United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

General Technical Report RMRS-GTR-266

January 2012

Modeling Wind Adjustment Factor 

and Midflame Wind Speed for 

Rothermel’s Surface Fire Spread Model

Patricia L. Andrews
You may order additional copies of this publication by sending 

your mailing information in label form through one of the following 

media. Please specify the publication title and number.

Publishing Services

Telephone (970) 498-1392

FAX (970) 498-1122

E-mail rschneider@fs.fed.us

Web site http://www.fs.fed.us/rmrs

Mailing Address Publications Distribution

Rocky Mountain Research Station

240 West Prospect Road

Fort Collins, CO 80526

Andrews, Patricia L. 2012. Modeling wind adjustment factor and midflame wind speed for 

Rothermel’s surface fire spread model. Gen. Tech. Rep. RMRS-GTR-266. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 39 p.

Abstract

Rothermel’s surface fire spread model was developed to use a value for the wind speed that affects 

surface fire, called midflame wind speed. Models have been developed to adjust 20-ft wind speed to 

midflame wind speed for sheltered and unsheltered surface fuel. In this report, Wind Adjustment Factor 

(WAF) model equations are given, and the BehavePlus fire modeling system is used to demonstrate 

WAF calculation and effect on modeled fire behavior. There are differences in implementation of the 

same basic wind adjustment models in various fire behavior applications, including the Fireline Handbook and FARSITE. Differences are due to assumptions such as tree shape and rules for transition 

from sheltered to unsheltered conditions. Specifics are given for differences among WAF tables and 

calculation applications. This technical documentation is useful to analysts, system developers, fire 

weather meteorologists, and those who are interested in model background and foundation.

Keywords: wildland fire, fire behavior, fire danger rating, fire modeling, fire spread

Author

Patricia L. Andrews is a Research Physical Scientist with the Fire, Fuel, and Smoke 

Science Program at the Fire Sciences Laboratory in Missoula, Montana. She has 

been at the Fire Lab since 1973, serving as Project Leader of the Fire Behavior 

Research Work Unit from 1992 to 1996. Her research focus is fire behavior 

prediction and fire danger rating.

Acknowledgments

I thank the following people for helpful review comments: Larry Bradshaw, Jason 

Forthofer, Chuck McHugh (U.S. Department of Agriculture, Forest Service), and John 

Saltenberger (U.S. Department of the Interior, Bureau of Land Management). I also 

thank Faith Ann Heinsch (U.S. Forest Service), who provided valuable technical 

help and assisted with document preparation.

The BehavePlus fire modeling system computer program is available from http://

www.FireModels.org.
Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Rothermel’s Surface Fire Spread Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Fire Model Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Midflame Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Wind Adjustment Factor (WAF) Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

WAF and Midflame Wind in Fire Modeling Systems . . . . . . . . . . . . . . . . . . . . . . 4

Influence of Wind Adjustment Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Equations to Calculate WAF in BehavePlus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Log Wind Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Unsheltered WAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Sheltered WAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Sheltered Versus Unsheltered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Definition of Midflame Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

WAF Implementation in BehavePlus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Midflame Wind Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Input WAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Calculate WAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Other Implementations of WAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

WAF Table Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

BehavePlus Help System Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Baughman and Albini (1980) Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Rothermel (1983) Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Fireline Handbook (NWCG 2006) Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Scott (2007) Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

WAF Calculation Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

FARSITE Fire Area Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

FuelCalc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

BEHAVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

FVS-FFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

WAF for NFDRS................................................. 35

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Model Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Modeling Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Model Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 1

Introduction ____________________

Wind is among the most important influences on 

wildland fire. Fire behavior is strongly affected by wind 

speed and direction, which vary in time at the scale on 

the order of hours, minutes, and even seconds. The wind 

that affects wildland fire is influenced by terrain and 

vegetation. In addition to horizontal changes across the 

landscape, wind speed varies vertically with height above 

the ground. Determining an appropriate wind speed to 

use in modeling surface fire spread, flame length, and 

intensity is not a trivial task.

Fire modeling plays an important role in wildland 

fire and fuel management. For example, the National 

Fire Danger Rating System (NFDRS) (Deeming and 

others 1977) indices indicate the level of fire danger for 

public information and suppression preparedness. The 

FARSITE fire area simulator (Finney 1998) can be used 

to model growth of ongoing wildfires for tactical fire 

suppression decisions. The Fire Program Analysis (FPA) 

system includes modeling hypothetical fires to evaluate 

various funding levels (USDA/USDOI 2001; Finney 

and others 2011). The BehavePlus fire modeling system 

(Andrews 2011) can be used to develop prescription 

windows based on desired fire behavior and escaped 

fire contingency planning.

Rothermel’s surface fire spread model (Rothermel 

1972) is at the core of most fire behavior, fire danger, 

fuels management, and fire decision support systems 

in the United States. The Rothermel model includes 

the effect of fuel, fuel moisture, slope, and wind speed 

on surface fire rate of spread and intensity. Wind speed 

appropriately has a significant effect on modeled fire 

behavior. The fire model uses “midflame” wind speed, 

the wind that affects the surface fire.

Wind speed varies with height above the ground. The 

wind speed at midflame height is generally less than the 

wind speed above that level. Local winds generated from 

general winds aloft are slowed by surface friction near 

the surface, often producing a velocity profile as shown 

in figure 1.

The U.S. standard for fire weather wind is 20 ft above 

bare ground, or 20 ft above the vegetation (figure 2). 

The wind adjustment factor (WAF) adjusts the 20-ft 

wind speed to a midflame wind. Midflame wind speed 

is 20-ft wind speed multiplied by WAF. We use the term 

“wind adjustment factor” rather than “wind reduction 

factor” to avoid confusion in interpretation. A WAF 

of 0.40 reduces the 20-ft wind by 60 percent.

Figure 1—General wind velocity profile near 

surface (from Rothermel 1983).
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

The various standards for the height of measured and 

forecasted wind speed should be kept in mind when determining midflame wind (figure 3). Wind measurements 

are taken at 20 ft above the ground at fire weather stations. 

Fischer and Hardy (1972) defined the 

standard for station location as level, 

cleared areas that are not influenced 

by surrounding vegetation. At that 

time, because daily fire weather readings were taken manually, stations 

were generally located near an office. 

Remote Automatic Weather Stations 

(RAWS) now allow location in areas 

more representative of fire weather, 

possibly on slopes where it is more 

difficult to find a site not influenced 

by surrounding vegetation. For fire 

weather applications, the U.S. National Weather Service (NWS) usually 

forecasts 20-ft wind. In some cases, 

spot weather forecasts are given for 

midflame (eye level) wind. NWS forecasts and observations for the public 

are at the 10-m height, the World 

Meteorological Organization (WMO) 

standard. The Oklahoma Mesonet 

includes wind speed at 10 m as a core 

parameter and wind speed at 2 m as a 

supplemental parameter (mainly used 

for agricultural purposes) (McPherson 

and others 2007). Portable Fire RAWS 

have masts about 1.8 m (6 ft) above the ground (Bradshaw 

and others 2003). Handheld measurements are taken in 

the field using a hand-held anemometer, thus the term 

“eye level.”

Figure 2—Twenty-ft wind speed is the wind 20 ft above bare ground, surface fuels, or trees. Midflame wind is the wind that affects 

a surface fire, as required by Rothermel’s surface fire spread model.

Figure 3—Various height standards are used for wind measurements and forecasts.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 3

This paper describes models and methods for determining WAF. The influence of WAF and of the 

definition of “midflame height” on fire model results 

is demonstrated. The vertical difference in wind velocity can be significant. Midflame wind speed may be 

only 10 percent of the wind speed 20 ft above the tree 

tops. Of the many variables related to wind speed as 

it affects fire behavior, this report addresses only the 

vertical adjustment of a single wind velocity value. 

Wind adjustment factor does not address the effect of 

landscape on 20-ft wind speed and direction, nor does 

it deal with the various time averaging methods.

An effective user of wildland fire modeling systems 

is aware of model assumptions and of the effect of user 

selections on results. While it is possible to use models 

as a black box, it is not recommended. This report will 

be useful to those who desire an understanding of the 

modeling behind wind adjustment factor and midflame 

wind speed at a technical level. In addition to interested 

practitioners and course developers, the intended audience 

consists of system developers, analysts, meteorologists, 

and researchers. Developers of fire modeling systems 

and methods should make informed decisions on how 

to include midflame wind and WAF. An analyst who is 

comparing predicted and observed fire behavior should 

be aware of the influence of the method of determining 

wind speed for the calculations. Meteorologists who have 

a basic understanding of how fire models use midflame 

wind speed and wind adjustment factor can potentially 

provide better fire weather forecasts. Researchers who 

are working on new fire behavior models can recognize 

limitations of current methods and develop improvements 

for the future.

 It is assumed that the reader is familiar with fire 

modeling concepts and terminology (for example, Rothermel 1983) and with the BehavePlus fire modeling 

system (Andrews 2007; Heinsch and Andrews 2010). 

BehavePlus is used as the basis for description of WAF 

and midflame wind speed and their effect on modeled 

surface fire behavior. Details of the WAF models (equations) as implemented in BehavePlus are also given. 

This is followed by a description of the use of WAF in 

other applications, such as FARSITE, NFDRS, and the 

Fireline Handbook (NWCG 2006), with comparison to 

the method used in BehavePlus. In the final section of 

this report, we discuss the limitations of WAF models 

and the need for additional research and development.

Rothermel’s Surface Fire Spread 

Model _________________________

Fire Model Development

Because midflame wind and WAF are meaningful 

only in the context of Rothermel’s surface fire spread 

model, we begin with relevant points on development 

of the model. The spread model is based, in part, on 

experimental burns in a wind tunnel and on a data set of 

fast spreading wildfires. The final model describes the 

wind input as “mean wind speed at midflame height” 

(Rothermel 1972: p.33).

Laboratory facilities allow the study of fires in a controlled environment, which is not possible in the field. 

Combustion facilities were designed to control airflow 

uniformly within the test areas. This means uniformity 

of the average flow across the test area as well as of the 

velocity fluctuations within the average flow (Rothermel 

1965, 1967). Data were collected with mean wind tunnel velocity set at 2, 4, 6, or 8 mi/h (Rothermel 1972; 

Rothermel and Anderson 1966). The wind velocity was 

therefore not related to the height of the flames, but was 

the average value for the uniform flow in the wind tunnel.

In order to expand the range of conditions beyond 

what was possible in the laboratory, Rothermel also used 

a data set from fast spreading grass fires in Australia 

(McArthur 1969). Fifteen data points provided forward 

rate of spread for “average wind velocity at 33’ in the 

open” (10-m wind). Specific fuel data were not available; fuel bed depth was assumed to be 1 ft. While not 

specifically stated, a comparison of figure 19 and figure 

20 in Rothermel (1972) indicated that 10-m wind was 

multiplied by 0.4 to estimate midflame wind. (This is 

equivalent to WAF of 0.46 for 20-ft wind.)

It is not technically possible to define the height of 

midflame wind in the field in terms that match the data 

used for fire model development. As is the case for all 

models, developing WAF models involves assumptions 

and simplifications, which are described in this report.

Midflame Wind

While the term “midflame” wind indicates the wind at 

the midpoint of the flame, that is not a precise definition. 

The fire model was developed to use information available 

before the fire burns (Rothermel 1972). Midflame wind 

speed is the average wind velocity that affects surface 
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

fire spread. The term “midflame” was coined to make 

the distinction between the “free wind” at 20 ft or 10 m 

above the vegetation and the reduced wind that is used 

in calculating surface fire spread rate. Because it was 

defined specifically for Rothermel’s surface fire spread 

model, the term “midflame” applies only to surface fire, 

not crown fire.

If the height for the wind velocity depended specifically on flame height, then a calculation of flame height 

would be needed to determine the wind speed used in the 

calculation of flame height. This would not be acceptable 

and is not the case. The model for WAF is based on fuel 

bed depth, while the flame height for fires in fuel of that 

depth can vary significantly based on fuel moisture, wind 

speed, and slope. According to Rothermel (1983), “The 

model is complete in the sense that no prior knowledge 

of a fuel’s burning characteristics is required. All that is 

necessary are inputs describing the physical and chemical 

makeup of the fuel and the environmental conditions in 

which it is expected to burn.... Environmental inputs 

are mean wind velocity and slope of terrain.”

Weather measurements taken on-site with a belt weather 

kit or other hand-held device are at eye level. This is an 

appropriate estimate of midflame wind, as described by 

Rothermel (1983). He cautions that the location of the 

measurements must be reconciled with the fire location, 

with consideration of topography, sheltering, and time 

of day.

Rothermel (1983: preface) stated that it took 10 years 

to develop the spread model and another 10 years to learn 

how to obtain the inputs and interpret the outputs for 

field application. Development of concepts and models 

for wind adjustment factor to convert 20-ft wind speed 

to midflame wind speed is one of the steps taken to make 

the spread model useful for field applications.

Wind Adjustment Factor (WAF) 

Models ________________________

In some cases, a value for midflame wind speed can be 

used directly to calculate surface fire behavior. In other 

cases, the wind at 20 ft (or at 10 m) must be converted to 

an appropriate midflame value. Models for wind adjustment factor are based on a log wind profile, surface fuel 

depth, and sheltering from overstory vegetation at the 

site. The model gives an average wind adjustment over 

a height range, not at a specific height above the ground. 

WAF tables based on those models are an alternative to 

direct use of the model calculations. In selecting an appropriate WAF value from a table, a user can consider 

factors not in the models, such as the effect of surrounding 

vegetation as well as the potential of wind to penetrate 

the overstory due to location on the terrain (for example 

ridge top versus valley bottom). WAF models and methods 

address only the vertical adjustment of 20-ft wind speed 

to midflame wind speed. The effect of terrain on 20-ft 

wind speed is a separate topic.

Albini and Baughman (1979) developed the modeling 

foundation for WAFs for wildland fire. In their introduction, they noted the problem with the definition of midflame height: “The poorly defined ‘midflame’ windspeed 

can be approximated by using a spatially averaged value 

of the windspeed over an appropriate height range.” Their 

work dealt only with the steady, undisturbed windfield 

and its influence on fire in surface fuels. No account was 

taken of wind direction or the influence of the fire on 

the wind speed. Furthermore, flat terrain and uniform 

continuous vegetation cover were assumed.

Albini and Baughman (1979) presented mathematical 

models for wind characteristics above a vegetative cover 

that is a single-stratum fuel (grass, brush, etc.) and for 

wind under a forest canopy. There are two models, one 

for sheltered and one for unsheltered WAF. We use the 

term “unsheltered from the wind” rather than “exposed 

to the wind” to avoid confusion with characterization of 

understory exposure to the sun in discussing fuel moisture.

While the work of Albini and Baughman (1979) is 

appropriately listed as a reference for most applications 

of WAF, there are differences in implementation that are 

described in this report.

WAF and Midflame Wind in Fire 

Modeling Systems_______________

The original implementation of the Rothermel surface 

fire spread model was in the 1972 NFDRS (Deeming 

and others 1972). Observed 20-ft wind speeds from fire 

weather stations (Fischer and Hardy 1972) and forecasted 

winds were reduced to midflame wind speed for the calculation of fire danger indices using a WAF of 0.5 for all 

nine fuel models. The updated 1978 NFDRS assigned a 

WAF to each of the 20 fuel models (Deeming and others 

1977).

The Rothermel model was first available for fire behavior applications in the FIREMOD program (Albini 1976a) 

and the nomographs (Albini 1976b). In FIREMOD, midflame wind speed was the required input. The nomographs 

for the 13 fuel models (Anderson 1982) were initially 

based on the assumption that wind speed at midflame 

height was half of the 20-ft wind speed. They were later 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 5

revised to be based on midflame wind speed rather than 

20-ft wind speed to allow for variable WAF (Rothermel 

1983). A reformatted set of nomographs that includes an 

additional 40 fuel models (Scott and Burgan 2005) was 

also based on midflame wind speed (Scott 2007).

Baughman and Albini (1980) developed the original 

WAF tables for the 13 standard fire behavior fuel models. 

The authors included guidance on selection of sheltered 

versus unsheltered values based on the effect of topography, for example “fuels on high ridges where trees offer 

little shelter from wind.” The table was changed for presentation in the S-590 Advanced Fire Behavior course, as 

described by Rothermel (1983). Other versions of WAF 

tables are available in the Fireline Handbook (NWCG 

2006), in the nomograph publication (Scott 2007), and 

in the BehavePlus Help system.

The first computerized fire modeling system to include 

calculation of an unsheltered wind adjustment factor was 

the TSTMDL (test model) program of the BEHAVE fire 

behavior prediction and fuel modeling system (Burgan and 

Rothermel 1984). The fuel modeling portion of BEHAVE 

provided a means of developing custom fuel models to 

meet needs not satisfied by the 13 standard fuel models.

The FARSITE fire area simulator was an early geospatial 

implementation of Rothermel’s surface fire spread model 

and continues to be widely used. The input data layer of 

20-ft wind is automatically adjusted to midflame wind 

based on surface fuel bed depth and overstory values for 

each pixel. The WAF calculation method in FARSITE 

is also used in FlamMap (Finney 2006), FSPro (Finney 

and others 2010), and FPA.

The BehavePlus fire modeling system offers the 

option of direct entry of midflame wind speed or of 

entering 20-ft (or 10-m) wind speed and either entering 

or calculating WAF. The focus of this report is WAF 

modeling in BehavePlus followed by a description of 

other implementations.

Use of models for fire behavior prediction initially 

relied on use of nomographs (nomograms) and tables 

and on expert opinion on using appropriate values. The 

target user was an experienced fire behavior analyst 

who was able to choose a WAF by considering not only 

vegetation conditions at the site, but surrounding vegetation and terrain. That approach, however, is not feasible 

for geospatial modeling systems that necessarily rely on 

calculated WAF values.

Variation in fuel bed depth, stand characteristics, flame 

length and height, and wind speed and direction can 

be more important influences on fire behavior than the 

WAF. Nevertheless, modeling of fire behavior requires 

a repeatable method of calculating WAF based on a description of the fuel and vegetation (not on anticipated 

flame length or height).

Influence of Wind Adjustment 

Factor _________________________

Example runs using BehavePlus illustrate the influence 

of WAF on fire behavior modeling. (Specifics of WAF 

determination are given in following sections.) English 

units match the source documents of Albini and Baughman 

(1979) and Baughman and Albini (1980). BehavePlus 

also allows the use of metric units.

The first example (figure 4) shows calculated surface 

rate of spread and flame length for three values of WAF, 

for a fire on flat ground in fuel model 2 with 5 percent 

dead fuel moisture, 75 percent live fuel moisture, and a 

range of 20-ft wind speeds. Fuel model 2 is a grass type 

that can be an appropriate choice for an open ponderosa 

pine stand with annual grass understory or scattered 

sage within grasslands (Anderson 1982). WAF of 0.2 

would be used for fuel that is sheltered from the wind 

by overstory. WAF of 0.4 would be used if there is no 

overstory, leaving the surface fuel unsheltered from the 

wind. A high WAF value of 0.7 (low wind reduction) is 

also included for comparison. For this example, a 20-ft 

wind of 12 mi/h results in midflame wind speed of 2.4 

mi/h under the canopy and 4.8 mi/h in the open. The 

calculated rate of spread is three times as high in the 

open (37.9 ch/h) as under the canopy (12.5 ch/h). For a 

20 mi/h 20-ft wind, rate of spread using WAF of 0.7 is 

253 ch/h, which is much higher than the 93 ch/h based 

on an appropriate WAF of 0.4 for unsheltered fuels. The 

influence of WAF on flame length is similar. Plots show 

calculated rate of spread and flame length for the three 

WAF values for 20-ft wind speeds from 4 to 20 mi/h.

When modeled fire behavior is used in support of fire 

management decisions, due attention should be given 

to WAF. As with all aspects of modeling, the user is 

responsible for understanding model assumptions and 

limitations, selecting proper input, and interpreting and 

applying results appropriately.

Prescribed fire objectives, for example, might include 

an acceptable level of tree mortality. The mortality models 

in BehavePlus are based, in part, on scorch height, which 

is a function of flame length, which is, in turn, dependent 

on midflame wind speed. Midflame wind can be estimated 

from 20-ft wind using models for WAF that depend on 

the sheltering of the wind by the overstory.
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Figure 4—BehavePlus run showing the effect of WAF of 0.2 (sheltered fuel), 0.4 (unsheltered fuel), and 0.7 (a high value for 

comparison) on calculated surface rate of spread and flame length.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 7

The BehavePlus run in figure 5 illustrates the sheltering effect of overstory and the resulting fire behavior. (A 

modeler should realize that a change in overstory would 

also affect fire behavior in other ways, such as a change 

in fine dead fuel moisture due to a change in exposure to 

the sun.) The calculated flame length and scorch height 

are based on the same fuel model and fuel moisture. The 

effects of three values of canopy cover are shown in the 

resulting WAFs. In this example, fuels are sheltered from 

the wind for 40 and 80 percent canopy cover, but are 

unsheltered at 20 percent cover. At 20 percent cover, the 

models show 80 percent mortality compared to only 7 

percent mortality at 80 percent cover.

Figure 5—Canopy cover affects calculated WAF, which affects flame length and scorch height, and, 

ultimately, probability of mortality.
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Equations to Calculate WAF in 

BehavePlus ______________________________

This section of the report is a documentation of the 

mathematical models used to do the calculations in 

BehavePlus. Complete model development can be found 

in the referenced publications.

WAF models in BehavePlus are based on the work of 

Albini and Baughman (1979) and Baughman and Albini 

(1980), using some assumptions made by Finney (1998). 

There are separate models for wind above surface fuel 

that is unsheltered from the wind by overstory and for 

wind that is sheltered by a forest canopy. Both models 

are based on a logarithmic wind profile. The unsheltered 

WAF is based on an average wind speed from the top of 

the fuel bed to a height of twice the fuel bed depth. The 

sheltered WAF is based on the assumption that the wind 

speed is approximately constant with height below the 

top of a uniform forest canopy. Sheltered WAF is based 

on the fraction of crown space occupied by tree crowns. 

The unsheltered WAF model is used if crown fill portion 

is less than 5 percent. Midflame wind speed is 20-ft wind 

multiplied by the WAF.

Following are the equations used in BehavePlus. Background and explanation are in following sections.

The unsheltered WAF is

WWWWWW = 1.83

ln 20 + 0.36HH

0.13HH

[1]

where

H = fuel bed depth, ft

The sheltered WAF is

WWWWWW = 0.555

ffff ln 20 + 0.36HH

0.13HH

[2]

f = F ⋅ CR = fraction of the volume under the 

canopy top that is filled with tree crowns 

(crown fill portion)

F = CC/3 = fraction of the canopy layer filled 

with tree crowns

CC = canopy cover, fraction

CR = crown ratio, fraction

The sheltered WAF model is used if crown fill portion 

(f) is greater than 5 percent. 

Log Wind Profile

The WAF models are based on the vertical distribution 

of wind speed. Albini and Baughman (1979) determined 

the wind speed above a vegetative cover using the logarithmic wind profile in the following form (Sutton 1953; 

Albini and Baughman 1979: p. 1).

UU! = UU∗

KK ln

zz − DD!

zz!

[3]

where

U

_

 = average wind speed at height z

U* = friction velocity

K = 0.4 (the von Kármán constant)

z = height above ground

D0 = zero-plane displacement

z0 = roughness length

Values for D0 and z0 were determined to be related to 

the vegetation height H as D0 =0.64H and z0 = 0.13H. The 

wind speed at a height x above the top of the vegetation 

is then

UU!!! = UU∗

KK ln

HH + xx − 0.64HH

0.13HH

[4]

A universal dimensionless wind profile can be applied 

to any vegetation, from short grass to tall trees. A plot 

of relative height [(H + x)/H] versus relative wind speed 

(UH+X/UH) is shown in figure 6. The dashed line represents an assumed extension of the wind profile into the 

vegetation cover (as described in the “Sheltered WAF” 

section) (Baughman and Albini 1980).

The relationship between the wind speed at 20 ft above 

the vegetation and the wind at the top of the vegetation 

(Albini and Baughman 1979: p. 3) is

UU!

UU!"!!

= 1

ln 20 + 0.36HH

0.13HH

[5]

where

UH = wind speed at the top of the vegetation

U20+H = wind speed at 20 ft above the top of the 

vegetation

H = vegetation height, ft
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 9

Unsheltered WAF

The midflame wind speed over surface fuel that is not 

sheltered from the wind by overstory is determined by 

averaging the wind over a height range over the fuel. The 

average wind speed over the height range from H to 

H + HF (Albini and Baughman 1979) is

UU

UU!"!!

= 1 + 0.36HH HH!

ln 20 + 0.36HH

0.13HH

ln

HH! HH + 0.36

0.13 − 1 [6]

where

U

=

 = average wind speed from to H and HF

H = surface fuel bed depth, ft

HF = flame extent above the fuel (from the top of 

the fuel bed to the top of the flame), ft

Albini and Baughman (1979) gave a plot of various 

WAFs for flame extensions above the fuel bed as the 

ratio HF/H.

Baughman and Albini (1980) used the relationships 

to develop WAF tables for the 13 standard fuel models, 

which were described by Anderson (1982). Given that 

“midflame height” is not well defined, the authors 

didn’t define specific criteria, but in most cases used 

HF = H to develop their WAF table. BehavePlus 

follows the assumption made by Finney (1998) in 

developing FARSITE, that the flame extension above 

the fuel bed depth is equal to the fuel bed depth. The 

curve for HF/H = 1 in figure 7 is the relationship in 

BehavePlus (see figure 19).

Figure 6—Wind profile (from Baughman and Albini 1980)

Figure 7—Unsheltered WAF is the 

average wind speed acting on a flame 

extending above a uniform surface fuel 

layer (Albini and Baughman 1979). 

BehavePlus uses the curve labeled 1, 

indicating the flame extension above 

the fuel is equal to the fuel bed depth.
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Figure 8—Unsheltered WAF is the average wind speed from the top of the fuel bed to twice the 

fuel bed depth divided by the wind speed at 20 ft above the top of fuel bed.

For HF = H in equation [6], the unsheltered WAF is 

calculated as

WWWWWW = UU

UU!"!!

= 1.36

ln 20 + 0.36HH

0.13HH

ln

1.36

0.13 − 1 [7]

which can be simplified as

WWWWWW = 1.83

ln 20 + 0.36HH

0.13HH

[8]

Figure 8 shows the relationship of fuel bed depth, flame 

extent above the fuel, flame height, and the height range 

over which the wind is averaged to find unsheltered WAF.

Sheltered WAF

The sheltered WAF adjusts the wind speed at 20 ft 

above the top of the overstory vegetation to the wind that 

influences a fire burning through the surface fuel under 

a forest canopy.

Albini and Baughman (1979) modeled the variation of 

wind speed with height for air flow through and under a 

forest canopy based on the assumption that below some 

height, near but below the top of the uniform forest 

canopy, the wind speed is approximately constant with 

height. Foliage within the live crowns provides a bulk 

drag force that resists the airflow. The model is based 

on the portion of volume under the canopy that is filled 

with tree crowns.

The fraction of the canopy layer filled with tree crowns 

is F. The canopy layer is measured from the top of the 

canopy to the bottom of the live crowns. Albini and 

Baughman (1979) stated that F is approximated by the 

product of crown cover and a fraction accounting for the 

tapering of crowns that results in additional void volume 

higher in the canopy. The authors did not provide a means 

of calculating F, but used 0.4 for dense stands and 0.1 

for open stands.

BehavePlus uses the assumption made by Finney (1998) 

for FARSITE that the tree crown is conical shaped, the 

volume of which is one-third that of a cylinder with the 

same base. BehavePlus does not include the additional 

factor of π/4 used in FARSITE to account for the additional void resulting from a square horizontal packing of 

circular crowns. The fact that an area cannot be filled 100 

percent by circles is a minor model limitation compared 

to the actual variation in overstory.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 11

The portion of the canopy layer filled with tree crowns 

(F) is calculated as

F = CC/3 [9]

where

CC = fraction canopy cover, horizontal coverage

The portion of volume under the canopy top that is 

filled with tree crowns (f) includes consideration of crown 

ratio, as described by Albini and Baughman (1979). (In 

FARSITE, f = F and CR = 1.0.)

ff = CCCC⋅ FF [10]

where

CR = crown ratio

Table 1 gives values from Albini and Baughman (1979) 

of tree characterization for sheltering of surface fuels 

from the wind. For comparison, tables 2 and 3 show the 

portion of the canopy layer filled with tree crowns (F) and 

the crown fill portion (f) values calculated in BehavePlus 

using equations [9] and [10]. The ranges of values are 

similar.

The ratio of the wind under the canopy UC to the wind 

speed at the top of the canopy UH (Albini and Baughman 

1979: p. 8) is

UU!

UU!

= 0.555

ffff

[11]

Table 2—Portion of the canopy layer filled 

with tree crowns (F) calculated 

from percent canopy cover (CC), 

equation [9].

CC F

20 0.07

40 0.13

60 0.20

80 0.27

100 0.33

Table 3—Crown fill portion (f) values calculated from crown 

ratio (CR) and percent canopy cover (CC), equations [9] and [10].

f

CC

20 40 60 80 100

CR

0.2 0.01 0.03 0.04 0.05 0.07

0.4 0.03 0.05 0.08 0.11 0.13

0.6 0.04 0.08 0.12 0.16 0.20

0.8 0.05 0.11 0.16 0.21 0.27

1.0 0.07 0.13 0.20 0.27 0.33

Table 1—Values from Albini and Baughman (1979) for sheltered WAF.

Shade-tolerant trees Shade-intolerant trees

Young Mature Young Mature

Dense Open Dense Open Dense Open Dense Open

F, fraction of the canopy layer

 occupied by tree crowns 0.4 0.1 0.4 0.1 0.4 0.1 0.4 0.1

CR, crown ratio, fraction 0.8 0.9 0.6 0.7 0.4 0.7 0.2 0.5

f = F x CR, volume filling 0.32 0.09 0.24 0.07 0.16 0.07 0.08 0.05

fraction (crown fill portion)
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

The ratio of wind under the canopy to wind at 20 ft 

above the canopy top (Albini and Baughman 1979: p. 9) 

is then

WWWWWW = UU!

UU!"!!

= 0.555

ffff ln 20 + 0.36HH

0.13HH

[12]

Baughman and Albini (1980) plotted this equation 

as “ratio of wind speed under canopy (midflame) to 

wind speed at 20 ft above canopy top” versus “height 

of uniform canopy top, ft” for three values they labeled 

“fraction of canopy layer filled with tree crowns.” The 

plot was based on the f value while, in error, the curves 

were labeled with a description of F. The curves should 

be labeled as “portion of volume under the canopy top 

that is filled with tree crowns.” A plot of equation [12] 

with the corrected curve label is given in figure 9.

An alternate plot of equation [12] shows WAF for a 

range of f values for several canopy height values 

(figure 10). The canopy fill portion has a greater influence on the results than does canopy height.

Sheltered WAF is a function of canopy height (CH), 

canopy cover (CC), and crown ratio (CR). The plot in 

Figure 9—Ratio of wind speed within (and 

below) forest canopy to wind speed 20 ft 

above canopy top (adapted from Albini 

and Baughman 1979).

Figure 10—Sheltered WAF for a range of 

crown fill portion and several values of CH.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 13

figure 11 shows the relationship for CH = 100 ft for ranges 

of CC and CR. The resulting high WAF values for low 

canopy cover, especially for low crown ratio, show that 

it is not appropriate to apply the sheltered model for the 

full range of values. For an extreme of 1 percent CC and 

0.2 CR, the calculated WAF is 1.66, meaning that midflame wind is greater than 20-ft wind, which is obviously 

inappropriate. The question is when to use the sheltered 

versus unsheltered WAF model.

Sheltered Versus Unsheltered

Albini and Baughman (1979) presented two WAF 

models, one for sheltered fuel and one for unsheltered 

fuel. The authors did not give specific rules for choosing 

which model to use, but Baughman and Albini (1980) 

gave guidance (see figures 17 and 25), which works for 

tables based on human judgment. However, an explicit 

rule is needed for computer implementation.

Given a description of the overstory, the question is 

whether the fuel is sheltered or unsheltered from the 

wind. Keep in mind that the models are based on the 

assumption of a uniform canopy on flat ground. A few 

trees would have little effect and the surface fuel would 

be considered unsheltered from the wind (figure 12).

For both sheltered and unsheltered conditions, the 20-ft 

wind is multiplied by the WAF to give midflame wind. 

Recall that “20-ft wind” is the free wind at 20 ft above 

the top of the vegetation. That is 20 ft above the top of 

surface fuel unsheltered from the wind and 20 ft above 

the tree tops for surface fuels that are sheltered from the 

wind.

Figure 11—Sheltered WAF for a range of CR and CC, CH is 100 ft. Very low 

CC values give unreasonably high WAF values. It is therefore not appropriate 

to use the sheltered model for the entire range of canopy closure values.

Figure 12—Classification of surface fuel as sheltered or unsheltered depends on the overstory at the site. At some point, the 

trees have little influence and the fuels are considered unsheltered from the wind.
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Albini and Baughman (1979) gave an example table 

for crown fill portion (f). They described the graph of 

sheltered WAF (see figure 9) as “plotted for the typical 

and extreme values of f ”—5, 15, and 32 percent. Given 

that the authors considered 5 percent an extreme value 

for f, that value was selected as the cutoff point for the 

model in BehavePlus. BehavePlus uses a criterion of 5 

percent or greater crown fill portion to indicate sheltered 

conditions. Crown fill portion is determined from both 

canopy cover and crown ratio.

Figure 13 shows the canopy cover and crown ratio values 

that result in a crown fill portion (f) greater than or less 

than 5 percent, indicating whether the fuel is considered 

sheltered or unsheltered. (The FARSITE criterion for 

sheltered conditions is CC>0.)

Definition of Midflame Height______

The definition of midflame height has a substantial 

influence on modeling results. It is not reasonable to 

base midflame height directly on the height of the flames, 

since the objective is to calculate fire behavior from a 

description of the pre-fire environment, including wind 

speed. As described above, a consistent method of defining midflame wind to determine a WAF is the average 

from the top of the fuel bed to twice the fuel bed depth 

for unsheltered fuel. Because wind is assumed to be 

constant with height under the canopy, the definition of 

midflame height is not critical for sheltered conditions.

The definition of midflame wind as the average wind 

speed over a height range determined by the fuel bed 

depth, however, cannot be applied to wind measurements, 

which are taken at a specific height.

In table 4, we compare the calculation of WAF for 

unsheltered fuel based on the assumption that midflame 

wind speed is defined as the average wind from the top 

of the fuel bed to twice the height of the fuel bed (as in 

BehavePlus and FARSITE) to other definitions of midflame heights. We calculate a WAF for a height twice the 

fuel bed height, and for heights of 4 ft and 5.5 ft, which 

might be hand-held (eye level) height for some people. 

We also look at a 2-m height, which is available on the 

Oklahoma Mesonet (McPherson and others 2007).

To illustrate the effect of various definitions of midflame height on modeled WAF and fire behavior, consider 

a fire in fuel model GS2 (moderate load, dry climate 

grass-shrub) that has a fuel bed depth of 1.5 ft. Look at 

the unsheltered WAF for various heights, the resulting 

midflame wind speed for a 20-ft wind of 15 mi/h, and 

the calculated rate of spread and flame length. For midflame height defined as the height above the ground, use 

equation [4] for the ratio of (Uz = Ux+H) to U20+H to find 

WAF.

In all cases, we use the same fuel model (GS2), fuel 

moisture (dead 5 percent and live 75 percent), slope (zero), 

and 20-ft wind (15 mi/h). The average wind from 1.5 to 

3 ft above the ground is the definition of midflame wind 

used in BehavePlus (equation [6]), resulting in a WAF of 

0.39, midflame wind speed of 5.9 mi/h, rate of spread of 

31ch/h, and flame length of 5.8 ft. Midflame wind at all 

other selected heights is higher. At 5.5 ft, WAF is 0.68, 

midflame wind is 10.2 mi/h, rate of spread is 66 ch/h, 

and flame length is 8.3 ft.

Figure 14 illustrates values in table 4, showing the 

log wind profile with the wind speeds at various heights 

compared to the average wind for the height range used 

in the WAF model.

Figure 13—In BehavePlus, the unsheltered WAF model is 

used for crown fill less than 5 percent. 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 15

Table 4—WAF from the definition of midflame wind as the average over a height range based on Albini and Baughman (1979) 

and used in BehavePlus and FARSITE as compared to other midflame heights based on the same log wind profile, 

equation [4].

“Midflame” height Height above Midflame wind = 

above ground the fuel, Calculated WAF WAF x (20-ft Rate of

(fuel bed depth = 1.5 ft) H = 1.5 ft (eq. [4]) wind = 15 mi/h) spreada

, ch/h Flame lengtha, ft

1.5 to 3 ft 0 to 1.5 0.39 5.9 31 5.8

(average from top of fuel 

to twice the fuel bed depth)

3 ft 1.5 0.50 7.5 43 6.8

(twice the fuel bed depth)

4 ft 2.5 0.59 8.8 54 7.5 

(shorter person hand-held)

5.5 ft 4.0 0.68 10.2 66 8.3

(taller person hand-held)

6.56 ft 5.06 0.72 10.8 72 8.6

(2 m)

a

Fuel model GS2, dead moisture 5 percent, live moisture 75 percent, slope 0 percent

Figure 14—Illustration of height and wind speed values in table 4. For a fuel bed 

depth of 1.5 ft, WAF is based on an average wind speed from 1.5 to 3.0 ft.
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

WAF Implementation in 

BehavePlus ____________________

The BehavePlus fire modeling system integrates many 

mathematical models for fire behavior, fire effects, and the 

fire environment, including models for WAF (Andrews 

2007). In calculating surface fire behavior, the BehavePlus 

system’s SURFACE module offers three options:

• direct entry of midflame wind speed,

• entry of WAF and 20-ft (or 10-m) wind speed, and

• calculation of WAF for entered 20-ft (or 10-m) wind 

speed.

Each of these options is applied with examples in the 

following sections. All are based on 20-ft wind, which is 

the basis of the WAF models. When a BehavePlus user 

enters a 10-m wind speed, the value is divided by 1.15 

to get 20-ft wind (Turner and Lawson 1978).

Most of the following examples are based on the effect 

of WAF on calculated surface fire rate of spread and flame 

length. Those outputs from the SURFACE module can be 

used as inputs to other modules (Andrews 2009; Heinsch 

and Andrews 2010). WAF therefore affects scorch height 

(SCORCH), tree mortality (MORTALITY), transition to 

crown fire (CROWN), spotting distance from a winddriven surface fire (SPOT), safety zone size (SAFETY), 

size of a point source fire (SIZE), and fire containment 

(CONTAIN).

Midflame Wind Speed

It is common for a user to specify values for midflame 

wind speed, skipping the step of adjusting 20-ft wind to 

midflame height. This option is useful for several applications, including the following:

• Calculation of fire behavior from wind speed measured at a site or from a spot weather forecast of eye 

level wind.

• Exercising the fire spread model to examine sensitivity to input values: wind, slope, fuel, and moisture.

• Comparison of fuel models for a range of wind 

speeds, as done by Scott and Burgan (2005).

• Examination of fuel treatment options for constant 

midflame wind speed and fuel moisture.

As is the case with all aspects of fire behavior prediction 

and use of BehavePlus, the user is responsible for making 

good decisions. Recognition of the role of WAF cannot 

be ignored, even when it is neither input nor calculated.

Comparison of fire behavior in two fuel models 

for a range of midflame wind speeds should include 

consideration of sheltering effects on the wind. Some 

fuel models such as fuel model 9, long needle litter, 

are generally sheltered, while others such as fuel model 

SH5, high load, dry climate shrub (chaparral), are often 

unsheltered. In figure 15, we compare the calculated 

rate of spread for the two fuel models for the same 

moisture and slope values for a range of midflame wind 

speed from 0 to 20 mi/h. The plot does compare the 

role of the fuel models in the mathematical fire model. 

What might seem like a reasonable midflame wind 

speed might imply an unrealistic 20-ft wind speed. 

Midflame wind of 10 mi/h for unsheltered chaparral 

would be equivalent to a realistic 20 mi/h 20-ft wind 

(WAF = 0.5). A 10 mi/h midflame wind on a litter fire 

under a closed canopy would, however, be equivalent 

to an unreasonable 100 mi/h at 20 ft above a closed 

canopy (WAF = 0.1). Just because the program lets 

a user enter a value, it doesn’t mean that the value is 

reasonable or appropriate for fire behavior calculation.

When midflame wind speed is used in the calculations, it 

is used as specified and is not adjusted according to flame 

height. Figure 16 shows calculated flame lengths for a 

midflame wind of 6 mi/h for fuel models SH3 (moderate 

load, humid climate shrub) and SH6 (low load, humid 

climate shrub). The calculated flame length for fuel 

model SH6 is 11 ft at 5 percent dead moisture and 2 ft 

at 28 percent dead moisture. The same midflame wind 

speed is used in all cases. Wind speed is not adjusted according to the calculated flame length. Midflame wind is 

the wind that affects surface fire spread. It is customary 

to think of eye level wind as midflame wind.

Input WAF

In BehavePlus, if wind is specified at the 20 ft or 10 m 

height, the WAF can be entered directly. This allows for 

the use of any WAF values, such as those developed by 

Norum (1983). Direct entry of WAF permits judgment 

and consideration of factors not included in the calculations. The original application of WAF was through use 

of tables and guidelines and relied on expert opinion. 

That method remains a valid approach when possible.

While calculations are based on a description of the 

vegetation at the site under the assumption of a uniform 

canopy cover on flat ground, a person can consider the 

effect of terrain and non-uniformity on sheltering from 

the wind. This is appropriate if the objective is to predict 

the behavior of an ongoing wildfire or to model potential 

fire behavior for a prescribed fire. While the point modeling in BehavePlus leaves room for the role of expert 

opinion, geospatial systems such as FARSITE must rely 

on calculated WAF.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 17

Figure 15—Two fuel models are compared for a range of midflame wind speed. High midflame 

winds are reasonable for unsheltered shrub fuels (sh5) but not for sheltered litter fuels (9).

Figure 16—Comparison of flame length for two fuel models for a range of dead fuel moisture 

values for constant live moisture, slope, and midflame wind speed. The midflame value is as 

specified and is not adjusted according to the height of the flames.
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

WAF tables and guidelines are included in the 

BehavePlus Help system (figure 17). The table gives the 

unsheltered WAF for the 53 standard fuel models. The 

diagram is a visual display of the sheltering conditions 

given on the table. Even if the canopy is continuous for 

an area, surface fuel on a ridge top might be unsheltered 

from the wind. The diagram addresses only adjustment 

of 20-ft wind to midflame height based on sheltering 

conditions. It does not relate to the effect of terrain on 

20-ft wind.

Figure 4 showed the effect of WAF of 0.2, 0.4, and 

0.7 on surface fire rate of spread in fuel model 2. Note 

on the tables that WAF of 0.2 represents fully sheltered, 

open stands, and the unsheltered WAF for fuel model 2 

(with a fuel bed depth of 1 ft) is 0.4. Note also that none 

of the WAF on the table is greater than 0.5.

Figure 17—From the BehavePlus Wind Adjustment Factor help window. The diagram illustrates 

sheltering conditions described on the table. The unsheltered WAF is based on fuel bed depth.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 19

Calculate WAF

BehavePlus offers the option of calculating WAF using the two mathematical models—one for unsheltered 

surface fuel and the other for surface fuel that is sheltered 

from the wind by overstory. Whether or not the fuel is 

sheltered depends on overstory characteristics. The option of calculating WAF is appropriate if the uniformity 

assumption is valid and if there is need for repeatability 

based on a model without adjustment for human judgment. In addition, the calculation option can be used 

to understand the WAF models, as we illustrate in this 

section.

In the BehavePlus run in figure 18, only WAF was 

calculated (no fire behavior or fire effects calculations). 

All 53 standard fuel models were selected and canopy 

cover was set to zero. The result is unsheltered WAF 

based on only fuel bed depth. If The BehavePlus output 

was presented to one decimal place, values would agree 

with those in the table in figure 17. Note that the unsheltered WAF for fuel model 2 is 0.36, which corresponds 

to 0.4 on the table in figure 17. The WAF for fuel model 

4 is 0.55 in figure 18 and 0.5 in figure 17 because the 

calculated WAF to three decimal places is 0.547, which 

rounds to 0.5. Similarly, WAF is 0.547 for fuel models 

SH5 and SH7 and is 0.447 for SB4.

Figure 18—Calculated unsheltered WAF to two decimal places for the 53 standard fuel models.
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

The BehavePlus run in figure 19 uses the option of the 

surface fuel description entered as fuel parameters rather 

than as fuel model. Because BehavePlus is designed to 

request only relevant input, when only WAF is calculated, 

the only fuel parameter on the worksheet is fuel bed 

depth. For canopy cover of zero, the unsheltered WAF 

is calculated and plotted for a range of fuel bed depths.

The BehavePlus run in figure 20 shows calculated 

WAF for fuel model 2 for canopy cover of 40 percent, 

canopy height of 50 ft, and crown ratio values from 0.1 

to 0.9. BehavePlus provides intermediate values to allow 

examination and understanding of the calculations. The 

table shows that for crown ratio 0.1 and 0.3, the crown 

fill portion is less than 5 percent, so the unsheltered WAF 

is calculated. For crown ratios of 0.5, 0.7, and 0.9, the 

sheltered model is used. The plot shows the shift from 

unsheltered to sheltered conditions.

Calculated WAF is based on surface fuel bed depth if 

there is no or sparse overstory and on a description of 

the overstory if it exists at the site. There is, therefore, a 

discontinuity in the results. The switch between sheltered 

and unsheltered model in BehavePlus occurs at crown 

fill portion 5 percent.

Recall that the height of the 20-ft wind is 20 ft above 

the top of the surface fuel for unsheltered fuel or 20 ft 

above the tree tops (figure 2). The step change is unsettling, but it is a reflection of the modeling foundation 

and the switch from one model to the other.

Figure 19—Unsheltered WAF is a function of fuel bed depth.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 21

Figure 20—WAF for fuel model 2 for CH of 50 ft, CC of 40 percent, and a range of CR values. 

Results show the switch between the unsheltered and sheltered WAF models that occurs when 

crown fill is greater than 5 percent.
22 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

A person using BehavePlus to calculate WAF should 

look at ranges of values to see when a change in conditions causes the change from sheltered to unsheltered. It 

is best not to do just a single calculation.

BehavePlus uses a crown fill portion of 5 percent 

as the indicator of sheltered versus unsheltered fuel. 

Calculation of crown fill portion is based on canopy 

cover and crown ratio. Figure 21 is a plot of crown 

fill portion for canopy cover from 0 to 100 percent for 

several crown ratio values. A dashed line was added to 

the BehavePlus plot to indicate 5 percent crown fill. 

Surface fuel is considered unsheltered from the wind 

for less than 5 percent crown fill. Note that for crown 

ratio of 0.1, even for canopy cover of 100 percent, the 

surface fuel is unsheltered from the wind. For crown 

ratio of 0.9, fuel is unsheltered for canopy cover less 

than about 17 percent.

Canopy cover is defined as horizontal coverage. Jennings 

and others (1999) described the difference between canopy 

closure and canopy cover. Crown ratio is crown length 

divided by tree height. Figure 22(a) shows 25 percent 

canopy cover, and figure 22(b) shows four crown ratios. 

Each stylized tree in this example has the same base, 

which would correspond to the same canopy cover.

In figure 23, we look at the effect of canopy cover of 

60, 80, and 100 percent, all of which lead to the sheltered 

case, with crown fill portion greater than 5 percent. 

Fuel model plays no role in the calculation of sheltered 

WAF. The plot shows the sheltered WAF for crown 

ratios of 0.3 to 1.0.

Figure 21—Crown fill portion is a function of canopy cover and crown ratio. Surface fuel is 

considered unsheltered for crown fill portion less than 5 percent.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 23

Figure 22—Crown fill portion is a function 

of canopy cover and crown ratio. (a) CC 

is horizontal coverage (25 percent in this 

example). (b) CR is crown length divided 

by tree height.

Figure 23—Sheltered WAF is 

calculated for a range of crown 

ratios for three values of canopy 

cover.
24 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Other Implementations of WAF ____

WAF modeling in BehavePlus is used as the focus for 

describing and comparing WAF in other applications. Not 

all fire modeling systems that include surface fire behavior 

are included here. FOFEM, for example, requests flame 

length as input, so it does not request a wind speed to 

use in calculations (Reinhardt and others 1997). FCCS 

(Ottmar and others 2007), NEXUS (Scott 1998), and 

the nomographs (Rothermel 1983; Scott 2007) request 

midflame wind speed.

WAF Table Comparison __________

A summary of the differences in WAF table values is 

given, followed by a description of each application in 

the following sections.

Table 5 shows the unsheltered WAFs for the 13 standard 

fire behavior fuel models that were given on various tables 

compared to those in BehavePlus. The BehavePlus table 

and Scott (2007) give WAF values for the 53 standard fuel 

models, while the other sources include only the original 

13 fuel models. Inconsistencies are due to differences in 

calculations and, in some cases, only to rounding.

The sheltered and partially sheltered WAF tables are the 

same in BehavePlus, Rothermel (1983), and the Fireline 

Handbook (NWCG 2006). These values are compared 

in table 6 to those in the original table in Baughman and 

Albini (1980). Scott (2007) based the sheltered WAF on 

canopy cover (table 7).

Table 5—Comparison of unsheltered WAF for the 13 standard fire behavior fuel models in several applications.

 BehavePlus Baughman and Rothermel Fireline Handbook Scott

Fuel model table Albini (1980) (1983) (NWCG 2006) (2007)

1 0.4 0.36 0.4 0.4 0.36

2 0.4 0.36 0.4 0.4 0.36

3 0.4 0.44 0.4 0.4 0.44

4 0.5 0.55 0.6 0.5 0.55

5 0.4 0.42 0.4 0.4 0.42

6 0.4 0.44 0.4 0.4 0.44

7 0.4 0.44 0.4 0.4 0.44

8 0.3 0.36 0.4 0.4 0.28

9 0.3 0.36 0.4 0.4 0.28

10 0.4 0.36 0.4 0.4 0.36

11 0.4 0.36 0.4 0.4 0.36

12 0.4 0.43 0.4 0.4 0.43

13 0.5 0.46 0.5 0.5 0.46

Table 6—Comparison of sheltered WAF in several applications.

 BehavePlus,

Rothermel (1983), 

Baughman and Fireline Handbook

Shelter from the wind Albini (1980) (NWCG 2006)

Partially sheltered 0.25 0.3

Fully sheltered, sparse stands, shade-intolerant species 0.17 0.2

Fully sheltered, sparse stands, shade-tolerant species 0.14 0.2

Fully sheltered, dense stands, shade-intolerant species 0.12 0.1

Fully sheltered, dense stands, shade-tolerant species 0.08 0.1

Table 7—Sheltered WAF based on canopy 

cover from Scott (2007).

Canopy cover, percent WAF

	 5 <CC≤ 10	 0.30

	 10<CC≤ 15	 0.25

	 15<CC≤ 30	 0.20

	 30<CC≤ 50	 0.15

CC>50 0.10
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 25

BehavePlus Help System Tables

The unsheltered WAF table in the BehavePlus Help 

system is generated by the calculations implemented in 

the program. WAF values are rounded to tenths for the 53 

fuel models. The fuel bed depth for each WAF category 

allows application for custom fuel models (figure 24). 

(Prior to BehavePlus version 5.0.3, fuel model SH4 was 

incorrectly listed on the table with WAF = 0.4.) The 

sheltered values and guidelines are the same as those in 

Rothermel (1983).

Figure 24—WAF tables from BehavePlus help system.
26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Baughman and Albini (1980) Tables

The original WAF table from Baughman and Albini 

(1980) is shown in figure 25. The authors used the models 

of Albini and Baughman (1979) for average wind speed 

over a height range from H to H+HF (equation [6]) to find 

an unsheltered WAF for each of the 13 fuel models. The 

authors did not specifically describe how they reached 

the 13 values, but stated the following:

Each midflame windspeed obtained by use of table 

3 [Wind reduction table] implies a midflame height. 

For example, consider a fuel model 3 and the corresponding reduction factor of 0.44. From table 4 

[Stylized fuel models], fuel model 3 is found to be 

2.5 ft high grass. These values of 0.44 and 2.5 ft are 

used to enter figure 2 [figure 7 in this paper] where 

the ratio of the flame height to the fuel bed height is 

found to be about 1. Thus the flame height extends 

about 2.5 ft above the tall grass. The flame height of 

other fuel models can be found in a similar fashion.

In the above quote, the authors used the term “flame 

height” to mean the flame extension above the fuel bed 

(HF), rather than the distance from the ground to the top 

of the flame. Baughman and Albini (1980) used flame 

extension above the fuel bed equal to fuel bed depth 

(HF/H = 1, HF = H) for all but fuel models 8 and 9. For 

those fuel models, fuel bed depth is 0.2 ft and the WAF on 

their table is 0.36. Working backward, as they described, 

this translates to HF/H = 2.5 and HF = 0.5.

Table 8 is a comparison of WAF values on Baughman 

and Albini’s table (figure 25) to those calculated using

equation [6], as is done in BehavePlus (figure 18). There 

is a difference only for fuel models 8 and 9.

Figure 25—Original WAF table from Baughman and Albini (1980)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 27

Rothermel (1983) Tables

The original WAF tables were adjusted for presentation 

in the S-590 course, as presented in Rothermel (1983) 

(figure 26). The unsheltered WAF values were based 

on Albini and Baughman’s (1979) unsheltered model 

(equation [6]). As a means of estimating midflame height, 

Rothermel (1983) calculated flame height for zero wind 

and slope and 8 percent dead moisture and 100 percent 

live moisture for the 13 fuel models. This method was 

based on the author’s statement that “We have found 

that even though flame length varies considerably with 

changes in wind speed, flame height is not as variable.” 

He used the calculated flame heights as the flame extension above the fuel bed (HF) in equation [6].

Rothermel’s (1983) calculations and tables are compared to those from BehavePlus in table 9. Rothermel 

did calculations to hundredths and then used them as 

guidance for setting WAF values in tenths for the table. 

Some table values are not strict rounded values from the 

calculations. For example, the calculated WAF for fuel 

model 8 of 0.32 is given as 0.4 on the table.

However, the results are similar to those based on the 

assumption used in BehavePlus (and FARSITE) that 

flame height (from the ground) is twice the fuel bed 

depth (flame height = HF+H). When the WAF calculated 

in BehavePlus is rounded to tenths, there is a difference 

between the Rothermel (1983) table and the BehavePlus 

table for only fuel models 4, 8, and 9. Fuel models 8 and 

9 are generally sheltered, so the unsheltered WAF would 

rarely be used.

Table 8—Unsheltered WAF values for the 13 standard fire behavior 

fuel models in Baughman and Albini (1980) compared to 

those calculated in BehavePlus. There is a difference in 

the results for fuel models 8 and 9 (in bold).

 WAF from WAF Calculated

Fuel Fuel bed Baughman and by BehavePlus, 

model depth, ft Albini (1980) eq. [6], for HF/H = 1

1 1.0 0.36 0.36

2 1.0 0.36 0.36

3 2.5 0.44 0.44

4 6.0 0.55 0.55

5 2.0 0.42 0.42

6 2.5 0.44 0.44

7 2.5 0.44 0.44

8 0.2 0.36 0.28

9 0.2 0.36 0.28

10 1.0 0.36 0.36

11 1.0 0.36 0.36

12 2.3 0.43 0.43

13 3.0 0.46 0.46

Figure 26—WAF table from Rothermel (1983).
28 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

The sheltered and partially sheltered values are a simplification of those in Baughman and Albini (1980). A 

comparison of values was given in table 6.

Fireline Handbook (NWCG 2006) 

Tables

A table of WAFs is in the Fireline Handbook (figure27). 

The sheltered and partially sheltered WAF values are the 

same as those in BehavePlus and Rothermel (1983), but 

there are some differences in the unsheltered WAF values. 

WAF fuel model 4 matches the table in BehavePlus and 

differs from Rothermel (1983), while the WAF for fuel 

models 8 and 9 differ from the BehavePlus table and 

match those in Rothermel (1983).

Scott (2007) Tables

A table of unsheltered WAF values for the 53 fuel 

models to two decimal places is given in Scott (2007) 

(figure 28). Those values are based on the same calculation 

as that used in BehavePlus and match those in figure 18. 

The BehavePlus Help system table rounds WAF values 

to tenths and organizes it according to fuel bed depth.

Figure 27—WAF table in the Fireline Handbook 

(NWCG 2006).

Table 9—Basis of the Rothermel (1983) calculation of unsheltered WAF compared to that used by BehavePlus. There 

are differences in the results for fuel models 4, 8, and 9 (in bold).

Rothermel (1983) BehavePlus

 Fuel bed Flame WAF WAF calculation

Fuel depth (H), heighta

 (F), Flame height/ eq. [6] WAF WAF eq. [6] 

model ft ft fuel depth HF = F table table HF = H

1 1.0 0.99 0.99 0.36 0.4 0.4 0.362

2 1.0 1.6 1.6 0.42 0.4 0.4 0.362

3 2.5 2.7 1.1 0.45 0.4 0.4 0.440

4 6.0 4.9 0.80 0.55 0.6 0.5 0.547

5 2.0 0.92 0.46 0.35 0.4 0.4 0.418

6 2.5 1.4 0.56 0.37 0.4 0.4 0.440

7 2.5 1.4 0.56 0.38 0.4 0.4 0.440

8 0.2 0.37 1.8 0.32 0.4 0.3 0.275

9 0.2 0.90 4.5 0.44 0.4 0.3 0.275

10 1.0 1.6 1.6 0.41 0.4 0.4 0.362

11 1.0 1.1 1.1 0.37 0.4 0.4 0.362

12 2.3 2.7 1.2 0.45 0.4 0.4 0.431

13 3.0 3.7 1.2 0.48 0.5 0.5 0.469

a

Wind speed 0 mi/h, dead moisture 8 percent, live moisture 100 percent, slope 0 percent. Flame height is used as flame extension 

above the fuel bed in the WAF model.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 29

Figure 28—WAF values for the 53 standard fuel models in 

Scott (2007).
30 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

Scott (2007) also gives a table of sheltered WAF 

based on canopy cover (figure 29). Values are based 

on Albini and Baughman (1979), equation [2], with 

CR = 1. Figure 30 shows a comparison of the table and 

the calculation. BehavePlus also uses equation [2] to 

calculate sheltered WAF, but allows variable values 

for crown ratio. While Scott (2007) used the sheltered 

model when canopy cover was greater than 5 percent, 

BehavePlus uses the criteria of crown fill portion greater 

than 5 percent.

Figure 29—Sheltered WAF values based on 

canopy cover in Scott (2007).

Figure 30—Comparison of sheltered WAF in Scott (2007) table compared to calculation 

using equation [2] with CR of 1.0.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 31

WAF Calculation Comparison _____

Previous sections addressed WAF values available 

for use in tabular form. Some fire modeling systems 

include a calculation of WAF. Table 10 is a summary of 

methods of calculating WAF in Behave Plus, FARSITE, 

FlamMap, FSPro, FPA, FuelCalc (Scott, personal communication), Nomographs for the 13 fuel models, the 

BEHAVE TSTMDL program, and FVS-FFE (Reinhardt 

and Crookston 2003). The models for sheltered and 

unsheltered WAF and the criteria of which to use are 

given in the following sections.

FARSITE Fire Area Simulator

The FARSITE fire area simulator models fire growth 

under varying conditions of fuel, topography, wind, and 

fuel moisture. The basic FlamMap function involves 

independent fire behavior calculations for each point 

on the landscape under set environmental conditions. 

The modeling in FARSITE and FlamMap is used in 

other systems, including FSPro and FPA. All use the 

method of calculating WAF described by Finney (1998).

FARSITE and FlamMap calculate WAF for each pixel 

based on the fuel and vegetation values assigned to that 

pixel. While gridded wind data can account for the effect 

of terrain on 20-ft wind speed and direction, the adjustment to midflame wind speed does not consider the 

degree of sheltering of surface fuel due to penetration of 

the wind into the canopy based on position on the slope 

or on surrounding canopy cover.

The BehavePlus method of calculating unsheltered 

WAF was based on that developed for FARSITE (equation [6]) with the assumption that the flame extension 

above the fuel bed is equal to the fuel bed depth, or that 

the flame height is twice that of the fuel bed depth. WAF 

is based on the average wind speed from the top of the 

fuel bed to twice the fuel bed height. The wording in the 

FARSITE publication, however, might be misleading: 

“For nonforested areas midflame windspeed is reduced 

to a nominal height equal to twice the fuel bed depth.” 

FARSITE uses the average over the height range, not 

the wind at the set height. As shown by the example 

calculations in table 4, there is a noteworthy difference 

in WAF for the average from the top of the fuel bed to 

twice that height and WAF for the point at twice the fuel 

bed depth.

The basic calculation of sheltered WAF in FARSITE 

is the same as that in BehavePlus (equation [2]) but with 

CR = 1. While there was not an error in the FARSITE 

program, the publication contained an error (Finney 1998 

[revised 2004]: p. 18). The constant 0.3066 should have been 

0.555 for the equation in metric units. The printed copy of 

the revised paper in 2004 contained the same error, but the 

current online version of the publication is correct.

Table 10—Summary of methods of calculating WAF in various systems.

 Source Unsheltered Sheltered Sheltered condition

BehavePlus eq. [6], HF = H eq. [2], based on eq. [9] and [10],

CC, CR, and C if F>5%

FARSITE, eq. [6], HF = H eq. [2], based on CC>0

FlamMap, FSPro, CC and CH; CR=1

FPA

FuelCalc eq. [6], HF = H eq. [2], based on If WAF(sheltered)<

CC, CR, and CH WAF(unsheltered)

Nomographs 0.5 0.5 N/A

(Albini 1976)

TSTMDL program eq. [6], HF = flame N/A N/A

in the old lengtha

BEHAVE system

FVS-FFE 0.5 Interpolated from CC>5%

five points; based 

on CC 

a

Wind speed 0 mi/h, dead moisture 8 percent, live moisture 100 percent, slope 0 percent
32 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

In FARSITE, crown fill portion (fraction) is

ff = CCCC

100

ππ

12

where

CC = canopy cover, percent

This differs from the calculation in BehavePlus (equations [9] and [10]) in two ways. While both methods 

assume tree crowns are conical, FARSITE includes an 

additional factor of π/4 to account for gaps in a square 

horizontal packing of circular crowns. This addresses 

the issue of not being able to achieve 100 percent cover 

with circles. In addition, FARSITE calculates crown fill 

fraction based on canopy cover without the influence of 

crown ratio. FARSITE uses CR = 1 and f= F.

The significant effect of crown ratio on WAF in 

BehavePlus was shown in figures 20, 21, and 23. Figure31

compares the intermediate calculation of crown fill 

portion (f) as a function of canopy cover for BehavePlus 

and FARSITE.

The square versus circular base assumption has a minor 

impact. For crown ratio of one, the difference between 

BehavePlus and FARSITE crown fill portion (f) is due 

to the π/4 factor. The resulting WAF in FARSITE is 

1.13 times greater than that in BehavePlus, based on a 

different crown fill portion (f) in equation [12].

FARSITE uses the sheltered WAF model whenever 

crown cover is greater than zero. For a very low CC of 1 

percent, WAF is 0.74 (see figure 11), a value much higher 

than the WAF that would be calculated for unsheltered 

fuel. (Sheltered WAF should not be higher than unsheltered WAF.) Although it might seem rare that an area 

would be categorized with a canopy cover of 1 percent, 

FARSITE users often change zero canopy cover to a very 

low value to enable spotting, possibly not realizing that 

the change also affects WAF and calculated surface fire 

behavior.

While BehavePlus and FARSITE use the same basic 

WAF equations, the following is a summary of the differences in implementation:

• BehavePlus uses CC, CH, and CR to find the crown 

fill portion used in the sheltered WAF calculation. 

FARSITE uses the same equation with CR = 1.

• FARSITE uses the unsheltered WAF model only if 

canopy cover is zero. BehavePlus uses the unsheltered WAF model if crown fill portion is less than 

5 percent.

• FARSITE includes a π/4 factor that is not in 

BehavePlus to account for gaps in a square horizontal 

packing of circular crowns.

Figure 31—BehavePlus uses CR in the calculation of crown fill portion and, thus, of 

WAF. FARSITE always uses CR = 1.0 and includes an additional factor of π/4 to account

for gaps in a square horizontal packing of circular crowns.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 33

FuelCalc

FuelCalc is a software package that calculates initial 

fuel quantities, simulates a variety of fuel treatment scenarios, and then calculates potential fire behavior. Wind 

speed is entered at the 20-ft height. WAF is calculated 

based on surface fuel and overstory using the same basic 

Albini and Baughman (1979) models as implemented 

in BehavePlus. The difference is in the determination of 

whether to use the sheltered or unsheltered WAF model.

In order to avoid the step change in calculated WAF that 

occurs in BehavePlus with the change from the sheltered to 

the unsheltered model (see figure 20), FuelCalc calculates 

both sheltered and unsheltered and uses the minimum 

WAF value. The difference occurs for stands with less 

than 5 percent crown fill portion that are categorized 

as unsheltered in BehavePlus but might be modeled as 

sheltered in FuelCalc. Recall that the sheltered model is 

based on the assumption that wind is constant with height 

under the top of the canopy.

Figure 32 shows WAF values calculated by FuelCalc 

for canopy height of 100 ft; crown ratio of 0.1, 0.5, and 

0.9; and canopy cover from 0 to 100 percent. The dotted 

portion of the curves shows where results are different in 

BehavePlus. For this example, for CR = 0.1, BehavePlus 

would produce WAF = 0.39, the unsheltered value, even 

for canopy cover of 100 percent, which has a crown fill 

portion of 2.6 percent. FuelCalc assigns WAF = 0.23—the 

minimum of the sheltered value 0.39 and the unsheltered 

value 0.23.

Table 11 shows selected values from the plot with a 

comparison of BehavePlus and FuelCalc WAF values 

and associated flame length. When there is a difference, 

FuelCalc produces lower modeled fire behavior due to 

the sheltered assumption and the lower WAF (greater 

reduction).

Figure 32—WAF values calculated 

by FuelCalc. Canopy height is 100 ft. 

There are differences from BehavePlus 

(dotted portion of the curves) when 

crown fill portion is less than 5 percent 

and the sheltered WAF is less than the 

unsheltered WAF value.

Table 11—Comparison of FuelCalc and BehavePlus WAF values and associated flame length. When there is a difference (bold), FuelCalc results are lower.

FuelCalc BehavePlus

Canopy cover, Midflame wind, Flame Lengthb

, Midflame wind, Flame Lengthb,

percent WAFa mi/h ft WAF mi/h ft

5 0.39 7.8 12.1 0.39 7.8 12.1

10 0.33 6.6 10.9 0.39 7.8 12.1

20 0.23 4.6 8.8 0.39 7.8 12.1

30 0.19 3.8 7.8 0.39 7.8 12.1

40 0.17 3.4 7.3 0.39 7.8 12.1

50 0.15 3.0 6.8 0.15 3.0 6.8

60 0.14 2.8 6.6 0.14 2.8 6.6 

a

 Canopy height 100 ft, crown ratio 0.5

b

Fuel model GR5 (low load, humid climate grass), dead moisture 5 percent, live moisture 75 percent, 20-ft wind 20 mi/h, slope 0 percent. 
34 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

BEHAVE

The BEHAVE fire behavior prediction and fuel modeling system was the predecessor to the BehavePlus fire 

modeling system. WAF was not calculated in the fire 

behavior prediction portion of BEHAVE (Andrews 1986; 

Andrews and Chase 1989). The fuel modeling program 

TSTMDL calculated an unsheltered WAF for custom 

fuel models (Burgan and Rothermel 1984). WAF was 

calculated as described in (Rothermel 1983). If a standard fuel model was saved as a custom fuel model, the 

reported WAF could be different due to the adjustments 

that Rothermel made to the calculated values to produce 

his table. The method of calculating WAF in the TSTMDL program of BEHAVE is different from that used 

in BehavePlus.

FVS-FFE

The fire and fuels extension to the forest vegetation 

simulator (FVS-FFE) simulates fuel dynamics and potential fire behavior over time, in the context of stand development and management. It is used to better understand 

and display the consequences of alternative management 

actions. Wind is not a critical component of the modeling, given that a default value of 20 mi/h can be used 

for 20-ft wind speed. The 20-ft wind speed is converted 

to midflame wind speed by multiplying it by what the 

authors call “a correction factor” based on the canopy 

cover in the stand (figure 33). For canopy cover from 0 

to 5 percent, WAF is 0.5. Unsheltered WAF is therefore 

constant and does not depend on fuel model or surface 

fuel bed depth. A comparison of the FVS-FFE values to

equation [2], with CR = 1 is shown in figure 34.

Figure 33—WAF values used in FVSFFE are a function of CC. Results are 

interpolated between the six points as 

labeled on the plot. From Reinhardt and 

Crookston (2003). The authors used 

the term “Wind Correction Factor” for 

“Wind Adjustment Factor” and “Canopy 

Closure” for “Canopy Cover.”

Figure 34—WAF values used in FVSFFE compared to calculated values 

according to equation [2] with CR = 1.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 35

WAF for NFDRS _________________

To this point, we have discussed fire behavior modeling 

applications, which produce specific fire behavior values 

such as rate of spread and flame length. NFDRS, on the 

other hand, produces indices to indicate seasonal fire 

potential. Fire weather stations are located in the open, 

with no trees and no deep surface fuel. NFDRS is based 

on a worst-case assumption, so it doesn’t specifically 

consider sheltered conditions. The basis of the NFDRS 

calculations is the Rothermel fire spread model, which 

requires wind at the midflame height. Wind is measured 

and forecast for NFDRS at 20 ft above the ground.

The 1972 NFDRS (Deeming and others 1972) was 

replaced by a major update in 1978 (Deeming and others 

1977). A 1988 update offered additional options to the 

1978 system (Burgan 1988). The current system allows 

users to select either 1978 fuel models or 1988 fuel models. The WAF values for the 1978 and 1988 NFDRS and 

the retired National Fire Management Analysis System 

(NFMAS) (USDA Forest Service 1983), which used 

NFDRS fuel models, are given in table 12.

The 1972 NFDRS used WAF (which was called r) of 

0.5 for the nine fuel models. Schroeder and others (1972: 

p. 83-92) selected that value after in-depth analysis 

similar to that of Albini and Baughman (1979): “The 

ratio of the 6 meter windspeed to the mid-flame height 

windspeed gave values of r between 0.4 and 0.6 for all 

the fuel models over the range of windspeeds considered 

in the National Fire-Danger Rating System. Because of 

the general nature of the fuel models, the variability of 

r, we have chosen to set r equal to 0.5 until such time 

as fuel descriptions and meteorological instrumentation 

permit these fine distinctions to be properly utilized.”

The 1978 NFDRS assigned a WAF to each of the 20 

fuel models (Cohen and Deeming 1985). (The table of 

1978 fuel model descriptions in Burgan [1988] incorrectly 

listed the WAF for fuel model E as 0.5.) These WAF values were assigned prior to Albini and Baughman (1979) 

publishing the modeling basis for WAF calculations used 

for fire behavior prediction. The grouping for NFDRS was 

described as WAF = 0.6 for grass models, 0.5 for shrub 

and brush models, and 0.4 for timber models (Bradshaw 

and others 1983).

The 1988 NFDRS optional update uses a constant WAF 

if the fuel model has no live woody component or if the 

live woody vegetation is designated as evergreen. While 

not listed in Burgan (1988), the 88 NFDRS constant 

WAF values in table 12 were taken from the computer 

code. There is a change from the 78 NFDRS values for 

fuel models L, N, P, Q, and U. A variable WAF is used 

Table 12—WAF values assigned to each of the NFDRS for the 1978 and 1988 system 

and for the now retired NFMAS system. 

 78 88 88 NFDRS

Fuel Model NFDRS NFDRS Min – Maxa NFMAS

A Western grasses (annual) 0.6 0.6 0.36

B California chaparral 0.5 0.5 0.5 – 0.5 0.55

C Pine-grass savanna 0.4 0.4 0.3 – 0.5 0.25

D Southern rough 0.4 0.4 0.4 – 0.4 0.12

E Hardwood litter (winter) 0.4 0.4 0.3 – 0.5 0.25

F Intermediate brush 0.5 0.5 0.5 – 0.5 0.42

G Short needle (heavy dead) 0.4 0.4 0.3 – 0.3 0.12

H Short needle (normal dead) 0.4 0.4 0.3 – 0.3 0.17

I Heavy slash 0.5 0.5 0.46

J Intermediate slash 0.5 0.5 0.43

K Light slash 0.5 0.5 0.36

L Western grasses (perennial) 0.6 0.5 0.44

N Sawgrass 0.6 0.5 0.5 – 0.5 0.55

O High pocosin 0.5 0.5 0.5 – 0.5 0.55

P Southern pine plantation 0.4 0.3 0.3 – 0.3 0.12

Q Alaska black spruce 0.4 0.2 0.2 – 0.3 0.63

R Hardwood litter (summer) 0.4 0.4 0.3 – 0.5 0.08

S Tundra 0.6 0.6 0.6 – 0.6 0.45

T Sagebrush-grass 0.6 0.6 0.6 – 0.6 0.42

U Western pines 0.4 0.3 0.3 – 0.3 0.12 

a

Used to calculate WAF that changes with season when live woody fuel is designated as deciduous. 

Blank cells indicate fuel models without live woody fuel.
36 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

for fuel types for which live woody fuel is designated as 

deciduous. WAF is set to its maximum value during the 

winter. (Higher WAF causes less wind reduction due to 

leaf drop and higher wind speeds.) WAF decreases during 

the spring and increases during the fall as a function of 

the woody greenness factor. WAF is set to its minimum 

value during the summer when shrubs are fully green. 

(Lower WAF causes more wind reduction and lower wind 

speeds.) Note in table 12 that the maximum is equal to 

minimum WAF for some fuel models, so the variable 

WAF is applied only to fuel models C, E, Q, and R. Note 

also that designating live woody fuel as deciduous for 

1988 fuel models G and H effectively changes the WAF 

from 0.4 to 0.3.

In addition, for all 1988 fuel models, if more than 0.1 

inch of precipitation occurred either on the current or 

the previous day, the WAF is multiplied by 0.3. “This 

correction reduces the sensitivity of the 1988 NFDRS 

to wind until the dead fuels have had at least 1 day of 

drying” (Burgan 1988). This is merely an artificial way 

to influence index values and is not based on any actual 

effect on the wind.

NFMAS was a fire planning system that used fire danger fuel models. Calculations were aimed at estimating 

fire behavior for the planning application rather than to 

produce fire danger indices as in NFDRS. The NFMAS 

WAF values in table 12 are given here for historical 

documentation. NFMAS has been retired and replaced 

by FPA, which uses fire behavior fuel models and the 

modeling in FlamMap.

Discussion _____________________

Of the many aspects of wind that affect fire behavior, 

WAF addresses only one. It reduces the wind speed at 20 

ft above the vegetation to a value at midflame height as 

needed by Rothermel’s surface fire spread model. Other 

means are used to determine variation in 20-ft wind speed 

and direction across the landscape, as well as temporal 

changes from gusts to front passages.

Decisions that are supported by fire modeling can be 

influenced by the WAF. It is important that a user be 

aware of the technical foundation of the models with all 

of the associated assumptions and limitations.

Model Limitations

As is the case with all models, WAF models are based 

on simplifying assumptions. Following is from the section “Applicability of Results” in Albini and Baughman 

(1979):

The numerical results derived in this report are 

subject to the restrictions listed below. The reader is 

cautioned to be certain that the specified conditions 

prevail when using these results.

1. Flat terrain has been assumed throughout. If the 

terrain has substantial slope or roughness, the 

windfield will reflect this fact, and substantial 

deviations from these results may occur.

2. Adequate fetch to establish a uniform friction layer 

has been assumed. The definition of “adequate” 

fetch is the subject of current research (Shaw 

1977), so numerical limits cannot be stated at 

this time. But near forest edges, lakeshores, or 

transitions in surface vegetation cover, the results 

given here may not be accurate.

3. A “well-behaved” windfield is modeled by the 

relationships herein. If the windfield is not steady, 

but fluctuates significantly in speed, direction, 

or both, the friction layer will be continually in a 

transient state as it responds to the forces at play. 

During the periods of such variability the results 

given here may not be applicable.

4. Any interaction between the fire and the windfield 

that substantially influences the speed or direction 

of the wind should invalidate these results. We 

have dealt here with a windfield whose structure 

is dominated by the influence of the vegetation 

cover on the friction layer and any factor that 

disturbs this condition negates the validity of the 

results given here.

An additional limitation is the assumption of a neutral 

stable atmosphere near the ground. And convective slope 

winds have a different velocity profile than that shown in 

figure 1. Rothermel (1983) noted that “the type of wind 

driving the fire is very important and must be known to 

make proper interpretation of midflame windspeed.”

The model for WAF, based on average wind speed over 

a height range, is consistent with the wind tunnel data 

used in developing the spread model. Alternate definitions 

of midflame height at a specific height, using the same 

wind profile, give significantly different WAF results 

(see table 4). There will always be differences between 

the model results and a wind measurement taken at eye 

level because the observation is not an average over a 

height range.

It is well known that many influencing factors are not 

considered in the simple Albini and Baughman models. 

For example, Schroeder and Buck (1970) described the 

effects of vegetation on wind in their comprehensive 

“Fire Weather” publication. Among the influences they 

describe: “In forest stands that are open beneath the main 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 37

tree canopy, air speed increases with height above the 

surface to the middle of the trunk space, and then decreases 

again in the canopy zone.” The WAF model is based on the 

assumption that wind is constant under the canopy.

Limitations of the WAF models should be considered 

in conjunction with limitations of all associated models 

and with ability to describe the required input values. 

Overstory trees, for example, are not uniformly shaped 

cones. The same canopy cover can result from many 

narrow trees or fewer broad ones. The surface fire spread 

model is based on the assumption that the surface fuels are 

uniform and continuous. Geospatial fire modeling systems 

use data for which conditions are assumed constant for 

each pixel (possibly 30 m across). It is the responsibility 

of a model user to be aware of all of the many limitations 

in appropriate application of model results.

Model limitations should also be considered in the 

context of application. Limitations of individual models 

(such as WAF) might be minor in the context of a fire 

growth simulation that involves thousands of calculations and variables. On the other hand, the limitation of 

the modeling to determine the point at which fuels are 

sheltered from the wind by overstory can be significant 

in fuel treatment assessments. BehavePlus is designed to 

encourage users to generate tables and graphs to examine 

the effect of input selections on results rather than merely 

supplying values for each input variable and getting a 

result from a single calculation.

Modeling Applications

When Albini and Baughman developed WAF models, 

fire behavior analysts predicted wildfire behavior using 

tables, graphs, and nomographs (Albini 1976b; Rothermel 

1983). There was an appropriate reliance on experience 

and judgment in determining sheltering of fuels from the 

wind to select a WAF value. That application continues. A 

user of nomographs, BehavePlus, or other similar systems 

can and should use judgment to determine WAF based 

not only on conditions at a point, but on wind direction, 

nearby vegetation, location on the slope, and the ability 

of wind to penetrate the overstory.

While judgment is an important component of predicting wildfire behavior, fire planning applications, including fuel hazard assessment, may be better served by the 

consistency and repeatability of models. Calculation of 

WAF can have a large impact on results, especially when 

conditions are close to the point of transition between 

sheltered and unsheltered fuel. In addition there is the 

limitation of using the canopy description only at the 

fuel treatment site to calculate WAF. Two stands with 

the same overstory and understory and the same 20-ft 

wind speed in reality could have different potential fire 

behavior due to the influence of surrounding vegetation on the wind. The midflame wind speed in a small, 

thinned area surrounded by closed timber would differ 

from that in a large, thinned area adjacent to a meadow. 

A modeler should find the appropriate balance between 

calculations and judgment.

The use of fire models for wildland fire management 

has greatly expanded since the 1970s. Computer systems 

are ever more complex and sophisticated. A fire growth 

simulation system such as FARSITE necessarily relies on 

a calculated WAF. Judgment has no place in determining 

WAF for thousands of pixels. The original FARSITE used 

a wind field that changed in time but was constant over 

the landscape. As noted by Finney (1998) in describing 

his implementation of WAF calculations, “Variation 

would undoubtedly occur with stands of mixed species, 

multiple strata, different crown shapes, and tree arrangement. However, given the burden of additional input 

requirements, further refinement could not be justified 

without models of spatially dynamic wind fields that 

reflect the influence of topography, vegetation roughness, 

and neighboring vegetation structure.”

Models that adjust the 20-ft wind across the terrain are 

now available at the scale needed by FARSITE and other 

geospatial fire modeling systems (Wind Ninja and Wind 

Wizard) (Butler and others 2006; Forthofer and others 

2009). FARSITE then adjusts the spatially variable 20-ft 

wind speed to midflame height based on vegetation for 

each pixel and not adjacent pixels, wind direction, or 

terrain. The question of the burden of additional input 

requirements remains.

Model Needs

This report first described implementation of WAF 

models in BehavePlus followed by WAF in other applications, with emphasis on the differences. Even though 

most applications cite Albini and Baughman (1979), 

there are differences in the implementation and in the 

results. The question about the need for consistency 

among fire modeling systems should be asked in conjunction with an assessment of the impact of change. It 

may seem logical for all applications to use the same 

approach. But while an internal, transparent, change to 

equations might be easy for a programmer, a change in 

calculated results can be unsettling. The impact of redoing 

analyses, plans, and reports that used model results can be 

significant and perhaps unnecessary. Given differences 

in WAF modeling approaches and the impact on results, 
38 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012

fire modeling results should be reported along with the 

source (reference, computer program, version number, 

etc.), and possibly model assumptions.

A good solution would be a new and improved WAF 

model that is well documented and can uniformly be 

implemented in a range of applications that require midflame wind speed. Such an improvement could overcome 

some of the current limitations such as the transition 

between models for unsheltered and sheltered fuels and 

consideration of adequate fetch and terrain on penetration 

of the wind through the overstory.

While wind profile modeling is useful to all fire behavior models, current WAF modeling is aimed at the 

requirements of the Rothermel surface fire spread model. 

Future fire models might be able to utilize or even require 

wind speed and direction that vary in time and space.

A modeling effort inherently requires assumptions on 

what is relevant to the effort. Uncertainties in fuel characterization, fuel moisture variation, and complex wind 

behavior should be considered. There is a large range in 

fire modeling approaches and products. At one extreme 

are CFD (computational fluid dynamics) models that 

model specifics of air flow through the canopy, if there 

are fuel and vegetation data to feed the model. At the 

other extreme are non-computer based guides for use on 

the fireline. The need continues for general guidelines 

that incorporate influencing factors for fire behavior and 

safety.

Albini and Baughman’s WAF modeling has served 

us well. Their models are included in applications that 

go well beyond the initial tables designed to be used 

by an experienced fire behavior analyst who can make 

adjustments based on judgment. Albini and Baughman 

(1979) stated that “... this effort is seen as a small first 

step in the direction of a more complete description of 

the complex phenomena with which we are dealing.” 

References _____________________

Albini, Frank A. 1976a. Computer-based models of wildland fire 

behavior: a user’s manual. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 68 p.

Albini, Frank A. 1976b. Estimating wildfire behavior and effects. 

Gen. Tech. Rep. INT-30. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment 

Station. 92 p.

Albini, Frank A.; Baughman, Robert G. 1979. Estimating windspeeds 

for predicting wildland fire behavior. Res. Pap. INT-221. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 12 p.

Anderson, Hal E. 1982. Aids to determining fuel models for estimating fire behavior. Gen. Tech. Rep. INT-122. Ogden, UT: U.S. 

Department of Agriculture, Forest Service, Intermountain Forest 

and Range Experiment Station. 22 p.

Andrews, Patricia L. 1986. BEHAVE: fire behavior prediction and 

fuel modeling system–BURN subsystem, part 1. Gen. Tech. Rep. 

INT-194. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Research Station. 130 p.

Andrews, Patricia L. 2007. BehavePlus fire modeling system: past, 

present, and future. In: Proceedings of 7th Symposium on Fire and 

Forest Meteorology; 2007 October 23-25; Bar Harbor, ME. Boston, 

MA: American Meteorological Society. 13 p.

Andrews, Patricia L. 2009. BehavePlus fire modeling system, version 

5.0: Variables. Gen. Tech. Rep. RMRS-GTR-213WWW Revised. 

Fort Collins, CO: Department of Agriculture, Forest Service, Rocky 

Mountain Research Station. 111 p.

Andrews, Patricia L. 2011. Do you BEHAVE? Application of the 

BehavePlus fire modeling system. In: Proceedings of 3rd Fire Behavior and Fuels Conference; 2010 October 25-29; Spokane, WA. 

Missoula, MT: International Association of Wildland Fire. 17 p.

Andrews, Patricia L.; Chase, Carolyn. H. 1989. BEHAVE: fire behavior prediction and fuel modeling system–BURN subsystem, 

part 2. Gen. Tech. Rep. INT-260. Ogden, UT: U.S. Department of 

Agriculture, Forest Service, Intermountain Research Station. 93 p.

Baughman, Robert G.; Albini, Frank A. 1980. Estimating midflame 

windspeeds. In: Proceedings of Sixth Conference on Fire and Forest Meteorology; 1980 April 22-24; Seattle, WA. Washington, DC: 

Society of American Foresters: 88-92.

Bradshaw, Larry S.; Deeming, John E.; Burgan, Robert E.; Cohen, 

Jack D. 1983. The 1978 National Fire-Danger Rating System: 

technical documentation. Gen. Tech. Rep. INT-169. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 44 p.

Bradshaw, Larry S.; Petrescu, Gene.; Grenfell, Isaac. 2003. An initial analysis of relationships between 2- and 10-minute averaged 

winds at 10, 6, and 1.8 meters: implications for fire behavior and 

danger applications. In: 2nd International Wildland Fire Ecology 

and Fire Management Congress and 5th Symposium on Fire and 

Forest Meteorology; 2003 November 16-20; Orlando, FL. Boston, 

MA: American Meteorological Society. 5 p.

Burgan, Robert E. 1988. 1988 revisions to the 1978 National FireDanger Rating System. Res. Pap. SE-273. Asheville, NC: U.S. 

Department of Agriculture, Forest Service, Southeastern Forest 

Experiment Station. 39 p.

Burgan, Robert E.; Rothermel, Richard C. 1984. BEHAVE: fire 

behavior prediction and fuel modeling system–FUEL subsystem. 

Gen. Tech. Rep. INT-167. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment 

Station. 126 p.

Butler, Bret W.; Finney, Mark; Bradshaw, Larry; Forthofer, Jason; 

McHugh, Chuck; Stratton, Rick; Jimenez, Dan. 2006. WindWizard: 

a new tool for fire management decision support. In: Andrews, P. 

L.; Butler, B. W., comps. Fuels management—How to measure 

success: conference proceedings; 2006 March 28-30; Portland, OR. 

Proc. RMRS-P-41. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 787-796.

Cohen, Jack D.; Deeming, John E. 1985. The National Fire-Danger 

Rating System: basic equations. Gen. Tech. Rep. PSW-82. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific 

Southwest Forest and Range Experiment Station. 16 p.

Deeming, John E.; Burgan, Robert E.; Cohen, Jack D. 1977. The 

National Fire-Danger Rating System–1978. Gen. Tech. Rep. INT39. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 63 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-266. 2012 39

Deeming, John E.; Lancaster, James W.; Fosberg, Michael A.; Furman, 

R. William; Schroeder, Mark J. 1972. National Fire-Danger Rating 

System. Res. Pap. RM-84. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range 

Experiment Station. 165 p.

Finney, Mark A. 1998 [revised 2004]. FARSITE: Fire Area 

Simulator—model development and evaluation. Res. Pap. RMRSRP-4. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station. 47 p. Corrected version available: http://www.fs.fed.us/rm/pubs/rmrs_rp004.pdf [September 

29, 2011].

Finney, Mark A. 2006. An overview of FlamMap fire modeling 

capabilities. In: Andrews, P. L.; Butler, B. W., comps. Fuels management—How to measure success: conference proceedings; 2006 

March 28-30; Portland, OR. Proc. RMRS-P-41. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station: 213-220.

Finney, Mark A.; Grenfell, Isaac C.; McHugh, Charles W.; Seli, Robert 

C.; Trethewey, Diane; Stratton, Richard D.; Brittain, Stuart. 2010. 

A method for ensemble wildland fire simulation. Environmental 

Modeling and Assessment. 16(2): 153-167.

Finney, Mark A.; McHugh, Charles W.; Grenfell, Isaac C.; Riley, 

Karin L.; Short, Karen C.2011. A simulation of probabilistic wildfire risk components for the continental United States. Stochastic 

Environmental Research and Risk Assessment. 25(7): 973-1000.

Fischer, William C.; Hardy, Charles E. 1972. Fire-weather observer’s 

handbook. Agric. Handb. 494. Ogden, UT: U.S. Department of 

Agriculture, Forest Service, Intermountain Forest and Range 

Experiment Station. 152 p.

Forthofer, Jason; Shannon, Kyle; Butler, Bret. 2009. Simulating diurnally driven slope winds with WindNinja. In: Proceedings of the 8th 

Symposium on Fire and Forest Meteorology; 2009 October 13-15; 

Kalispell, MT. Boston, MA: American Meteorological Society. 10 p.

Heinsch, Faith Ann; Andrews, Patricia L. 2010. BehavePlus fire 

modeling system, version 5.0: Design and Features. Gen. Tech. 

Rep. RMRS-GTR-249. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 111 p.

Jennings, Stephen B.; Brown, Nick D.; Sheil, Douglas. 1999. Assessing forest canopies and understorey illumination: canopy closure, 

canopy cover and other measures. Forestry. 72(1): 59-73.

McArthur, Alan G. 1969. The Tasmanian bushfires of 7th February, 

1967, and associated fire behavior characteristics. In: Mass fire 

symposium: the technical cooperation programme, Panel N2 working group. Forest Research Institute, Canberra, Australia. 23 p.

McPherson, Renee A.; Fiebrich, Christopher A.; Crawford, Kenneth 

C.; [and others]. 2007. Statewide monitoring of the mesoscale environment: a technical update on the Oklahoma Mesonet. Journal 

of Atmospheric and Oceanic Technology. 24(3): 301-321.

National Wildfire Coordinating Group [NWCG]. 2006. NWCG fireline handbook appendix B: fire behavior. PMS 410-2/NFES 2165. 

Boise, ID: National Interagency Fire Center. 124 p.

Norum, Rodney A. 1983. Wind adjustment factors for predicting fire 

behavior in three fuel types in Alsaka. Res. Pap. PNW-309. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Forest and Range Experiment Station. 5 p.

Ottmar, Roger D.; Sandberg, David V.; Riccardi, Cynthia L.; Prichard, 

Susan J. 2007. An overview of the Fuel Characteristic Classification 

System–quantifying, classifying, and creating fuelbeds for resource 

planning. Canadian Journal of Forest Research. 37(12): 2383-2393.

Reinhardt, Elizabeth D.; Crookston, Nicholas L. 2003. The Fire and 

Fuels Extension to the Forest Vegetation Simulator. Gen. Tech. Rep. 

RMRS-GTR-116. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 209 p.

Reinhardt, Elizabeth D.; Keane, Robert E.; Brown, James K. 1997. 

First Order Fire Effects Model: FOFEM 4.0, user’s guide. Gen. Tech. 

Rep. INT-GTR-344. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Research Station. 65 p.

Rothermel, Richard C. 1965. Low airspeed differential pressure integrating system. Res. Note INT-37. Ogden, UT: U.S. Department 

of Agriculture, Forest Service, Intermountain Forest and Range 

Experiment Station. 8 p.

Rothermel, Richard C. 1967. Airflow characteristics–wind tunnels and 

combustion facilities–Northern Forest Fire Laboratory. Missoula, 

MT: U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 32 p.

Rothermel, Richard C. 1972. A mathematical model for predicting 

fire spread in wildland fuels. Res. Pap. INT-115. Ogden, UT: U.S. 

Department of Agriculture, Forest Service, Intermountain Forest 

and Range Experiment Station. 40 p.

Rothermel, Richard C. 1983. How to predict the spread and intensity 

of forest and range fires. Gen. Tech. Rep. INT-143. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 161 p.

Rothermel, Richard C.; Anderson, Hal E. 1966. Fire spread characteristics determined in the laboratory. Res. Pap. INT-30. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 34 p.

Schroeder, Mark J.; Buck, Charles C. 1970. Fire weather...A guide 

to application of meteorological information to forest fire control 

operations. Agric. Handb. 360. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 229 p.

Schroeder, Mark J.; Fosberg, Michael A.; Lancaster, James W.; 

Deeming, John E.; Furman, R. William. 1972. Technical development of the National Fire-Danger Rating System. Review Draft. 

Unpublished paper on file at: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station, Fire Sciences 

Laboratory, Missoula, MT. 151 p.

Scott, Joe H. 1998. NEXUS: a system for assessing crown fire hazard. 

Fire Management Notes 59(2): 20-24.

Scott, Joe H. 2007. Nomographs for estimating surface fire behavior 

characteristics. Gen. Tech. Rep. RMRS-GTR-192. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 119 p.

Scott, Joe H. 2010. [Email to Patricia L. Andrews]. July 20. FuelCalc 1.0 Reference Guide, Draft. On file at: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station, 

Fire Sciences Laboratory, Missoula, MT; Fire, Fuel, and Smoke 

Science Program files.

Scott, Joe H.; Burgan, Robert E. 2005. Standard fire behavior fuel 

models: a comprehensive set for use with Rothermel’s surface fire 

spread model. Gen. Tech. Rep. RMRS-GTR-153. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 72 p.

Shaw, Roger H. 1977. Secondary wind speed maxima inside plant 

canopies. Journal of Applied Meteorology. 16(5): 514-521.

Sutton, O. Graham. 1953. Micrometeorology. New York: McGrawHill. 333 p.

Turner, Jack A.; Lawson, Bruce D. 1978. Weather in the Canadian 

Forest Fire Danger Rating System: a user’s guide to national 

standards and practices. Information Report BC-X-177. Victoria, 

British Columbia, Canada: Environment Canada, Canadian Forest 

Service, Pacific Forest Research Centre.

U.S. Department of Agriculture [USDA], Forest Service. 1983. Fire 

management analysis and planning handbook. For. Serv. Handb. 

5109-19. Washington, DC: U.S. Department of Agriculture, Forest 

Service. 100 p.

U.S. Department of Agriculture [USDA]; U.S. Department of the 

Interior [USDI]. 2001. Developing an interagency, landscape-scale 

fire planning analysis and budget tool, Report to the National Fire 

Plan Coordinators. 




Federal Recycling Program Printed on Recycled Paper

The Rocky Mountain Research Station develops scientific information 

and technology to improve management, protection, and use of the 

forests and rangelands. Research is designed to meet the needs of 

the National Forest managers, Federal and State agencies, public and 

private organizations, academic institutions, industry, and individuals. 

Studies accelerate solutions to problems involving ecosystems, range, 

forests, water, recreation, fire, resource inventory, land reclamation, 

community sustainability, forest engineering technology, multiple use 

economics, wildlife and fish habitat, and forest insects and diseases. 

Studies are conducted cooperatively, and applications may be found 

worldwide.

Station Headquarters

Rocky Mountain Research Station

240 W Prospect Road

Fort Collins, CO 80526

(970) 498-1100

Research Locations

Reno, Nevada

Albuquerque, New Mexico

Rapid City, South Dakota

Logan, Utah

Ogden, Utah

Provo, Utah

The U.S. Department of Agriculture (USDA) prohibits discrimination in all of its 

programs and activities on the basis of race, color, national origin, age, disability, 

and where applicable, sex (including gender identity and expression), marital 

status, familial status, parental status, religion, sexual orientation, political 

beliefs, genetic information, reprisal, or because all or part of an individual’s 

income is derived from any public assistance program. (Not all prohibited bases 

apply to all programs.) Persons with disabilities who require alternative means 

for communication of program information (Braille, large print, audiotape, etc.) 

should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD).

To file a complaint of discrimination, write to: USDA, Assistant Secretary for Civil 

Rights, Office of the Assistant Secretary for Civil Rights, 1400 Independence 

Avenue, S.W., Stop 9410, Washington, DC 20250-9410. Or call toll-free at 

(866) 632-9992 (English) or (800) 877-8339 (TDD) or (866) 377-8642 (English 

Federal-relay) or (800) 845-6136 (Spanish Federal-relay). USDA is an equal 

opportunity provider and employer.

www.fs.fed.us/rmrs

Flagstaff, Arizona

Fort Collins, Colorado

Boise, Idaho

Moscow, Idaho

Bozeman, Montana

Missoula, Montana