertical. The fact that 

the flames of a crown fire stand so erect is a direct result of 

the powerful buoyancy associated with the large amount 

of energy released in the flame front (fig. 8-1 C). Radiation 

from the crown fire wall of flame can produce painful burns 

on exposed skin at more than 100 m from the fire edge 

(Albini 1984).

Given the difficulty of gauging the horizontal depth of 

the burning zone in a crown fire, flame height constitutes 

a more easily visualized dimension than flame length. 

However, efforts to objectively estimate flame heights of 

crown fires is complicated by the fact that sudden ignition 

of unburned gases in the convection column can result in 

flame flashes that momentarily extend some 100 m or more 

into the convection column aloft; one such flame flash was 

photographically documented that extended almost 200 

m above the ground (Sutton 1984). Such flashes can easily 

result in overestimates of average flame heights, which usually range from about 15 to 45 m on high-intensity crown 

fires (Byram 1959b). Average flame heights of crown fires 

are thus generally regarded as being about two (fig. 8-1 C) 

to three times the stand height. 

Crown Fire Area and Perimeter Growth 

For forest fires of today to become large, they typically have 

to involve some degree of crowning. A common axiom is 

that 95 percent of area burned is generally caused by less 

than 5 percent of the fires. When a forest fire at the very 

minimum doubles its spread rate after the onset of crowning, the area burned for a given period will be at least four 

times what would have been covered by a surface fire. In 

other words, the area burned is proportional to the rate of 
GENERAL TECHNICAL REPORT PNW-GTR-854

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spread increase (following the transition to crowning) to 

a power of 2.0 (McArthur 1965). Thus, if a fire triples its 

rate of advance after crowning, the area burned will be 

nine times the size it would have been had it remained as a 

surface fire (i.e., 3.02 = 9).

Other than dry and plentiful fuels, the principal 

ingredients for major crown fire runs are strong, sustained 

winds coupled with extended horizontal fuel continuity. 

The Hayman Fire that occurred along the Colorado Front 

Range, for example, burned close to 25 000 ha during its 

major run on June 9, 2002, and eventually grew to nearly 

56 000 ha towards the end of the month (Graham 2003). 

Under favourable conditions, crown fires on level to gently 

undulating terrain have been documented to cover in excess 

of 70 000 ha in a single, 10-hour burning period (Kiil and 

Grigel 1969) and up to a third that much in mountainous 

areas (Anderson 1968).

Assuming continuous fuels, including no major barriers 

to fire spread, and no change in wind and fuel moisture 

conditions, the forward spread distance of a crown fire can 

be determined by multiplying its predicted rate of spread 

by a projected elapsed time. Provided the wind direction 

remains relatively constant and the fire environment is 

otherwise uniform, wind-driven surface and crown fires 

typically assume a roughly elliptical shape (Alexander 1985, 

Anderson 1983, Van Wagner 1969) defined by its length-tobreadth ratio (L:B) (fig. 8-9), which in turn is a function of 

windspeed (fig. 8-10). The L:B associated with crown fires 

generally ranges from a little less than 2.0 to a maximum 

of approximately 8.0 in exceptional cases (Folweiler 1937). 

Simple estimates of potential crown fire size in terms of 

area burned and perimeter length can be made on the basis 

of the forward spread distance and L:B (fig. 8-11).

This simplistic picture of fire growth as described 

here is applicable to cases involving a point source ignition 

(e.g., an escaped campfire or lightning fire start) or perhaps 

Figure 8-9—Schematic diagram of a simple elliptical fire growth 

model (after Van Wagner 1969). The point of ignition is at the 

junction of the four area growth zones. 

Figure 8-10—Length-to-breadth ratio of elliptical-shaped fires on 

level terrain as a function of windspeed, as used in the Canadian 

Forest Fire Behavior Prediction System (Forestry Canada Fire 

Danger Group 1992) and in Rothermel’s (1991) guide to crown 

fire behavior in relation to experimental fire and wildfire observations given in Alexander (1985) and Forestry Canada Fire Danger 

Group (1992). The Beaufort scale for estimating 6.1-m open 

windspeeds is presented in table 8-4.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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a breach in an established control line, involving unidirectional winds and is generally limited to a 1- to 8-hour 

projection period. This approach is thus not appropriate to 

estimating crown fire growth when the perimeter becomes 

highly irregular in shape with the passage of time as a 

result of changes in wind direction, fuel types, and terrain 

characteristics (e.g., Rothermel et al. 1994).

One particularly dangerous synoptic fire weather situation worth highlighting with respect to crown fire behavior 

is the case of the dry cold frontal passage (Schroeder and 

Buck 1970). In the Northern Hemisphere, winds ahead 

of an approaching dry cold front generally shift from the 

southeast to south, and finally to the southwest. As the cold 

front passes over an area, winds shift rapidly to the west, 

then northwest. Windspeeds increase in strength as a front 

approaches, and usually become quite strong and gusty 

when the front passes over an area. This can result in a long 

crown fire run in a north-northeast direction followed by a 

fire’s entire right flank crowning in an east-southeast direction at an even greater rate of spread and intensity (DeCoste 

et al. 1968, Simard et al. 1983, Wade and Ward 1973).

Crown Fire Spotting Activity

Spotting or mass ember transport can be an important 

mechanism determining a crown fire’s overall rate of spread 

under certain conditions. Its general effect on crown fire 

rate of spread is determined by the density of ignitions and 

distances these ignitions occur ahead of the main fire. These 

two characteristics are intimately linked, with density 

typically decreasing with distance from the main advancing 

flame front.

The effect of spotting on the overall spread and growth 

of a wildland fire is dependent on topography and fuel 

distribution. In certain fuel types, the propagation of active 

crown fires is linked to high-density, short-range spot fires 

occurring up to 50 m or so ahead of the main advancing 

flame front followed by their subsequent coalescence. 

Figure 8-11—Area burned (A) and perimeter length (B) of an elliptical-shaped crown fire as a function of forward spread distance and 

windspeed on level terrain. The Beaufort scale for estimating 6.1-m open windspeeds is presented in table 8-4.
GENERAL TECHNICAL REPORT PNW-GTR-854

120

Figure 8-12—Minimum separation distance required for a newly 

ignited spot fire to avoid being overrun by the main flame front 

of an advancing crown fire as a function of rate of spread and 

ignition delay (adapted from Alexander and Cruz 2006). Ignition 

delay represents the elapsed time between a firebrand alighting, 

subsequent ignition, and the onset of fire spread.

Under such conditions, the overall fire spread is dictated by 

spotting as well as radiative and convective heat transfer 

mechanisms associated with the crowning phase (Taylor et 

al. 2004). In situations involving heterogeneous fuel type 

distributions and complex topography, spotting will allow 

the main advancing fire front to quickly bypass areas with 

low spread potential (e.g., downslope runs, pure hardwood 

stands in summer, discontinuous fuels) thereby effectively 

advancing the horizontal extent of the fire’s “head.” Spotting from crown fires is also effective in breaching major 

barriers to fire spread, including large water bodies and 

other nonfuel areas (e.g., rock slides, barren ground).

When fire environment conditions are uniform and 

winds aloft are favorable, spotting can contribute to the 

overall spread and growth of crown fires provided the spot 

fires are able to burn independently of the main advancing fire front. In most high-intensity wildfires that involve 

crowning, spot fires originating out ahead of the advancing 

flame front are typically overrun and thus incorporated into 

the larger fire perimeter before they are able to develop and 

spread independently, or otherwise be influenced by the 

main fire (e.g., in-draft winds). For a crown fire spreading 

at a rate of 50 m/min or 3 km/h and burning under homogeneous fuel, weather, and topographic conditions, spotting 

distances would, depending on the ignition delay, have to 

exceed approximately 500 to 700 m (fig. 8-12) to have the 

potential to increase a fire’s overall rate of spread through a 

“leapfrog” type of effect. If there are sufficient spot fires at 

or just beyond this distance that can rapidly coalesce, this 

“mass ignition” effect will temporarily lead to the formation 

of pseudo flame fronts with greatly increased flame heights 

(Wade and Ward 1973).

Spotting distances of up to about 2 km are commonly 

observed on wind-driven crown fires in conifer forests, but 

spotting distances close to 5 km have been documented 

as well. Spot fire distances of 6 to 10 km were reported to 

have occurred in the Northern Rocky Mountains during 

the 1910 and 1934 fire seasons. The occurrence of spotting 

distances greater than 5 km require a specific combination 

of convection column strength and vertical wind profile. For 

a viable firebrand to travel such distances, a large amount of 

energy needs to be released (associated with the postfrontal 

combustion of large fuels) to transport the firebrands at 

significant heights. Spotting distances of this magnitude are 

likely to be associated with isolated peaks of fire intensity, 

such as those occurring in an upslope run, that will inject 

large quantities of firebrands in the plume. An atmospheric 

profile with very strong winds aloft is also necessary to 

considerably tilt the convection column and allow for 

significant drift of the firebrand after it leaves the plume. 

Under exceptional circumstances, spotting distances greater 

than 10 km have been described. Especially noteworthy is 

the 16- to 19-km spot fire distances associated with the 1967 

Sundance Fire in northern Idaho (Anderson 1968), which 

were quite possibly caused by massive fire-induced vortices.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

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Models, Systems, and Other Decision Aids 

for Predicting Crown Fire Behavior

Rothermel Guide to Predicting Size and Behavior 

of Crown Fires

Rothermel (1972) developed a model for predicting surface 

fire rate of spread and intensity that still forms the basis for 

the vast majority of guides and computerized decision supports for predicting fire behavior in use today in the United 

States. He acknowledged that his model was not applicable 

to predicting the behavior of crown fires because the nature 

and mechanisms of heat transfer between the two spread 

regimes were quite different. In the mid to late 1970s, the 

general guidance to gauging whether crowning was possible 

or not was to use the predicted surface fireline intensity or 

flame length (table 8-1). There was no method at that time 

for predicting the spread rate of crown fires, but by the early 

1980s, the suggestion was being made to assume that crown 

fire rate of spread would be two to four times that of the 

predicted surface fire rate of spread of Anderson’s (1982) 

Fire Behavior Fuel Model 10 (Rothermel 1983).

The 1988 fires in the Great Yellowstone Area are 

generally regarded as the impetus for developing a more 

robust method of predicting crown fire behavior in conifer 

forests (Alexander 2009), although such a general need 

had been recognized for many years (e.g., USDA FS 1980). 

Rothermel (1991) produced such a guide for the northern 

Rocky Mountains or mountainous areas with similar fuels 

and climate using currently available information, including 

the method of estimating fine dead fuel moisture content 

(table 8-2) given in Rothermel (1983). The core component 

of his method or approach was a simple correlation derived 

from eight wildfire observations of crown fire rate of spread 

and the corresponding predictions from his surface fire rate 

of spread model (i.e., a 3.34 multiplier as opposed to 2.0 to 

Table 8-2–Predicted fine dead fuel moisture content (FDFM) 

as a function of ambient air temperature and relative humidity 

assuming >50-percent shading between 1200–1600 hours 

during May–July

 Air temperature 

Relative humidity 0 – 9 10 – 20 21 – 31 32 – 42 >43

Percent	 	 	 Degrees Celsius

0–4 4 4 4 4 4

5–9 5 5 4 4 4

10–14 5 5 5 5 5

15–19 6 6 5 5 5

20–24 7 7 6 6 6

25–29 8 8 7 7 7

30–34 8 8 8 7 7

35–39 9 9 8 8 8

40–44 10 9 9 9 9

45–49 10 10 10 10 10

50–54 10 10 10 10 10

55–59 11 11 11 11 11

60–64 12 11 11 11 11

65–69 12 12 11 11 11

70–74 13 12 12 12 12

75–79 14 13 13 13 13

The FDFM values are used in the Rothermel (1991) crown fire rate of spread model and 

in the Cruz et al. (2004, 2005) models for predicting crown fire occurrence and crown 

fire rate of spread. Adapted from Rothermel 1983.
GENERAL TECHNICAL REPORT PNW-GTR-854

122

4.0 as suggested earlier). Rothermel (1991) also included an

adjustment factor (1.7) for estimating the near-maximum

crown fire rate of spread associated with upslope runs or

sudden surges in crown fire activity.

Rothermel (1991) emphasized that his statistical model

for predicting the spread rate of wind-driven crown fires

was a first approximation and that more research was

needed to strengthen the analysis. At the time, he did not

explicitly include any specific criteria for determining the

onset of crowning other than in the most general terms (e.g.,

examine the fire weather forecast).

Rothermel (1991) considered his predictive methods

were not applicable to plume-dominated crown fire. However, he did end up incorporating Byram’s (1959b) ratio

of the power of the fire versus power of the wind concepts

(Nelson 1993) into his guide so as to distinguish the conditions favorable for plume-dominated crown fires as opposed

to wind-driven crown fires. Neither Byram’s (1959b) criteria

nor Rothermel’s (1991) adaptation of Byram’s criteria have

been evaluated for their robustness.

Rothermel’s (1991) guide included a suggested method

of predicting the flame lengths of crown fires. However,

neither his suggestion nor the approach of others seems to

work consistently well based on comparisons against data

from experimental crown fires. Furthermore, his model for

predicting the L:B of crown fires based on windspeed does

not appear to produce realistic results in light of observational evidence (fig. 8-10).

U.S. Fire Modeling Systems

Since the late 1990s, a number of existing and newly

developed decision-support systems have either separately

implemented or linked Rothermel’s (1972, 1991) surface and

crown rate of fire spread models with Van Wagner’s (1977a,

1993) crown fire transition and propagation criteria. These

include:

•	 BehavePlus (Andrews et al. 2008)

•	 FARSITE (Finney 2004)

•	 NEXUS (Scott and Reinhardt 2001)

Figure 8-13—An example of the differences in the critical midflame windspeeds required for the onset of crowning resulting

from the implementation of Van Wagner’s (1977a) crown fire

initiation model in various U.S. fire modeling systems for fuel

models 2 and 10 as described by Anderson (1982) (adapted from

Cruz and Alexander 2010). The following environmental conditions were held constant: slope steepness, 0 percent; fine dead fuel

moisture, 4 percent; 10-h and 100-h time lag dead fuel moisture

contents, 5 and 6 percent, respectively; live woody fuel moisture

content, 75 percent; and live herbaceous fuel moisture content,

75 percent. The associated 6.1-m open winds would be a function

of forest structure and can be approximated by multiplying the

midflame windspeed by a factor ranging between 2.5 (open stand)

and 6.0 (dense stand with high crown ratio) (Albini and Baughman

1979).

•	 Fire and Fuels Extension to the Forest Vegetation

Simulator (FFE-FVS) (Reinhardt and Crookston

2003)

•	 Fuel Management Analyst (FMA) Plus (Carlton

2005)

•	 FlamMap (Finney 2006)

To this list, we can also add two additional geographic

information system-based decision-support systems, namely

ArcFuels (Ager et al. 2011) and the Wildland Fire Decision

Support System (WFDSS) (Pence and Zimmerman 2011).
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Figure 8-15–Observed rates of spread of experimental active crown fires and wildfires that exhibited extensive active crowning 

versus predictions based on Rothermel’s (1991) crown fire rate of spread model (adapted from Cruz and Alexander 2010). The 

dashed lines around the line of perfect agreement indicate the ± 25-percent error interval. 

Figure 8-14—Observed head fire rates of spread >1.0 m/min associated with prescribed burning experiments in ponderosa pine forests 

of Yosemite National Park, California, versus predictions based 

on the Rothermel (1972) surface fire rate of spread model for fuel 

model 9-hardwood litter as described by Anderson (1982) 

(adapted from van Wagtendonk and Botti 1984). The dashed lines 

around the line of perfect agreement indicate the ± 25-percent 

error interval. Similar prediction trends were observed in mixedconifer pine, mixed-conifer fir, and true fir forest fuel types.
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These modeling systems are extensively used for fire 

operations, planning, and research. A recent review of the 

use of many of these fire modeling systems in several 

simulation studies examining fuel treatment effectiveness, 

revealed that many users are unaware of a significant 

underprediction bias that exists within these systems 

when it comes to assessing potential crown fire behavior 

in conifer forests of western North America (Cruz and 

Alexander 2010). The principal sources of this underprediction bias have been shown to include (i) incompatible model 

linkages (fig. 8-13), (ii) use of surface and crown fire rate 

of spread models that have inherent underprediction biases 

themselves (figs. 8-14 and 8-15), and (iii) a reduction in 

crown fire rate of spread based on the use of unsubstantiated crown fraction burned functions (fig. 8-16). The use 

of uncalibrated custom fuel models to represent surface 

fuelbeds was also identified as a fourth potential source 

of bias (Cruz and Fernandes 2008). The underprediction 

tendency was found to occur as well with the crown fire 

rate of spread model in the Fuel Characteristic Classification System (Schaaf et al. 2007). The Cruz and Alexander 

(2010) review also highlighted some issues with the manner 

in which users have been handling certain inputs in their 

crown fire modeling (i.e., foliar moisture content, canopy 

base height, and canopy bulk density) and in some perceived 

shortcomings of the two windspeed-based crown fire hazard 

indices developed by Scott and Reinhardt (2001).

Canadian Forest Fire Behavior Prediction System

Do alternative methods exist for predicting crown fire 

behavior? The Canadian Forest Fire Behavior Prediction 

(FBP) System (Forestry Canada Fire Danger Group 1992, 

Taylor et al. 1997, Wotton et al. 2009) constitutes one such 

possibility, at least for certain regions of the United States 

possessing fuel complexes structurally similar to those 

found in adjacent areas of Canada. The FBP System is a 

module of the larger Canadian Forest Fire Danger Rating 

System (CFFDRS), which also includes the Canadian Forest 

Fire Weather Index (FWI) System (Van Wagner 1987). 

Some states have adopted all or part of the CFFDRS (e.g., 

Alaska, Minnesota, Michigan).

Figure 8-16–Comparison of the effect of crown fraction burned 

(CFB) functions on rate of fire spread developed by Scott and 

Reinhardt (2001) and Finney (2004) as used in various U.S. fire 

modeling systems (e.g., NEXUS, FFE-FVS, FARSITE, FlamMap) 

in relation to the Rothermel (1972, 1991) surface and crown fire 

rate of spread models and Van Wagner’s (1977a) criteria for the 

critical minimum spread rates for crown fire initiation (ROSinitiation) and active crowning (ROScritical) for the Anderson (1982) Fire 

Behavior Fuel Model 2–Timber (grass and understory) with a wind 

reduction factor of 0.2 (Albini and Baughman 1979), a canopy bulk 

density of 0.1 kg/m3

, and canopy base height of 1.5 m (adapted 

from Cruz and Alexander 2010). A CFB function is not employed 

in BehavePlus. The following environmental conditions were held 

constant: slope steepness, 0 percent; fine dead fuel moisture, 6 

percent; 10-h and 100-h time lag dead fuel moisture contents, 7 and 

8 percent, respectively; live woody fuel moisture content, 75 percent; live herbaceous fuel moisture content, 75 percent; and foliar 

moisture content, 140 percent. The dashed portion of the Rothermel 

(1991) curve represents output below the original data set bounds 

for rate of spread. The Beaufort scale for estimating 6.1-m open 

windspeeds is presented in table 8-4.
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The FBP System provides estimates of head fire 

spread rate, fuel consumption, fireline intensity, type of fire 

description (table 8-3), and with the aid of an elliptical fire 

growth model, it gives estimates of fire area, perimeter, and 

perimeter growth rate as well as flank and backfire behavior 

characteristics for 16 major fuel types, 11 of which are 

subject to crowning (i.e., 7 coniferous and 4 mixed-wood 

types). The FBP System includes functions for the acceleration in rate of fire spread for a point source ignition to a 

quasi-steady-state equilibrium (McAlpine and Wakimoto 

1991), including a prediction of the elapsed time to crown 

fire initiation. Emphasis is placed on the influences of fire 

weather (i.e., fuel moisture and wind) on potential fire 

behavior for a given fuel type and the mechanical effects 

of slope steepness. The FBP System forms the basis for 

a major component of PROMETHEUS—the Canadian 

wildland fire growth simulation model (Tymstra et al. 2010), 

which is similar to FARSITE.

The FBP System is similar in many respects to predictive systems currently used in the United States. The 

principal difference is in the technical basis. The Rothermel 

(1972) surface fire model is based largely on laboratory fires 

and physical theory. The FBP System, on the other hand, 

is largely empirically based, representing the culmination 

of nearly 30 years of outdoor experimental burning work 

in major Canadian fuel types coupled with monitoring and 

documentation of numerous high-intensity wildfires. 

Other Empirically Based Approaches

Another possibility in lieu of the Canadian FBP System 

is the suite of empirically based models for predicting fire 

behavior incorporated into the Crown Fire Initiation and 

Spread (CFIS) software system (Alexander et al. 2006). 

These models are based largely on a reanalysis of the 

experimental fires carried out as part of developing the 

Canadian FBP System. The main outputs of CFIS are:

• Likelihood of crown fire initiation or occurrence based 

on two distinct approaches, one of which relies on 

the CBH or certain components of the Canadian FWI 

System (Cruz et al. 2003b), whereas the other is determined by the fine dead fuel moisture (table 8-2), CBH, 

windspeed, and an estimate of surface fuel consumption (Cruz et al. 2004) (fig. 8-17). 

• Type of crown fire (passive crown fire or active crown 

fire) and its associated rate of spread based on fine dead 

fuel moisture, CBD and windspeed (Cruz et al. 2005) 

(figs. 8-18 and 8-19).

• Minimum spotting distance required to increase a 

crown fire’s overall forward rate of spread assuming 

a point ignition and subsequent fire acceleration to 

Table 8-3—Type of fire as a function of the Initial Spread Index (ISI) component of the 

Canadian Forest Fire Weather Index (FWI) System for the coniferous (C) forest fuel types 

found in the Canadian Forest Fire Behavior Prediction System 

 Surface Passive Active

Fuel type Descriptive name fire crown fire crown fire

C-1 Spruce-lichen woodland <8 9 to 15 >16

C-2 Boreal spruce <1 2 to 7 >8

C-3 Mature jack or lodgepole pine <9 10 to 15 >16

C-4 Immature jack or lodgepole pine <2 3 to 8 >9

C-5 Red and white pine <25 26 to 40 >41

C-6 Conifer plantation <8 9 to 17 >18

C-7 Ponderosa pine/Douglas-fir <15 16 to 30 >31

The ISI is a relative numerical rating that combines the effects of fine fuel moisture (based on past and current weather 

conditions) and windspeed on the expected rate of fire spread. In the above tabulation, level terrain, a foliar moisture 

content of 97 percent, and a buildup index (BUI) of 81 to 120 is assumed. The BUI component of the FWI System is a 

relative numerical rating of the fuel available for combustion based on fuel dryness as determined by past and current 

weather conditions (Van Wagner 1987). In addition, a canopy base height of 7.0 m has been assigned to fuel type C-6. 

Adapted from Taylor et al. 1997.
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Table 8-4—Beaufort scale for estimating 6.1-m open windspeeds 

Wind Windspeed

class range Description Observed wind effects

	 km/h

1 <5 Very light Smoke rises nearly vertically. Leaves of quaking aspen in constant motion; 

 small branches of bushes sway; slender branchlets and twigs of trees move 

 gently; tall grasses and weeds sway and bend with wind; wind vane barely 

 moves.

2 6 to 11 Light Trees of pole size in the open sway gently; wind felt distinctly on face; loose 

 scraps of paper move; wind flutters small flag.

3 12 to 19 Gentle breeze Trees of pole size in the open sway very noticeably; large branches of pole- 

 size trees in the open toss; tops of trees in dense stands sway; wind extends 

 small flag; a few crested waves from on lakes.

4 20 to 29 Moderate breeze Trees of pole size in the open sway violently; whole trees in dense stands 

 sway noticeably; dust is raised in the road.

5 30 to 39 Fresh Branchlets are broken from trees; inconvenience is felt walking against 

 wind.

6 40 to 50 Strong Tree damage increases with occasional breaking of exposed tops and 

 branches; progress impeded when walking against wind; light structural 

 damage to buildings.

7 51 to 61 Moderate gale Severe damage to tree tops; very difficult to walk into wind; significant 

 structural damage occurs.

8 >62 Fresh gale Surfaced strong Santa Ana; intense stress on all exposed objects, vegetation, 

 buildings; canopy offers virtually no protection; windflow is systematic in 

 disturbing everything in its path.

Adapted from Rothermel 1983.

an equilibrium rate of spread based on the presumed 

crown fire rate of spread and ignition delay (Alexander 

and Cruz 2006) (fig. 8-12).

The primary models incorporated into CFIS have been 

evaluated against both outdoor experimental fires and wildfire observations and shown to be reasonably reliable (e.g., 

Alexander and Cruz 2006, Cronan and Jandt 2008, Stocks et 

al. 2004). The CFIS does allow one to evaluate the impacts 

of proposed fuel treatments on potential crown fire behavior 

based on the ability to manipulate three characteristics of a 

forest fuel complex (i.e., available surface fuel load, CBH 

and CBD) using silvicultural techniques.

The CFIS system is considered most applicable to 

free-burning fires that have reached a pseudo steady state, 

burning in live, boreal, or near-boreal type conifer forests 

found in western and northern North America (i.e., they are 

not applicable to insect-killed or otherwise “dead” stands). 

Furthermore, the models underlying CFIS are not applicable 

to prescribed fire or wildfire situations that involve strong 

convection activity as a result of the ignition pattern. Level 

terrain is assumed, as the CFIS does not presently consider 

the mechanical effects of slope steepness on crown fire 

behavior, although this is being planned for in a future 

version of the system. 

Physically Based Models

Physically based models are formulated on the basis of the 

chemistry and physics of combustion and heat transfer processes involved in a wildland fire. They range in complexity 

from models for calculating rate of fire spread based solely 

on the radiation from the flaming front (e.g., Albini 1996) 

to three-dimensional models coupling fire and atmospheric 

processes. Examples of the latter include FIRETEC (Linn 

et al. 2002), FIRESTAR (Dupuy and Morvan 2005), and 

Wildland Fire Dynamics Simulator (WFDS) (Mell et al. 
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Figure 8-17—The likelihood of crown fire occurrence as a function of canopy base height and windspeed for two fine dead fuel 

moisture levels, assuming a surface fuel consumption of 1.0 to 2.0 kg/m2

, based on the Cruz et al. (2004) probability model. The 

horizontal dashed line in each graph represents the approximate threshold value for the onset of crowning. The Beaufort scale for 

estimating 6.1-m open windspeeds is presented in table 8-4.

2007). Physically based models hold great promise in being 

able to advance our theoretical understanding of wildland 

fire dynamics and could possibly be used for operational 

prediction of wildand fire behavior in the future (Sullivan 

2009b). By their completeness, these models should be able 

to predict the development, the demise or cessation, spread 

rate, fuel consumption, intensity, and flame dimensions of 

crown fires in relation to any combination of fuel, weather, 

and topographic variables. In recent years, these models 

have been extensively used as research tools to evaluate 

the effect of canopy fuel structure on crown fire dynamics. 

Such modelling efforts could possibly allow one to investigate the effect of fuel treatments on crown fire potential. 

Nonetheless, the capacity of these models to describe crown 

fire behavior is still open to question given that no evaluation against any empirical crown fire data set has been 

undertaken to date to our knowledge.

What is quite likely to happen is the continuing 

mergence of empirical and physically based approaches 

(Sullivan 2009b, Van Wagner 1985). An example of such 

an approach is the semiphysically based crown fuel ignition 

model (CFIM) developed by Cruz et al. (2006b) to predict 

the onset of crowning based on fundamental heat transfer 

principles. A series of submodels that take into account 

surface fire characteristics along with canopy fuel properties are used to predict the ignition temperature of canopy 

fuels above a spreading surface fire. An evaluation of CFIM 

has been undertaken involving a sensitivity analysis of input 

parameters, comparison against other similar models under 

different burning conditions, and testing against outdoor 

experimental fires (Cruz et al. 2006a). Results have been 

favorable and provided new insights into the factors controlling the initiation of crown fires.
GENERAL TECHNICAL REPORT PNW-GTR-854

128

Figure 8-18–Threshold conditions for passive versus active crown fire spread in terms of windspeed and fine dead fuel moisture for 

two canopy bulk density levels based on the Cruz et al. (2005) crown fire rate of spread models and Van Wagner’s (1977a) criteria for 

active crowning. The Beaufort scale for estimating 6.1-m open windspeeds is presented in table 8-4.

Figure 8-19—Passive and active crown fire spread rates as a 

function of windspeed and fine dead fuel moisture for a canopy 

bulk density of 0.1 kg/m3

 based on the Cruz et al. (2005) crown fire 

rate of spread models. The vertical “kinks” in the fine dead fuel 

moisture curves are considered to represent the windspeed thresholds between passive and active crowning. The Beaufort scale for 

estimating 6.1-m open windspeeds is presented in table 8-4.
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Another example of the merging of empirical and 

physical modelling approaches was the International Crown 

Fire Modeling Experiment (ICFME). One of the objectives 

of this experimental burning program carried out in the 

Northwest Territories of Canada from 1995 to 2001 (Alexander 2005) was to test a newly developed, deterministic 

physical model for predicting crown fire rate of spread 

(Albini 1996, Butler et al. 2004). Measurements of flame 

radiometric properties and temperatures allowed for the 

parameterizing of the heat transfer components in Albini’s 

(1996) crown fire rate of spread model. Model evaluation 

indicated that the model predicted the relative response of 

fire spread rate to fuel and environmental variables, but it 

consistently overpredicted the magnitude of the spread rates 

observed on the ICFME crown fires.

Not all physically based models for predicting wildland 

fire spread specifically take into account the effects of 

spotting in increasing a fire’s rate of spread. The effects of 

spotting on a fire’s overall rate of advance are implicitly 

accounted for in both the FBP System and the Rothermel 

(1991) crown fire rate of spread model as a result of the 

empirical nature of their development (i.e., the use of 

wildfire observations as a data source). This assumes, 

however, that the fuels are continuous. Neither approach 

indicates how barriers to fire spread are to be handled. 

Short-range spotting from a crown fire is presumably able 

to easily breach fuel discontinuities of up to 100 m in width 

(Stocks et al. 2004, Taylor et al. 2004). Nominal spotting 

from crown fires is undoubtedly capable of breaching even 

much wider barriers, perhaps up to 1000 m (Alexander et 

al. 2004). What is unknown, however, is how much of a 

reduction there will be in the head fire rate of spread as a 

result of the time delay involved (which might possibly be 

30 to 60 min or longer) for the fire to resume its forward, 

equilibrium rate of advance.

Albini (1979) developed a physically based model for 

predicting the maximum spotting distance from single or 

group tree torching that covers the case of intermediaterange spotting of up to perhaps 3.0 km. This model is 

included within the BehavePlus modeling system, and a 

manual procedure is given in Rothermel (1983). Rothermel 

(1991) pointed out at the time he prepared his guide that 

no model existed for predicting the spotting distances 

for running or active crown fires. Venkatesh et al. (2000) 

subsequently extended Albini’s (1979) model to the case of 

wind-driven crown fires. The result was a 20- to 25-percent 

increase in spotting distance. However, no testing of this 

model has been undertaken to date to our knowledge. The 

Venkatesh et al. (2000) model like the one developed by 

Albini (1979) provides a prediction of the firebrand transport distance. Determining whether a given ember or firebrand will actually cause a spot fire must still be assessed 

based on its ignition probability (e.g., Rothermel 1983).

More recently, an alternative predictive system has 

been put forth for estimating the maximum spotting distance from active crown fires as a function of the firebrand 

particle diameter at alighting based on three inputs, namely, 

canopy top height, free flame height (i.e., flame distance 

above the canopy top height), and the windspeed at the 

height of the canopy.2 Although the system has not been 

specifically validated, the estimates produced by the system 

appear realistic in light of existing documented observations.

Example of a Practical Application of Linking 

Empirical and Physically Based Models

Pine Plantation Pyrometrics (PPPY) is a new modeling 

system developed to predict fire behavior in industrial pine 

plantations over the full range of burning conditions in 

relation to proposed changes in fuel complex structure from 

fuel treatments (Cruz et al. 2008). The system comprises a 

series of submodels, including CFIM and elements of CFIS, 

that describe surface fire characteristics and crown fire 

potential in relation to the surface and crown fuel structures, fuel moisture contents, and windspeed (fig. 8-20). A 

case study application of the PPPY modeling system has 

highlighted the complex interactions associated with fuel 

2 Albini, F.A.; Alexander, M.E.; Cruz, M.G. Preciding the maximum 

potential spotting distance from an active crown fire. Under review.
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Figure 8-20—Flow diagram of the Pine Plantation Pyrometrics modeling system for predicting fire 

behavior in exotic pine plantations (adapted from Cruz et al. 2008). CAC is the criteria for active 

crowning (Van Wagner (1977a), CFROS is the crown fire rate of spread, and SFROS is the surface 

fire rate of spread.

treatments such as pruning and thinning have on surface 

and crown fire behavior potential (fig. 8-21). It is also 

noteworthy that no definite reduction or increase in rate of 

spread was identified. Although a direct evaluation of the 

system’s overall performance has yet to be undertaken, its 

main components have been evaluated against independent 

data sets. 

Implications for Fire and Fuel Management 

In the broadest sense, the general conditions favorable 

for the development of crowning in conifer forests have 
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131

• Strong prevailing winds or steep slopes.

In the past 25 years or so, these conditions have, in 

turn, been crudely codified in various forms suitable for 

use by field personnel. Other aspects of the fire environment may lead to an increase in crown fire potential but by 

themselves are not a major predisposing factor (e.g., low 

foliar moisture content, high foliar heat content, presence of 

flammable oils and resins in the needle foliage). 

Assuming a threshold level in dryness has been reached 

in the forest floor layer, the potential for crown fire development and spread would generally follow the daily diurnal 

cycle in fire weather conditions, typically peaking in late 

afternoon (Beck et al. 2002). However, crown fire activity 

can extend late into the day if fire weather conditions are 

favorable for maintaining the moisture content of fine, dead 

surface fuels at low levels (Hartford and Rothermel 1991).

Rothermel (1991) quite rightly pointed out that “Fires 

are seldom uniform and well behaved.” Given the chaotic 

nature of most extreme fire phenomena, can we expect the 

behavior of crown fires to ever really be truly predictable? 

That depends on how accurate you expect the prediction to 

be. Certainly the minute-by-minute movement of a crown 

fire will probably never be predictable. However, in looking 

at crown fire propagation across longer timeframes, for 

example (e.g., 30 min to several hours), the available data 

have shown that some models are very capable of predicting 

fire spread within a margin of error that is useful to fire 

managers. Nevertheless, given the coarseness and uncertainty associated with the inputs in the crown fire initiation 

and propagation models, managers should be wary of their 

use for near-real-time predictions of fire behavior. Underestimating the potential for the onset of crowning under 

conditions that would sustain active crown fire propagation 

can, in turn, lead to substantial underpredictions in crown 

fire rate of spread and fireline intensity.

Models or guides that have a good fundamental 

framework and a solid empirical basis presumably predict 

fire behavior well when used for conditions that are within 

the data range used in their development (Sullivan 2009a). 

Overestimates of fire behavior can easily be readjusted 

Figure 8-21—Head fire rate of spread as a function of windspeed 

for 12-year-old thinned (50 percent basal area reduction treatment) and unthinned pine plantation stands based on the Pine 

Plantation Pyrometrics (PPPY) modeling system (adapted from 

Cruz et al. 2008). The fuel complex characteristics for the thinned 

and unthinned stands were, respectively, surface fuel available 

for combustion, 1.1 and 0.5 kg/m2

; canopy base height, 1.7 and 

0.9 m; and canopy bulk density, 0.05 and 0.1 kg/m3

. Given an air 

temperature of 40 °C and a relative humidity of 20 percent, the 

fine dead fuel moistures for the surface litter were, in turn, judged 

to be 5 and 7 percent, respectively. Foliar moisture content was set 

at 100 percent in both cases, and level terrain was assumed. The 

Beaufort scale for estimating 6.1-m open windspeeds is presented 

in table 8-4.

been known for some time and also apply to nonforested 

fuel types as well that exhibit high rates of fire spread and 

fireline intensities or very long flame lengths (e.g., Butler 

and Reynolds 1997). These include:

• Continuous fine fuels in sufficient quantity and 

arrangement, both vertically and horizontally.

• A dry spell of sufficient length to reduce the moisture 

content of dead fuels to a uniformly low level coupled 

with high ambient air temperatures and low relative 

humidity.
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Box 4: 

Crown Fire Dynamics in Conifer 

Forests—A Summary of the 

Salient Points

Types of Crown Fires

Three kinds or classes of crown fire 

are recognized according to their 

degree of dependence on the surface 

phase of fire spread (i.e., passive, 

active, and independent, although the 

latter is generally regarded as a rare 

and short-lived occurrence).

Crown Fire Initiation

The amount of heat energy required 

in the form of convection and radiation to induce the onset of crowning is 

dictated by the canopy base height and 

foliar moisture content as manifested 

in the surface fire’s intensity. A rather 

abrupt increase in fire activity should 

be expected as a fire transitions from 

the surface to crown fire phase.

Crown Fire Propagation

Whether a passive or active crown 

fire develops following the onset of 

crowning depends on the spread rate 

after initial crown combustion and is, 

in turn, related to canopy bulk density. 

A minimum value of about 0.1 kg/m3

appears to represent a critical threshold for active crowning.

Box 3: 

Useful Links to Further 

Information

U.S. Fire Modeling Systems

• http://www.firemodels.org/

Canadian Forest Fire Danger Rating 

Systems

• http://frames.nbii.gov/cffdrs

• http://cwfis.cfs.nrcan.gc.ca/en_CA/

background/summary/fdr

Crown Fire Initiation and Spread 

System

• http://frames.nbii.gov/cfis

International Crown Fire Modeling 

Experiment

• http://frames.nbii.gov/icfme

• http://www.youtube.com/

watch?v=FYup7cYKE3w

• http://www.youtube.com/

watch?v=zvPa_yEEd4E

Joint Fire Science Program Crown 

Fire Synthesis 

Project

• http://www.fs.fed.us/wwetac/ 

projects/alexander.html

• http://www.myfirecommunity.net/

Neighborhood.aspx?ID=816
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133

Box 4: continued

Crown Fire Rate of Spread

At a minimum, a doubling or tripling 

in a fire’s rate of advance follows the 

onset of crowning. Wind-driven crown 

fires have been documented to spread 

at up to 100 m/min for several hours 

and in excess of 200 m/min for up 

to an hour. Although the mechanical 

effect of slope steepness on increasing 

a fire’s rate of spread is well known, 

fires in mountainous terrain generally 

do not spread nearly as far for a given 

period of time compared to those on 

flat topography.

Crown Fire Intensity and Flame 

Zone Characteristics

As a result of the increase in spread 

rate and fuel available for combustion, 

a fire can easily quadruple its intensity 

in a matter of seconds when crowning 

takes place (e.g., from 3000 kW/m 

to 12 000 kW/m). The resulting wall 

of flame, standing nearly erect, is on 

average up to two to three times the 

tree height and emits fierce radiation. 

Flame fronts commonly exceed 30 to 

45 m in depth.

Crown Fire Area and Perimeter 

Growth

The area burned by a crown fire is at 

least four to nine times that of a surface fire for the same period. 

Box 4: continued

Assuming unlimited horizontal fuel 

continuity, crown fires are capable of 

burning an area of up to 70 000 ha 

with a perimeter length of 160 km in a 

single burning period and have done so 

in the past.

Crown Fire Spotting Activity

Crown fires commonly display highdensity, short-range spotting (<50 m). 

Spotting distances of up to about 2.0 

km, although less common, are frequently seen on crown fires, resulting 

in normal barriers to fire spread being 

breached. Many spot fires are simply 

overrun by the main advancing flame 

front of a crown fire before they effectively contribute to an increase in the 

fire’s overall rate of advance. Cases of 

long-distance spotting in excess of 10 

km have been reported. 

Models, Systems, and Other 

Decision Aids for Predicting Crown 

Fire Behavior

The current set of guides and decisionsupport system for assessing potential 

crown fire behavior used in the United 

States are considered deficient in the 

absence of considerable adjustment on 

the part of trained and informed users 

(e.g., fire behavior analysts, long-term 

fire analysts). Alternative models and 

systems that have undergone far more 
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134

without serious consequences. However, underestimates can 

be potentially disastrous (Cheney 1981). In this regard, the 

underprediction trend in predictions of crown fire behavior 

evident in the existing fire modeling systems used in the 

United States is of concern on a number of fronts. For 

example, if a system predicts or simulates that a fire will 

behave as a moderate-intensity surface fire under extreme 

fire weather conditions, why would it be necessary to undertake any form of fuel treatment or even be concerned about 

the general flammability of an area? As for human safety, if 

people are led to believe that some stand structures will not 

support crowning under a given set of weather conditions—

but in actual fact they will—are they not putting themselves 

and others in grave danger?

It has been suggested that most wildland fire operations personnel base their expectations of how a fire will 

behave largely on experience and, to a lesser extent, on 

guides to forecasting fire behavior (Burrows 1984). Experienced judgement is certainly needed in any assessment 

of wildland fire potential, but it does have some limitations 

(Gisborne 1948). The same can be said for mathematical 

models and computerized decision-support systems. Given 

the present realities, practical knowledge and sound professional judgment coupled with experience is still needed and 

perhaps should take on an even more prominent role when 

it comes to adjusting, interpreting, and applying crown fire 

behavior predictions. Predicting wildland fire behavior is, 

after all, both an “art and a science.”

Wildland fire research has done much to contribute to 

our current understanding of crown fire behavior through 

laboratory experiments, outdoor experimental burning, 

numerical modeling, and wildfire case histories (box 3). 

Although operational fire behavior specialists have also 

made substantial contributions (e.g., Beighley and Bishop 

1990, Murphy et al. 2007), valuable information and 

insights are not being captured in a systematic way. The 

continuance of basic research into fire fundamentals is 

essential to gaining a complete understanding of crown fire 

behavior, but scientific knowledge alone will not be enough 

to develop a complete picture of crown fire dynamics. There 

is still an overriding need to bolster the efforts in observing crown fire behavior and completing the necessary case 

study documentation in order to evaluate new and existing 

predictive models of crown fire behavior. Such a program 

should be regarded as a shared responsibility between wildland fire research and fire management and be considered 

part of adaptive management (Alexander and Taylor 2010).

Future Outlook 

In discussing his dichotomous key for appraising crowning 

potential, Fahnestock (1970) indicated that “No technique 

is available for calculating the mathematical probability 

that a fire will crown under given conditions.” In turn, 

Kerr et al. (1971) considered that “In the foreseeable future 

there is little prospect of predicting the behavior of a fast 

spreading crown fire in timber over any extended period 

of time.” More recently, Agee (1993) stated, “The chances 

of firebrand spotting and crown fires can be estimated, but 

the behavior of crown fire is still relatively unpredictable.” 

In light of these comments, obviously much has been 

accomplished and experienced in the past 20 to 40 years 

Box 4: continued

extensive testing and requiring a 

minimum of inputs are available. 

Implications for Fire and Fuel 

Management

Operational fire management personnel can readily help themselves when 

it comes to being able to assess crown 

fire behavior by engaging in a more 

rigorous program of wildfire monitoring and case study documentation 

than has been undertaken to date.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

135

when viewed from the point of our current understanding 

and predictive capability with respect to crown fires (box 4).

Presumably, the future holds the same promise as the 

recent past provided we are willing to readily admit what 

we know and, more importantly, what we presumably still 

do not know about crown fires with respect to their environment, characteristics, and prediction. Several knowledge 

gaps have been alluded to throughout this summary. 

Furthermore, a good many basic wildland fire behavior 

research needs identified over 25 years ago, some of which 

are relevant to crown fires, have yet to be addressed (Albini 

1984). Research must be directed at both the operational 

products desired by fire and fuel managers, and the fundamental understanding that forms the basis for such end-user 

tools (Cohen 1990).

Further discoveries and advancements in understanding of crown fire dynamics in conifer forests will require 

a dedication in time, money, and staff (Blatchford 1972). 

In actual fact, new research into the complexities of crown 

fire phenomenology has already been initiated (e.g., Cruz 

and Alexander 2009). However, in the long run, scientific 

investigations into crown fire behavior might be best 

accomplished in the form of a collaborative, international 

research, development, and application effort (Christensen 

et al. 2007, Weber 1995). Networked, multidisciplinary 

teams that can build on extant understanding while creating 

new knowledge regarding the mechanisms associated with 

crown fire initiation and spread may provide the necessary 

platform.

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Acknowledgments

The Extreme Fire Behavior synthesis was funded by the 

Joint Fire Science Program (JFSP) Project Number 09-2-

01-11 and a contribution from Project Number 09-S-03-1. 

The following individuals reviewed the entire manuscript 

and provided insightful comments: Don Haines (U.S. Forest 

Service, retired), Robert Ziel (Program Coordinator, Lake 

States Fire Science Consortium) and John Snook (Fire 

Weather/Predictive Services Program Manager, Northern 

California Geographic Area Coordination Center). 

Each chapter was also peer reviewed, and we wish 

to acknowledge those individuals for their comments that 

significantly improved the document. Listed below are the 

chapter reviewers:

• Effects of Complex Terrain—Tom Rolinski (Fire 

Weather Program Manager, Southern California 

Geographic Area Coordination Center) and Kenn 

McCarty (California Department of Forestry and 

Fire Protection).

• Critical Fire Weather Patterns—Rick Ochoa 

(National Interagency Fire Center Predictive 

Services, retired), Heath Hockenberry (National 

Weather Service), John Saltenberger (Fire Weather 

Services Program Manager, Northwest Geographic 

Area Coordination Center) and Julia Rutherford 

(National Weather Service).

• Fire Interactions and Mass Fires—Two anonymous 

reviewers.

• Column/Plume Dynamics and Spot Fires—Joseph 

J. Charney (U.S. Forest Service, Northern Research 

Station) and Don Haines (U.S. Forest Service, retired) 

for plume section; Nicholas Gellie (Australian Fire 

Behavior Analyst) and Marty Alexander (Alberta 

School of Forest Science and Management, University 

of Alberta, Edmonton, Canada) for spotting section. 

• Vortices and Wildland Fire—Don Haines and two 

anonymous reviewers.

• Crown Fire—Dave Thomas (U.S. Forest Service, 

retired), Philip Omi (Professor Emeritus, Colorado 

State University), C. Hoffman (University of Idaho) 

and J. Kern (Florida Division of Forestry). 

The authors also thank Victoria Werth for her help in 

writing the final document summary and to Victoria Werth 

and Lynn Sullivan for editing and formatting the manuscript in preparation for publication.

Unit Conversion Factorsa

SI unit Multiplication factor English unit Inverse factor

Degrees Celsius (°C) 5/9 (°F - 32) = Degrees Fahrenheit (°F) (9/5 °C) + 32

Hectares (ha) 2.47 = Acres (ac) 0.405

Kilograms (kg) 2.205 = Pounds (lb) 0.454

Kilograms per cubic 0.624 = Pounds per cubic foot (lb/ft3

) 16.0

 meter (kg/m3)

Kilograms per square 0.205 = Pounds per square foot (lb/ft2) 4.88

 meter (kg/m2)

Kilograms per second 737.463 = Pounds per hour per square 0.001356

 per square meter foot (lb hr-1.ft2)

 (kg sec-1 m

-2)

Kilograms per square 4.46 Tons per acre (t/ac) 0.224 

 meter (kg/m2)

Hectopascals (hPa) 0.0145 = Pounds per square inch (lb/in2) 68.94
GENERAL TECHNICAL REPORT PNW-GTR-854

144

SI unit Multiplication factor English unit Inverse factor

Kilojoules per kilogram 0.430 British thermal units per 2.32

 (kJ/kg) pound (BTU/lb)

Kilometers (km) 0.621 = Miles (mi) 1.61

Kilometers per hour 0.621 = Miles per hour (mi/h) 1.61

 (km/h) 

Kilowatts per meter 0.289 = BTUs per second per foot 3.46

 (kW/m) (BTU/s-ft)

Megawatts (MW) 56,869 = BTUs per minute (BTU/min) 0.0000176

Centimeters (cm) 0.394 Inches 2.54

Meters (m) 3.28 Feet (ft) 0.305

Square meters (m2

) 10.764 = Square feet 0.0929

Meters per minute 3.28 Feet per minute (ft/min) 0.305

 (m/min) 

Meters per minute 2.98 = Chains per hour (ch/h) 0.335

(m/min) 

Meters per second (m/s) 3.28 = Feet per second (ft/s) 0.305

Number per hectare 0.405 = Number per acre (No./ac) 2.47

 (No./ha)

Square meters per 4.36 = Square feet per acre (ft2/ac) 0.230

 hectare (m2/ha)

a Factors are given to three significant digits. To convert an English unit to a Standard Inernationad Unites (SI) unit, multiply by the 

inverse factor given in the right-hand column. 

Common and Scientific Namesa

Common name Scientific name

Douglas-fir Pseudotsuga menziesii (Mirb.) Franco

Eucalyptus Eucalyptus spp.

Jack pine Pinus banksiana Lamb.

Lodgepole pine Pinus contorta Douglas ex. Louden

Ponderosa pine Pinus ponderosa C. Lawson

Red pine Pinus resinosa Aiton

Sand pine Pinus clausa (Chapm. ex Engelm.) 

Vasey ex Sarg.

Singleleaf pinyon Pinus monophylla Torr. & Frém.

Utah juniper Juniperus osteosperma (Torr.) Little

White pine Pinus strobus L.

Spruce Picea spp.

a

 Source: USDA NRCS 2008.
Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

145

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