J.S.D.A. FOREST SERVICE RESEARCH PAPER INT-56,1988 

INTERMOUNTAIN FOREST AND RANGE EXPERIMENT STATION 

OGOEN UTAH 84401 

This file was created by scanning the printed publication.

Errors identified by the software have been corrected;

however, some errors may remain.
THE AUTHOR 

HAL E. ANDERSON received his bachelor's 

degree in physics from Central Washington 

State College in 1952. He was employed 

by General Electric Company from 1952 

to 1961, except for 2 years of military 

duty, and worked in thermal and nuclear 

instrumentation. In 1961 he came to the 

Intermountain Station's Northern Forest 

Fire Laboratory at Missoula, Montana, as a 

member of the Fire Physics Project. He 

was Project Leader from 1962 to 1966. He 

is now Project Leader of the Fuel Science 

Research Work Unit and is responsible for 

research on fuel thermodynamics and combustion properties. 
U S. D. A. Forest Service 

Research Paper INT-56 

1968 

Hal E. Anderson 

INTERMOUNTAIN FOREST AND RANGE EXPERIMENT STATION 

Forest Service 

U. S. Department of Agriculture 

Ogden, Utah 84401 

Joseph F. Pechanec, Director 
Large forest fires are spectacular, usually 

destructive, frequently dangerous, but always 

exciting. Fire research organizations, particularly just after a spectacular wildfire, are often 

eager to organize mobile research teams to 

study and document fire behavior. Unfortunately, this approach to the study of wildfire 

is usually not very productive. There are several reasons why not. 

It is very difficult for a researcher to be in 

the right place at the right time when severe 

fire behavior occurs. Often he is remotely 

located on the opposite side of a situation, or 

access to the area of interest is limited or inefficient. There is always the danger that 

research personnel will interfere with tactical 

activities at the fire, primarily in the air. 

There is generally no effective way to properly instrument an area prior to severe fire behavior, simply because there is not enough 

time and one cannot predict precisely when 

or where such events will occur. 

The Sundance Fire was unique. Early on 

September 1, there was already a tremendous 

concentration of fire organization, supplies, 

equipment, and manpower in the vicinity because of a number of active project fires. An 

additional force of competent firefighting personnel was gathering to meet the expected 

need. The Sundance Fire was being fought by 

State and local forces but was expected to 

cause increasing difficulty, and preparations 

were underway to transfer the fire to the 

U. S. Forest Service. Accumulative weather 

buildup was extreme, the fire had a 4- 

mile exposed front, and the current weather 

was critical. Suddenly the fire made one tremendous run, increasing in size by more than 

50,000 acres in one burning period. Since 

there are communities at each end of the fire 

run and a road cutting across the center of it, 

and because firefighting crews were in the 

Pack River area preparing for a possible run of 

the fire, many observers saw portions of this 

spectacular fire run. The mere fact that the 

event was limited in time and seen by many 

people permitted the research team to reconstruct some of its characteristics. Interviews 

with fire control people, local residents, and a 

variety of other individuals provided firsthand 

observational knowledge of what happened. 

Most observations were subjective, but some 

included measurements. 

The Sundance Fire offered a rather unusual situation for analysis and we think the 

results, as given in this paper, have a great deal 

of value. Nevertheless, it must be recognized 

that similar studies might only be redundant. 

As long as studies are limited by conventional 

measuring methods, there is no real assurance 

that an organized effort to chase large fires 

would produce equal or better analyses. Certainly the development of remote sensing 

techniques or instrumentation could change 

the picture considerably, and might allow for 

improved data collection on wildfires. We believe that fire analyses are best when applied 

to special case fires. 

There is, of course, some value for the 

scientist to be gained through observation of 

wildfires. It sharpens his perspective on the 

real world and on the problems he is facing. 

We must be cautious, however, not to expect 

more than is realistic from observing wildfires, 

while at the same time, we need to consider 

improved methods for observing them. This 

is one way that fundamental and theoretical work now underway in laboratories can 

be related to real-life conditions in the forest. 

ARTHUR P. BRACKEBUSCH 

Chief of Northern Forest Fire Laboratory 
The author acknowledges with deep appreciation the cooperation 

and assistance provided by the following persons, each in his 

special field: 

DONALD M. FUQUAY for his leadership in organizing the general 

approach to the analysis and development of a presentation 

format. 

RICHARD C. ROTHERMEL for his leadership in the fieldwork of 

gathering, data and photographs, and interviewing personnel. 

ARTHUR P. BRACKEBUSCH for his background knowledge of 

the area and experience in fire behavior. 

JAMES K. BROWN for his responsible appraisal of the fuel complex and calculations of fuel distribution. 

WILLIAM H. FRANDSEN for his capable interpretation of the 

destruction of the Pack River Bridge and contributions in 

other parts of the analysis. 

DWIGHT S. STOCKSTAD for his consultation on the area and the 

fire behavior. 

ARNOLD I. FINKLIN for his thorough and careful collection and 

analysis of the meteorological data used in the weather interpretation. 

ORVAL W. GASTINEAU, Jr., and ROBERT D. SCHUETTE for 

their valuable assistance in tabulations, computations, and 

aerial photo analysis. 

BRYAN M. OWEN for his competent graphic rendition of fire 

phenomena conceptions and preparation of other charts and 

illustrations. 

HERMAN WITTMAN for his assistance in the photographic processing of photographs, graphs, and illustrations. 

Personnel of the Priest Lake Timber Protective Association, Kaniksu 

National Forest, and Division of Fire Control, Northern Region, 

U.S.D.A. Forest Service for provision of fire logs and maps, 

interviews and valuable information regarding the area, the 

timber, and the fire. 
. . Preface .................................................. n 

... Acknowledgments .......................................... m 

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

The Fire Area .............................................. 2 

The Fire Story ............................................. 3 

The Buildup ............................................. 3 

The Early Days - August 11 to 31 ................................ 4 

The Major Run - September 1. 1967 ............................... 5 

The Fire Phenomena ......................................... 10 

Weather ............................................... 10 

Rate of Spread ........................................... 10 

Fuel Loading and Consumption ................................. 10 

Intensity .............................................. 12 

Fire Log for September 1 .................................... 13 

Some Prominent Fire Features ................................... 17 

Influence of Terrain and Wind on Fire Spread ......................... 17 

Blowdown ............................................. 17 

Firebrand Activity ........................................ 18 

Fire Activities in Pack River ................................... 20 

Conclusions .............................................. 22 

Appendix 1: Fuel Determinations Used in the Analysis ..................... 23 

Appendix 2: Meteorological Factors in the Fire Run ....................... 25 

Appendix 3: The Pack River'Bridge as an Area Fire Intensity Indicator ............ 33 

Literature Cited ............................................ 37 
The Sundance Fire in northern Idaho 

stands as a giant among several big fires in the 

Northwest during the summer of 1967. The 

magnitude of this fire resulted from a combination of conditions seldom occurring simultaneously - a prolonged dry period, persistent high temperatures, sustained winds during 

the fire run, and an uncontrolled 4-mile fire 

front. These conditions produced a rapidly 

moving fire that burned over a large area and 

released an awesome quantity of energy. In a 

period of only 9 hours this fire traveled 16 

miles, engulfed more than 50,000 acres, released millions of heat units, snapped and toppled hundreds of trees, and destroyed much of 

the storage capacity of the watersheds. 

The Sundance Fire started its major run on 

September 1, 1967, at approximately 2:00 

p.m. (1400 P.d.t.1 and by 11:OO p.m. (2300 

P.d.t.) it had completed the run. Shortly 

after the major run, personnel from the 

Northern Forest Fire Laboratory were assigned to the fire staff as fire behavior officers, to predict fire behavior and advise the 

control teams. At the same time, other NFFL 

personnel were assigned to a research team 

responsible for investigating the physical phenomena of this fire and for documenting its 

nistorical development. Their investigation 

was actually a detective job, since only evidence remaining on and near the burned area, 

eyewitness accounts, and records kept by the 

various agencies were available for interpreting the events. This paper presents the best 

estimates of fire phenomena that could be 

made by the research team, using the available 

information. The progress of the fire, its intensity, the fuel being consumed, the attendant weather events, and other special conditions were analyzed from a multitude of separate observations gathered from both official 

sources and individuals. 

When a forest fire "blows up," the people 

directly involved are busy moving equipment, 

materials, and firefighters to new locations, or 

planning new attack procedures, and their impressions are often confined to a single area or 

phase of the fire's activity. In reconstructing 

the fire's movements, any one person's 

account would have been of limited value. 

The magnitude of this fire was such that 

many different people observed it at many 

different locations. One of the earliest jobs 

for the research field team was to interview as 

many people as possible who had observed or 

participated in the fire activities on, September 1. Interviews from 24 people contributed 

to the determination of fire front location, 

rate of spread, type of fire, and energy release 

rate at specific times. 

The investigating team has attempted to 

sort out the details of a violently eventful 

period of time and construct from them a coherent account of the phenomena of a particular fire. A general conception of what was 

happening where and when as the fire progressed is necessary to understanding of the 

physical phenomena. Therefore this paper 

begins with a description of the geographical 

area and a narrative of the movement of the 

fire. The approach to study of certain major 

factors in the fire development is then explained, and the values calculated from the 

data are given in a chronology corresponding 

to the fire's major run. A more detailed discussion of significant features of the fire then 

follows, leaving for the appendixes information on methods of calculation and experimental procedures that helped the investigators to draw their conclusions. 
Sundance Mountain, the point of origin for 

the fire, is on a spur off the Selkirk Divide 

east of the town of Coolin, Idaho. (Topographic relationships of the fire area are 

shown in figure 1; see also the foldout map at 

the back of this paper.) The fire up to SepMILES 

Figure 1. - Topographic diagram of the Sundance 

Fire area. 

tember 1 was confined to the lower Soldier 

Creek (elev. 3,600 ft.) and Lost Creek (4,800 

ft.) drainages (fig. 2), which lie west and 

southwest of Sundance Mountain. On September 1, it progressed northeast up to the 

Divide, engulfing Jeru Peak (6,394 ft.) and 

Hunt Peak (more than 7,000 ft.), and skirting 

south of Gunsight Peak (7,357 ft.). As may be 

noted in figure 1, the elevation drops from 

5,800 ft. at the saddles along the Divide to 

3,800 ft. on the Pack River at the north edge 

of the fire path and to 2,600 ft. at the south 

edge. Northeast out of Pack River is Apache 

Ridge, running to the southeast; here the fire 

path is bounded by Roman Nose Mountain on 

the north at 7,264 ft. and Dodge Peak on the 

south at 5,026 ft. Northeast of Apache Ridge 

the terrain slopes to the southeast and is cut 

by the major drainages of Falls Creek, Highland Creek, Ruby Creek, and Caribou Creek. 

The fire ended its run near Snow Peak (more 

than 4,000 ft.) and worked toward Bonners 

Ferry, Idaho, down Caribou Creek to an elevation of 2,800 ft. 

Figure 2. - Sundance Mountain, looking to the northeast, with Chase Lake in foreground and Selkirk Dide 

in near background. Lost Creek is in center and Soldier Creek drainage on left edge. 
THE BUILDUP 

The fire story begins in the months before 

the outbreak itself, in the buildup of fire 

danger during the summer of 1967. The fire 

season in northern Idaho developed quite normally, and in the first 3 months showed characteristics of an average fire season. The fire 

weather records of the Priest River Experimental Forest in northern Idaho, interpreted 

according to the National Fire-Danger Rating 

System, illustrate the seasonal buildup of fire 

danger. The record presented in figure 3 

shows the buildup index remaining below the 

previous record high for the period of 1954 to 

1966 until August 11, 1967. The continued 

buildup in 1967 from late June until midSeptember differs from the historical pattern, 

which has shown one or more breaks in buildup during this period. This year, day by day 

the situation became more serious, and restrictions were imposed to minimize the 

hazard of man-caused fires. 

1967 Buildup index - 

Early in August, a high pressure zone had 

become deeply entrenched over the northwestern United States and was effectively 

blocking the influx of moist maritime air 

from the west. No significant amount of precipitation occurred between August 11 and 

September 1. In the 62 days from July 10 to 

September 10, only 0.10 inch of rain was 

measured at Priest River Experimental Forest 

from four different showers. From August 11 

to August 31, the 10-day averages showed the 

temperatures were the highest, the humidities 

the lowest, and the fine fuel moisture content 

the lowest for the season. Relative humidity is 

estimated to have been below 35 percent continually for at least 72 hours prior to the fire 

run. 

On August 31, all stations in northern 

Idaho except those at high elevations had 

buildup indexes rated "extreme." Late on 

that day, a Pacific weather system was 

approaching the Washington coast. A warning 

I 5 I0 I520257 5 10 152025 1 5 10 I5202536 5 10 I5 202536 5 10 15 2025 5 10 15 202530 

MAY JUNE JULY I AUGUST 1 SEPT. I OCTOBER I 

Figure 3. - Buildup index for 1967 compared with previous record high for period of 19541966. 
was issued for northern Idaho to expect increasing southwest winds 18-25 m.p.h. on 

September 1. These winds did increase steadily during the morning of September 1 and 

continued until late that evening. 

THE EARLY DAYS - August 11 to 31 

This, then, was the weather picture. For 

the fire activity preceding the major run on 

September 1, we have information from the 

Priest Lake Timber Protective Association and 

the Division of Fire Control, Northern Region, 

U. S. Forest Service. On August 11, a lightning storm moved over the area causing 

five fires on or near Sundance Mountain. 

Three of the fires were discovered and attacked on the llth, and two were declared 

out that same day; the third was declared out 

on August 15. A fourth fire was discovered on 

August 20, and 37 men, including 12 smokejumpers, brought the fire under control that 

evening - 1830 - with 2 acres burned. The 

fire was declared out on August 25 but was 

monitored by air patrol for several days. 

At 2240 on August 23, fie number 5 

broke out near the Sundance Lookout. The 

fire was contained by the end of the day, 

August 24, with an estimated 35 acres 

burned. Suppression activities continued for 

the next 5 days. Late in the evening of August 

29, at 2220, Priest Lake Timber Protective 

Association Headquarters near Coolin, Idaho, 

received word the fire had jumped the line 

and was out of control. Men and equipment 

were evacuated to headquarters. The fire was 

observed rolling down the hill in the Lee 

Creek drainage. A northeast wind had prevailed throughout the day and it is assumed 

this wind, coupled with the normal nighttime 

downslope currents, resulted in a wind-driven 

fire moving downslope. Information available 

from observers indicated the winds were 

Figure 4. - Looking west at Sundance Mountain after the fue's night run toward Coolin on August 29. Lost 

Creek in foreground; Soldier Creek on right. 
20-25 m.p.h. on the fire, but there was calm 

at Cavanaugh Bay near Coolin. The ground 

and crown fires were observed to move as a 

single front, with spotting up to one-half mile 

ahead. The main advance took place between 

2230, August 29, and 0200, August 30, which 

would give an average rate of spread of 0.80 

m.p.h. 

At 0700, August 30, the total acreage 

burned was reported as over 2,000 acres. Very 

little enlargement of fire area occurred, and 

there were only a few minor runs in the afternoon. Both Soldier Creek and Lost Creek 

drainages were now threatened if the wind 

shifted to the southwest (fig. 4). On August 

31, heavy smoke obscured the fire most of 

the day, but a spot fire broke out across a 

dozer line about 1700 and resulted in an intense burnout of lower Lost Creek. The 

southwest winds were starting to exert some 

influence judging by ash fallout reported by 

residents near Naples, Idaho, 18 miles to the 

northeast. Eight bulldozers were opening 

roads in the McCormick and Jeru Creek drainages. 

From this point, the story is best told in 

hourly reports of fire progress, which can be 

followed on the foldout map showing the 

fire-edge lines. The hourly progress of the fire 

was determined from the eyewitness accounts 

obtained through interviews. The approximate time an individual was in an area and 

where he saw the fire or the fire front were 

synchronized with the other observations. 

From this consolidated information, a coherent description of the fire's rate of spread was 

developed. 

THE MAJOR RUN - September 1,1967 

0700. The fire area had enlarged to approximately 4,000 acres and the eastern fire 

edge extended from the ridge between East 

River and Lost Creek north around Sundance 

Mountain and down into Soldier Creek (see 

foldout map). At 1010 on September 1, the 

U. S. D. A. Forest Service was requested to 

take over fire suppression activities on the 

Sundance Fire. 

Up to 1300. Fire activity had not increased significantly as reported by the air 

patrol observer, who had flown close to the 

perimeter. A run along the ridge between East 

River and Lost Creek had occurred since 0700 

but was not burning intensely. 

1400. The fire had started its advance to 

the northeast (see foldout map), the initial 

activity being in Lost Creek, which is at a 

higher elevation than Soldier Creek. In Soldier 

Creek the fire moved in a northeasterly direction along contours and downslope from Sundance Mountain. At this time the fire was 

considered to be a ground fire, consuming 

most of the brush and some crown material. 

Bulldozers were continuing to open roads 

east of the Selkirk Divide. Action was started 

to pull the equipment back. A fire camp had 

been established in Pack River south of Jeru 

Creek near Lindsey Creek. 

1500. By now the fire had increased in intensity, crossed Soldier Creek to the northeast 

side, continued up the south side, and reached 

the ridgetop in Lost Creek southwest of Jeru 

Peak. The fire was spotting across the Divide 

south of Jeru Peak and coming back up the 

east exposure slopes. The main front was becoming more brush and crown fire, with the 

brush controlling spread. 

1600. Lost Creek had burned out and 

spreading had been stalled by the Divide. The 

fires burning upslope on the east side were 

influenced by indraft winds. In Soldier Creek 

the entire front had become a crowning fire. 

Winds were carrying firebrands across the 

Divide and spot fires were burning upslope. 

Fire-induced winds were approaching 80 

m.p.h., resulting in some timber blowdown. 

The ground fire was lagging the main front 

and the brush was providing continuity to sustain the crowning fire. 

Dozers working at the head of Jeru Creek 

prepared to move back to Pack River. At 

about 1600, a two-man rescue team left Pack 

River fire camp by pickup truck to bring out 

men working the head of McCormick Creek. 

1700. The Selkirk Divide slowed the fire 

front; however, firebrands were being cast to 

the northeast. The ground fire moved up to 
Figure 5. - Convection column above Sundance Fire looking east, at 1700 on September 1,1967. 

Figure 6. - Fire destruction at mouth of McCormick the Divide, meeting the spot fires burning up 

Creek. Logging area on right. the east slo~es between Jeru Peak and Hunt 

Peak. The cbnvection column (fig. 5) reached 

a height of 25,000 ft. at this time and was 

detected as a weak echo by the U. S. Weather 

Bureau radar at Missoula, Montana. 

The rescue team had entered McCormick 

Creek and reported the smoke column was 4 

to 6 miles. wide. The dozers had moved out of 

the heads of Jeru and Hellroaring Creeks. 

1800. During this period the fire intensity 

decreased as the small fuel sizes were consumed. The spot fires on the east portion of 

the fire front, being influenced by the prevailing wind, became consolidated and quickly 

developed into a crowning fire. The large advancement of the fire between 1700 and 1800 

may be seen on the foldout map. 

The rescue team paced the fire while returning to Pack River after being blocked by 

the fire in McCormick Creek. As the rescue 

team reached the mouth of McCormick Creek 

(fig. 6), they could see flames on the ridge 

just southwest of them. The Roman Nose 

lookout could not see any fire because of the 
Figure 7. - Blowdown and burnout in Pack River area. 

smoke pall. A scout had left the Pack River 

camp and was at Youngs Creek trying to 

establish radio contact with the rescue team. 

Sometime during this period, the fire overran 

the dozer boss and operator near Fault Lake 

at the head of McCormick Creek. As the rescue team reached the mouth of McCormick 

Creek (fig. 6), they could see flames on the 

ridge just southwest of them. 

1900. As the fire front reached the lower 

reaches of the ridge between McCormick and 

Homestead Creeks, it was slowed by the 

steeper slopes, indrafts coming up and down 

Pack River, and a suppressed wind effect at 

the lower elevations. The ground fire was 

probably lagging the main front with the 

brush controlling spread, but considerable 

crowning was still occurring. 

The rescue team moved down Pack River 

under the advancing front of the fire and met 

the scout at Youngs Creek. Winds were breaking boughs and tops out of nearby young 

timber. Considerable spotting over Pack River 

was observed by others but the rescue team 

and scout saw only one large firebrand. While 

at Youngs Creek they saw fire cross the road 

about one-fourth mile upriver. They moved 

out toward the fire camp and estimated they 

were 3 to 4 minutes ahead of the fire. Upon 

their return to the fire camp, radio contact was 

made with the Roman Nose lookout instructing him to meet a pickup truck in Falls Creek. 

The pickup left Sandpoint, Idaho, near 1900. 

2000. Shortly after 1900 the fire swept 

down Jeru Creek so that the entire 5-mile 

front was in the Pack River drainage (see foldout map). For the next half hour the fire 

front was nearly stalled due to the turbulent 

winds created by indrafts up and down Pack 

River and down McCormick Creek. Fire 

whirls were triggered by these winds and 

moved randomly in the area near the Pack 

River bridge and the junction of McCormick 

Creek and Pack River. The indrafts and fire 

whirls caused extensive blowdown and tree 

breakage, as may be seen in figures 7 and 8. 

The fire activity in this area must have been 

similar to the classical "fire storm" with its 

intense burnout and associated winds in a 

fixed area. 
Figure 8. - Valley fwe storm area. 

The Roman Nose lookout had left the area 

to meet the pickup in Falls Creek. At 1930 he 

encountered a large spot fire blocking any 

further advance toward Falls Creek and 

turned back to the lookout. About 20 minutes later at 1950 the entire west slope of 

Apache Ridge appeared to ignite simultaneously. Observers at the Pack River fire camp 

stated no flame was visible one minute and 

the next the whole slope was aflame. The turbulent winds, convective and radiant preheating, and the firebrands lofted by the whirls 

evidently contributed to the ignition process. 

The fire swiftly moved up the slope and developed into a crowning fire. Rate of spread accelerated as the fire approached the ridgeline 

and firebrands were cast far in advance of the 

front. The convection column reached an altitude of 31,000 ft. at 2000 (fig. 9). 

The pickup sent for the Roman Nose lookout entered Falls Creek about 1930 and was 

buffeted by extremely strong winds and dust 

devils as he headed for the Roman Nose Creek 

crossing. The dust devils occurred rather regularly and were estimated to be between 10 

and 25 ft. in diameter. 

2100. After sweeping over Apache Ridge 

and moving into Falls and Roman Nose drainages, the fire slowed. It was still a crowning fire, 

sometimes with flames moving horizontally 

through the crowns. Between about 2015 and 

2100, the lookout observed spot fires far ahead 

of the front in Falls Creek; large spot fires were 

also seen above Roman Nose Lake No. 1. 

Meanwhile the pickup had reached Roman 

Nose Creek crossing. The driver saw fire to the 

west and southwest moving toward him. Upon 

turning around and leaving the area, he saw 

firebrands and large embers flying horizontally. 

At 2025, a spot fire started at the mouth 

of Snow Creek and at 2045 another started in 

Myrtle Creek (see foldout map). These spot 

fires were at least 10 miles in advance of the 

main front. In addition, numerous firebrands 

were falling to the ground at the intermediate 

distances. The spot fires observed from Black 

Mountain burned as both heading and backing 

fires, and one was observed to spread suddenly in all directions, developing a radial pattern 

of fire spread. At the same time, the convection column rose to 35,000 ft. and the pilot 

observer felt it was continuing to build. 

Long-range firebrands were being transported 

by at least two possible mechanisms: 

1. By being carried aloft in the convection 

column, then entering the influence of the 
Figure 9. - Artist's conception of convection column, then about 3 1,000 ft . high, made from slide taken by 

jet pilot over fue near 2000 hours. 

prevailing wind and falling under the gravitational influence (8)' ; 

2. By being carried aloft in the convection 

column, then entering the influence of a vortex street and being stabilized in it. The vortex street moved on downstream of the strong 

convection column (1). (For a more detailed 

discussion of this point, see the section below 

titled "Firebrand Activity.") 

As the man on Roman Nose Lookout retreated to a rockslide, where he stayed until 

the fire had abated the following day, a 

crowning fire moved up Zuni Creek out of 

Pack River and carried over into Ruby Creek. 

2200. By this time the fire had moved 

from Ruby Ridge into Ruby Creek and additional spotting occurred on Caribou Ridge 

(see foldout map). The fire front had become 

broken, probably for several reasons, including the facts that more area had been logged 

over a long period of time, so little slash was 

present, and that the spot fires had not consolidated into a single front. Hence there was 

poor fuel continuity and the main fire front 

was deprived of fuel. Sporadic flareups were 

occurring, indicating the meeting of heading 

and backing fires or burnout of pockets of 

'~urnbers in parentheses refer to Literature Cited. 

fuel. At this time the fire was primarily a 

brush fire, with considerable crowning. 

2300 on into September 2, 1967. The fee 

became more affected by prior burnout and 

became a ground fire as it moved into Caribou 

Creek. The rate of spread had decreased although some crowning took place. The main 

front had become poorly defined because of 

reduced fuel continuity. Nearly 60 percent of 

the area could now be considered affected by 

previous burning. 

A spot fire northeast of the main fire zone 

was reported at 2345. This probably resulted 

from the spot fire at the mouth of Snow 

Creek. Various portions of the fire became 

individual fires that resulted in flank increases 

and an occasional minor run. This type of behavior continued until the fire was controlled. 

Weather conditions continued to improve 

during the night (see table 2, p. 16). Observations made in Ruby, Highland, and Falls Creeks 

at 0200 indicated the fire was cooling down. 

One of the greatest concerns at this time was 

the possibility of a wind shift to the northwest 

or southeast. A fire front 20 miles wide would 

have resulted; however, the wind held steady 

and the acreage increased only slightly, 4 percent, before control was achieved. 
From the fire story told in the preceding 

section, it is clear that a great deal was happening in a short time on the Sundance Fire. 

The investigating team has analyzed the fire 

records to determine conditions such as 

weather, rate of spread, fuel loading and consumption, and fire intensity. For purposes of 

comparison, some of these determinations 

will be presented as an hourly log of the fire's 

major run. 

WEATHER 

A detailed analysis of the atmospheric conditions leading up to the fire and through 12 

hours after its run was made by the meteorological personnel of the Fire Laboratory. Estimated hourly freeatmosphere conditions - 

wind, temperature, and humidity - both preceding and during the major run of the fire, 

were derived for 5,000 ft. m.s.1. (Freeatmosphere conditions are those which 

would be experienced at some height above 

the ground, removed from surface influences.) 

The account that has been given of the 

progress of the fire has emphasized the importance of the weather influences and has included a description of the weather preceding 

the major outbreak of the fire. An illustrated 

analysis of the general weather pattern associated with the fire is given in Appendix 2. Bihourly reports on weather conditions during 

the major run are given in the log. 

With regard to temperature, it should be 

noted that the quantity of heat released by 

the fire modified the reported temperatures 

considerably near the ground, as evidenced by 

small islands of desiccated foliage. On the 

other hand, the general winds on the fire were 

probably always lower than those calculated 

for free-atmosphere conditions except where 

accelerated by fire-induced forces. 

RATE OF SPREAD 

It is evident from the fire story given earlier that the Sundance Fire spread with unusual rapidity during certain periods, slowing 

somewhat in the intervals. In the Log (p. 13), 

calculations for rate of spread are given for 

each hour. These are average values corresponding to the time and distance interval 

shown on the foldout map. 

It is interesting that the fire-spread indices 

based on the National Fire-Danger Rating 

System, averaged for the stations in the area, 

predicted actual conditions quite well. This is 

evident from the following comparison: 

Predicted Actual 

(0800) (1600) 

August 31 30 3 6 

September 1 52 64 

September 2 60 5 2 

FUEL LOADING AND CONSLlMPTlON 

The forest burned by the Sundance Fire 

was composed of mixed conifer stands interspersed with logged areas (fig. 10). A variety 

of stand conditions ranging from young to 

overmature and poorly to heavily stocked 

existed before the burn. 

To determine fuel loading, measurements 

were made on nearby areas comparable to 

those burned, as well as on the burned area 

itself. The fuel complex was considered to 

contain three levels: ground litter, brush, and 

crown material. 

Timber inventories for the Kaniksu Working Circle were used to estimate the crown 

fuel weight, by a method based on research 

done by Fahnestock (3). Data from the State 

of Idaho indicated that timber distribution on 

private and public lands should be similar, so 

the timber estimates based on Forest Service 

records were applied to the whole fire area. 

The average crown fuel load was estimated at 

2.04 tons per acre. Field measurements 

yielded a value of 5.25 tons per acre in a 

heavily stocked stand. 

The brush level was estimated by field 

sampling at a loading of 2.7 tons per acre. 

Visual observations throughout the bum area 

indicated the brush loading could have varied 

from 1 to 20 tons per acre, but the estimated 

average should be reasonable. 
Figure 10. - Clearcut logging area (left foreground) at the site of the Sundance Fire. At this time, on August 

30, the fire was backing into Soldier Creek drainage. 

Ground litter loadings were obtained from 

field sampling and found to be near 20 tons 

per acre. Again, in some areas the loadings 

could have been as low as 1 ton per acre. 

In determinations of fuel loading, crown 

material was limited to foliage and branches 

under l/&inch diameter, since field inspection of residual stems indicated most of such 

material was consumed. Areas where larger 

material in the crowns was burned were evident, but were disregarded in the computations. The maximum size for brush material 

was set at one-fourth inch, based on field 

sampling. 

Ground litter loadings varied considerably, 

but the field-obtained values were accepted as 

representative. The effects of logging slash 

and its treatment on the fire's intensity are 

difficult to evaluate. About 50 percent of the 

untreated logging slash in the area was more 

than 8 years old. Most of the fresh slash was 

at the southwest end of the fire area and was 

burned the day before the major run. Slash 

areas, therefore, were not considered to be 

different from the timbered areas in groundlitter determinations. 

Details of the calculations of fuel loading 

are given in Appendix 1, along with a description of the measurements of moisture content. Samples of the fuels were taken to establish moisture content levels in each stratum. 

Percentage estimates of fuel consumption 

are given in the Log, at approximately 

hourly intervals. These percentages are based 

on information gathered during the interviews, research data, and the acquired knowledge of experienced firefighters. The amount 

of fuel from each of the three fuel levels consumed in the fire front was estimated after 

considering the terrain and wind conditions, 

the rate of spread, and the observations of 

eyewitnesses. For example, where the fire 

front is being carried by the brush level at a 

low rate of spread with little wind we might 

consider that 90-percent available ground 

+95-percent brush +20-percent crown fuel 
was being consumed in the fire front. The percentages apply to the size classes and loading 

as previously described. 

Laboratory work (7) has shown that the 

percent of ground fuel burned in the flame 

front decreases as windspeed increases. We 

estimated 20-percent ground fuel consumption in our calculations. The difference between this and the field-determined amount 

consumed is assumed to have been burned by 

sporadic flaming and glowing combustion following passage of the main fire front. 

Fire intensity values were calculated and 

are given in the Log at hourly intervals, 

along with values for maximum energy release 

from the total front. These, too, are average 

values corresponding to the time and distance 

intervals. 

As explained earlier, physical constants for 

loading and moisture content of strata were 

derived. These constants established the 

amount of fuel on the site that could contribute energy to the fire. The average energy 

content of fuel was set at 8,500 B.t.u./lb.; the 

heat required to remove the moisture and 

raise the fuel to ignition in each fuel level was 

then subtracted to arrive at an energy release 

potential (B.t.u./lb.). Values of the energy 

potential per unit were calculated by multiplying the loading, corrected to pounds per 

square foot, by the energy release potential. 

For any time interval of interest, the energy 

per unit area was combined with the percent 

consumption values to obiain the energy released per unit area. Fire intensity and total 

energy release rate were calculated by use of 

the rate-of-spread determinations and application of laboratory findings for residence time 

(time the flaming zone exists at one point).' 

The physical constants for the three fuel 

levels are given in table 1. 

The values for energy release rate are shown 

in figure 11, for a 12-hour period that includes 

the fire's major run. To arrive at our values, 

the residence time for the fire-controlling fuel 

level was combined with a rate of spread to 

establish a flame depth. The active combustion 

area was calculated by multiplying the fire 

edge length by the flame depth. The combination of the combustion area and the energy 

released per unit area divided by the residence 

time yielded the tot$ energy release rate per 

unit time (B.t.u./sec.). 

Fire intensity in B.t.u. per second per foot 

of fireline (fire edge) was calculated after 

Byram (2, chap. 3, p.79). Values were obtained by combining the summed energy per 

unit area and the rate of spread for a time 

interval. The fire intensity values (fig. 12) appear reasonable when compared with the 

limits established by Byram. He gives an 

'~nderson, Hal E. Heat transfer and fire spread. U.S.D.A. 

Forest Serv., intermountain Forest and Range Exp. Sta., 

Ogden, Utah. (In preparation.) 

Table 1. - Estimated fuel constants for the Sundance Fire area. 

Fuel Moisture Residence Energy release Energy per 

strata Loading content time potential1 unit area 

Tonlacre Percent - Min. B.t.u./lb. B.t.u./sq.ft. 

Ground 

litter 20 (max.) 12 1 .O 7,675 7,060 

1 (min.) 12 1 .O 7,675 384 

Brush 2.7 (ave.) 60 2 .O 7,718 957 

Crown 5.25 (max.) 145 0.33 6,773 1,630 

2.04 (min.) 145 0.33 6,773 636 

Corrected for moisture and heat to ignition. 
Tme of day lho~rtl Tlr~e of day ilioilril 

Figure 11. - Rate of energy release at fire front over Figure 12. - Fire intensity calculated by Byram's 

a 12-hour period on September 1. method for a 12-hour period on September 1. 

upper limit of 30,000 B.t.u./sec.-ft. for large 

wildfires, whereas we derived a value of 

22,500 B.t.u./sec.-ft. for the Sundance Fire. 

The values given here are only approximations, since the derived residence time is an 

estimate. Each level of fuel and each size of 

fuel has its own residence time, which may or 

may not overlap that of the next size. This 

means residence time may be underestimated 

because of the sequential ignition and burnout of fuel sizes in increasing order. This 

probability seems partially verified by the 

nature of the flame-depth values, which were 

determined using the assigned residence time 

and the deduced rate of spread. Flame depth 

varied from a minimum of 44.0 ft. to a maximum of 264.0 ft., a range that appears narrow 

for the existing conditions. 

FIRE LOG FOR SEPTEMBER 1 

0700. Fire area 4,000 acres; fire edge extends from ridge between East River 

and Lost Creek north around Sundance Mountain and down into 

Soldier Creek. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 14 m.p.h. 

SW; relative humidity 25 percent; dry-bulb temperature 71" F. 

Up to 1300. No significant increase in fire area. Increased activity in Lost 

Creek showed the fire front was becoming more active. Weak convec- 
tion column over burned area meant an increase in windspeed could 

have a prompt influence on rate of fire spread at the boundaries. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 20 m.p.h. 

SW; relative humidity 27 percent; dry-bulb temperature 77" F. 

1400. Fire advancing to northeast; mainly ground fire. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 21 m.p.h. 

SW; relative humidity 25 percent; dry-bulb temperature 78" F. 

Rate of spread. 0.75 m.p.h. 

Fuels. Main fire front: 20-percent ground +75-percent brush 

+lo-percent crown. 

Intensity. Maximum energy release rate from total front: 53 X lo6 

B.t.u./sec.; fire intensity: 1,000 B.t.u./sec.-ft. 

1500. Fire has advanced to ridge southwest of Jeru Peak; main front brush 

and crown fire. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 24 m.p.h. 

SW; relative humidity 22 percent; dry-bulb temperature 78" F. 

Rate of spread. 1.25 m.p.h. 

Fuels. Main fire front: 20-percent ground +90-percent brush 

+40-percent crown. 

Intensity. Maximum energy release rate: 100 X lo6 B.t.u./sec.; fire 

intensity: 5,400 B.t.u./sec.-ft. 

1600. Fire at Selkirk Divide; ground fire lagging main front, brush sustaining 

crowning fire. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 25 m.p.h. 

SW; relative humidity 20 percent; dry-bulb temperature near 77" F. 

Rate of spread. 1.6 m.p.h. 

Fuels. Main front: 10-percent ground +95-percent brush +80-percent 

crown. 

Intensity. Maximum energy release rate: 145 X lo6 B.t.u./sec.; fire 

intensity: 6,900 B.t.u./sec.-ft. 

1700. Fire moving slowly across Divide; convection column 25,000 ft. high. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 29 m.p.h. 

SW; relative humidity 20 percent; dry-bulb temperature 75" F. 

Rate of spread. Less than 1 m.p.h. 

Fuels. Main fire front: 30-percent ground +85-percent brush 

+20-percent crown. 
Intensity. Maximum energy release rate: 73 X lo6 B.t.u./sec.; fire 

intensity: 3,150 B.t.u./sec.-ft. 

1800. Fire has advanced on east front. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 33 m.p.h.; 

relative humidity 20 percent; dry-bulb temperature 73" F. 

Rate of spread. 2.5 m.p.h.; could have peaked at more than 5 to 8 

m.p.h., as measured by rescue team. 

Fuels. In frontal advance: 10-percent ground +90-percent brush 

+95-percent crown. 

Intensity. Maximum energy release rate: 358 X lo6 B.t.u./sec.; fire 

intensity: 11,300 B.t.u./sec.-ft. - highest level up to this time. 

1900. Fire has reached ridge between McCormick and Homestead Creeks; 

ground fire lagging main front, brush controlling spread, considerable 

crowning. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 37 m.p.h. 

SW; relative humidity 21 percent; dry-bulb temperature 71" F. 

Rate of spread. 1 m.p.h. 

Fuels. In frontal advance: &percent ground +95-percent brush 

+60-percent crown. 

Intensity. Maximum energy release rate: 72 X lo6 B.t.u./sec.; fire 

intensity: 3,400 B.t.u./sec.-ft. 

2000. Fire in Pack River drainage; "fire storm"; convection column 31,000 ft. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 44 m.p.h. 

SW; relative humidity 23 percent; dry-bulb temperature 68" F. 

Rate of spread. 6 m.p.h. approaching Apache Ridge. 

Fuels. In frontal advance: &percent ground +60-percent brush 

+95-percent crown. 

Intensity. Maximum energy release rate: 474 X 106 B.t.u./sec.; fire 

intensity: 22,500 B.t.u./sec.-ft. These are maximum values for the 

entire run. 

2100. Fire has advanced over Apache Ridge and into Falls and Roman Nose 

drainages; convection column 35,000 ft. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 45 m.p.h. 

SW; relative humidity 28 percent; dry-bulb temperature 66" F. Windspeed as checked by Roman Nose lookout was steady at 35 to 40 

m.p.h. 

Rate of spread. 2.5 m.p.h. 
Fuels. In fire front: 4-percent ground +YO-percent brush +80-percent 

crown. 

Intensity. Maximum energy release rate: 174 X lo6 B.t.u./sec.; fire 

intensity: 8,250 B.t.u./sec.-ft. 

2200. Fire at Ruby Ridge and Ruby Creek; spotting on Caribou Ridge; 

broken front; brush fire with considerable crowning. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 52 m.p.h. 

SW; relative humidity 31 percent; dry-bulb temperature 65" F. 

Rate of spread. 1.5 m.p.h. 

Fuels. Approximately 30 percent of fuel previously affected by spot 

fires; fuel available to front: 6-percent ground +80-percent brush 

+YO-percent crown. 

Intensity. Maximum energy release rate: 60 X lo6 B.t.u./sec.; fire 

intensity: 3,800 B.t.u./sec.-ft. 

2300 and on into September 2. Fire moving into Caribou Creek; main front 

poorly defined; fire cooling down. 

Weather. Free-atmosphere conditions at 5,000 ft.: Winds 49 m.p.h. 

SW; relative humidity 36 percent; dry-bulb temperature 63" F. With 

the winds decreasing and the relative humidity increasing, general conditions on the fire were improving as shown in table 2. Winds on the 

firewereabout 10 m.p.h. in Ruby, Highland, and Falls Creeks at 0200. 

Rate of spread. 0.5 m.p.h. or less. 

Fuels. In fire front: 6-percent ground +GO-percent brush +50-percent 

crown. 

Intensity. Maximum energy release rate: 7.8 X lo6 B.t.u./sec.; fire 

intensity: 735 B.t.u./sec.-ft. 

Table 2. - Estimates of free-atmosphere conditions from 2200, 

September 1, 1967, to 0600, September 2, 1967. - 

Time Wind (SW) Relative Dry-bulb 

humidity temperature 

M.p.h. Percent 

- 

OF. 
The run of the Sundance Fire and its end 

cannot be attributed to any single factor. The 

wide front, dry fuels, and winds certainly 

played major roles in its advance, but firebrand activity was also instrumental. The termination of the run was influenced by the 

decrease in windspeed and the increase in 

humidity, and again by the firebrand activity 

that caused burned-out areas ahead of the 

main front. In addition the terrain was sloping 

downhill and prior logging activities had removed part of the fuel continuity. Like any 

fire, this one was controlled by nature's infinite variety of conditions and their combinations. The best we can do is assess the influence of the obvious variables, estimate their 

relative importance, and attempt to describe 

how the fire probably responded to each. 

To some extent, this has been done in the 

preceding pages. However, some additional 

comments may be helpful with respect to certain outstanding features of the fire. 

INFLUENCE OF TERRAIN 

AND WIND ON FIRE SPREAD 

The rate of spread of this fire was strongly 

influenced by the terrain features and the 

wind, singly or in combination. If figure 13, 

showing the changes in rate of spread during 

the major run, is correlated with the topographic diagram (fig. I), the windspeeds reported in the Log, and the foldout map, the 

influence of terrain and wind may be clearly 

seen. 

In Soldier Creek the rate of fire-front advance can be attributed to the slope and wind. 

The upslope terrain, exposed to southwest airflow from across a broad valley, allowed a 

crowning fire to become established at a 

lower windspeed. The fire probably accelerated until it reached the Selkirk Divide 

where it was slowed by backing fire from the 

spot fires and its own induced winds. 

Down into Pack River along McCormick 

Creek and Homestead Creek the wind was 

strong enough to push a crowning fire downslope. However, at the lower elevations the 

winds had less effect, the slopes became more 

steep, and indrafts countered the surface wind 

so the fire moved at a slower rate. 

The sharp increase in rate of spread over 

Apache Ridge can be attributed to preheating 

of fuel, spotting, upslope, and movement of 

the front into the wind's influence. Massive 

spotting and high winds advanced the fire 

through Falls Creek. The fire moved on to 

lower elevations where the winds were not so 

effective and in some areas backing spot fires 

developed a firebreak that slowed the spread. 

In addition the relative humidity rose 

above 30 percent toward the end of the run 

and may have prevented some spotting 

activity. Experience indicates that spotting 

tends to diminish when relative humidities 

rise above 30 percent. 

BLOWDOWN 

The extent of timber blowdown prompted 

an investigation of the probable indraft maximum in selected areas. Work carried out by 

Lommasson (5) on mass fires or fire storms 

Time of day (hours) 

Figure 13. - Rate of fue spread during a 12-hour 

period on September 1. 
Figure 14. - Blowdown in Falls Creek area. 

has provided a means of estimating indraft It has been generally estimated that winds in 

winds. His equation also provided the means excess of 80 m.p.h. are needed to cause blowof determining whether our values for flame down. Owen Cramer of the U. S. D. A. Forest 

depth and energy release were reasonable or Service, Pacific Northwest Station, after viewnot. The equation is ing slides of the damage, informally estimated 

1 /J the winds to be near 120 m.p.h. The agree- 

(88 x104) ($) ment of our values with other estimates, convw = sidering the extrapolations we used, encourw- ages continued efforts to develop theories and 

mathematical descriptions of field-observed 

where phenomena. 

V, = windspeed, m.p.h. Fl REBRAND ACTIVITY 

2 = total energy release rate, B.t.u./sec. 

A = combustion area, sq. miles. 

Values for four areas where blowdown was 

evident in aerial photographs (fig. 14) were 

tested with this equation: 

Up Soldier Creek Vw = 80 m.p.h. 

Down Upper 

McCormick Creek Vw = 95 m.p.h. 

Over Apache Ridge Vw = 96 m.p.h. 

Across Falls Creek Vw = 80 m.p.h. 

Firebrands and spotting were evident 

throughout the fire run and are believed to 

have played an important role in the fire's 

behavior. From the pattern of rate of spread 

and the record of reported fires, it was apparent that firebrand material had been carried 10 to 12 miles in advance of the main fire 

front. Much debris was found northeast of the 

fire area (fig. 15) in the form of moss and 

lichen strands or wreaths, larch and hemlock 

cones, needles, cedar fronds, branches the size 

of a pencil, and pieces of bark up to 4% 

inches long and 1% inches wide. The fallout 
Figure 15. - Debris 

and possible 

frebtand material 

found at Bonners 

Ferry airport, 

September 1, 1967. 

of this debris was widespread over the area 

northeast of the fire. Much of it was scorched 

or blackened but the particles had not burned 

to completion. Neither had they successfully 

ignited their surroundings. Reports of burning 

material falling near residences in the 

Kootenai Valley were received but none of 

this material was positively identified as burning when it landed. 

What kind, size, and shape of burning 

material could most likely have been carried 

such distances? Work done in Spain by Tarifa 

(8) was examined. He investigated the flight 

paths of spheres, cylinders, and rectangular 

plates and their burning characteristics to 

determine the lifetime of firebrands and the 

probable range. Various species of wood were 

used and some study was made of pine cones 

and charcoal elements. 

In studying his results, we found that 

wood elements would have to be large and 

carried quite high in the convection column 

to have a 10-mile range. The burning process, 

according to Tarifa, is glowing combustion, 

not flaming. Flaming firebrands existed only 

at low windspeeds. Charcoal brands have 

much longer burning times than any of the 

woods or natural firebrands. Pine cones burn 

much like wood once the scales have burned 

off. 

For our study we had an estimate of the 

wind and the distance the brands had to 

travel. We calculated the drag coefficient, terminal velocity, and mean velocity of fall for 

four types of brands: (a) a charcoal sphere, 1 

inch in diameter; (b) a charcoal cylinder, 1.75 

inches long and 0.6 inch in diameter; (c) a flat 

charcoal plate, 1.1 inches square and 0.43 

inch thick; and (d) a pine cone about 3.75 

inches long and 2.2 inches in diameter. We 

used the techniques presented by Tarifa to 

calculate the maximum flight time possible 

for each brand. This we considered as the 

time the brand would remain burning; thus 

the particle would have to reach the ground 

within this time period to ignite other material. From the mean velocity of fall and time 

of flight, we could calculate the height at 

which a particle of the given size must be 

cast - that is, the ejection altitude. In turn, 

we could then calculate the vertical velocity 

in the convection column required to carry 
20 r Charcoalsphere - 17.0 mio 

lg t X " Cylinder - 19.3 min. 

0 " Plate - 14.5min. 

la (h Pinecone - 9.0 min 

either stabilize within the vortex or centrifuge 

out. Particles with low drag configurations 

(fig. 15) would tend to stabilize, and as the 

vortex street dissipated far downstream of the 

convection column, the suspended debris 

would "touch down" (1). 

Several behavior characteristics noted by 

the people interviewed support this possibility. It has been stated earlier that the pickup 

man for the Roman Nose lookout encountered very intense winds and what he described as "dust devils" in Falls Creek ahead 

of the fire. The occurrence of these dust 

devils was periodic. The same man encountered similar winds later in the Snow Creek 

area. The lookout on Black Mountain observed a spot fire which fanned out in all 

directions in a circular manner. The tree-fall 

pattern in Falls Creek suggests the trees fell 

toward the centerline of the fie path from 

each side. These pieces of evidence have encouraged the investigation of the potential effectiveness of this phenomena (1). HOrlzonIal range irntlerl 

Figure 16. - Range of various types of firebrand 

material. 

the brand to the proper height. The results of 

these calculations are shown in figure 16. 

Any of the charcoal brands could have 

traveled the 10 to 12 miles in a glowing state, 

whereas the pine cone could have transported 

fire up to 7 miles. With a convection column 

developing to over 31,000 feet, it appears 

reasonable that vertical velocities of 45 m.p.h. 

would have existed and material could have 

been carried to altitudes over 18,000 feet in 

the convection column. Winds of 44 m.p.h. or 

greater did exist and could have carried the 

brand once it tumbled out of the column. 

This method of firebrand transport appears 

quite reasonable but other possibilities do 

exist. Currently under investigation is the possibility that moving vortices are generated by 

the fire and prevailing wind. The swirling air 

or vortices would move on downstream carrying firebrands and debris with them. The intensity and range of these vortices would 

depend on the amount of heat "pumped" 

into them by the fire and strength of the 

wind. Particles carried aloft would tend to 

FIRE ACTIVITIES IN PACK RIVER 

The Pack River area has received the 

greatest attention by the news media because 

of its accessibility, the obvious destructive 

forces associated with the fire, and the dramatic burning of the Pack River bridge. This 

area was surveyed by the research team in an 

attempt to localize the forces the fie exerted; 

however, these forces appear to have been so 

dynamic and fluctuating that the evidence remaining is not coherent. 

Shortly before the fire entered this area we 

know men near the river felt very little wind 

but could hear it overhead. Yet trees that had 

toppled into the river had green unscorched 

boughs underwater and burned limbs above 

water. This indicated that strong winds were 

present in the valley bottom very shortly before the fire arrived, or before the fire ignited 

the crown material. Some downed trees 

showed no signs of burning on the underside, 

although the upper side was charred. 

Winds of different magnitudes and directions may be inferred from the pattern of 

blowdown and breakage. The tops of many 

trees were snapped out either by a sudden 

sharp rush of air or from whipping back and 
forth, while a great many trees fell under the 

force of a strong, sustained high wind. The 

trees with snapped tops were generally less 

than 2 ft. d.b.h., whereas those that fell were 

somewhat larger. Tops were snapped off at a 

nearly uniform height of 60 to 75 feet at a 

diameter of approximately 1.0 foot. Across 

open reaches along Pack River below the 

bridge the trees fell to the north or northeast, 

but at the mouth of McCormick Creek trees 

were blown to the east and southeast. Above 

McCormick Creek some blowdown was to the 

south and southwest. 

All of these clues suggest strong erratic 

winds just before the fire and the presence of 

strong fire whirls building and collapsing randomly in the area surrounding the Pack River 

bridge (fig. 17). 

The sequence of events leading to the 

destruction of the bridge was also examined. 

The fire burned out both approaches and left 

the center span intact with some of its paint 

hardly scorched. The metal plate treads on 

the approaches were found draped from the 

concrete piers to the ground, indicating that 

as the wooden stringers and decking collapsed, temperatures had reached 1,200" F. 

This temperature level is not unusual in a fire. 

Laboratory tests showed the design of the 

approaches provided sustained combustion in 

the space between stringers, and the approaches could have continued burning after 

the main fire had passed. The burning period 

probably extended over many hours until the 

decking broke up and the stringers collapsed. 

Then an increase in heat occurred as the material piled up on the ground, resulting in a period of flaming combustion followed by a long , 

period of glowing combustion. Both types of 

burningcontributed to the spalling of the concrete piers. Additional information on the 

Pack River bridge is provided in Appendix 3. 

Figure 17. - Fire destruction at Pack River Bridge. 
The spectacular run of the Sundance Fire 

on September 1,1967, appears to have been a 

result of the combination of dry fuels from a 

sustained drought, low humidities for over 72 

hours, increasing winds sustained for a period 

of 9 hours, and a 4-mile active fire front existing on the morning of September 1. The fire 

advanced 16 miles in 9 hours and created spot 

fires 10 to 12 miles northeast of the place of 

origin. Spotting activity increased during the 

day, reaching a maximum intensity during the 

period of highest winds in the late afternoon. 

Whereas spot fires early in the day contributed to the fire spread, late in the day a multitude of firebrands created fuel voids leading 

to the breakdown of the main front and contributing to the fire's termination. Other 

factors influencing the decreasing rate of 

spread were the increasing humidity after 

2200, the decreasing wind after 2200, the 

downslope direction of burning, and to some 

degree the disruption of fuel continuity due 

to prior logging activities in the area. 

The average rate of spread ranged from 1 

to 6 m.p.h. with brief periods of spread rates 

having higher or lower values. The fire intensity built up to 22,500 B.t.u./sec.-ft. of fire 

front and was releasing nearly 500 million 

B.t.u./sec. A convection column rose to 

35,000 ft. Extensive firebrand activity and 

debris were transported from the fire. These 

brands were either lifted as high as 18,000 feet 

and transported by the wind or carried in 

vortices produced by the wind blowing 

around the convection column. Fire-induced 

winds could have been greater than 95 m.p.h., 

and caused the extensive tree blowdown. 

Fire buildup indices rated the conditions as 

"extreme" and spread indices forecast agreed 

reasonably with actual conditions. The averaged spread index for September 1 was "very 

high" and when combined with a buildup 

index of "extreme" produced a rating of "extreme fire danger." The National Fire-Danger 

Rating System Handbook describes it as: 

EXTREME. -Fires under extreme conditions start quickly, spread furiously, and burn 

intensely. All fires are potentially serious. 

Development into high intensity burning will 

usually be faster and occur from smaller fires 

than in the very high danger class. Direct attack is rarely possible, and may be dangerous, 

except immediately after ignition. Fires that 

develop headway in heavy slash or in conifer 

stands may be unmanageable while the extreme burning condition lasts. Under these 

conditions the only effective and safe control 

action is on the flanks until the weather 

changes or the fuel supply lessens. 

This very aptly described the Sundance 

Fire. The present rating system estimated the 

conditions properly and provided warnings of 

what could be expected. 

This analysis of the Sundance Fire did not 

reveal any phenomena incompatible with 

existing knowledge and principles. The extension of laboratory findings and theoretical 

developments to a field situation resulted in 

estimates of burning rates, fire intensities, 

firebrand ranges, and induced windspeeds 

which are realistic. Evidence of various phenomena gathered during this study has heightened interest in additional research that will 

provide fuller explanations of such items as 

vortex generation, firebrand propagation, and 

spot fire ignition characteristics. The research 

people assigned to this study have gained valuable experience and we hope our interpretations will be useful and informative to other 

research and fire control personnel. 
FUEL DETERMINATIONS 

USED IN THE ANALYSIS 

As stated in the text, the fuel complex on 

the 55,910 acres burned by the Sundance Fire 

was divided into three levels: ground litter, 

brush, and crown material. The crown material consumed was composed of foliage and 

branches under 118-inch diameter based on 

observations made in the field, The brush 

level quantity was field sampled and the maximum size involved in the fire front was set at 

one-fourth inch. The ground litter was 

sampled in the field at two locations and 

visual estimates were made throughout the 

area. Because of findings in laboratory work 

on fuel consumption, the quantity of the 

total litter fuel involved in the front was set at 

20 percent. 

The crown weights were calculated on the 

basis of timber inventories made by the 

U, S. D. A. Forest Service. An assumption 

that State and private lands contain about the 

same variety of timber types was made after 

information received from agencies in Idaho 

had been reviewed. The number of acres occupied by each cover type on Forest Service 

land, based on the. 1961 forest inventory, is 

as follows: 

Cover type 

Larch 

Alpine fir 

Spruce 

Hemlock 

Douglas-fir 

White pine 

Cedar 

Trees 5 inches All Forest 

d.b.h. and above lands 

Acres Acres 

Lodgepole pine 395 395 

Ponderosa pine 170 170 

for the Kaniksu Working Circle were determined by the following d.b.h. classes: 5 to 9 

inches, 9 to 11 inches, 11 to 15 inches, 15 to 

19 inches, and over 19 inches. Crown weight 

per acre was then determined from 

Fahnestock's study for each d.b.h. class using 

trees of average d.b.h. to represent each class. 

The weight of needles was calculated assuming 24 percent of the crown weight is foliage. 

Branches less than one-eighth inch in diameter 

were assumed to make up 6 percent of the 

crown foliage. The weight of crown fuel less 

than one-eighth inch in diameter was calculated as 30 percent of the crown weight. 

(These percentages were derived by weighting 

averages by volume for each species present in 

the Sundance Fire area.) 

Crown weights (ovendry basis) for all 

species in the area are as follows: 

Timber Foliage plus 

size class Crowns Foliage branches < 118" 

Inches (lbs./acre) (lbs./acre) (lbs./acre) 

Total 13,020 3,299 4,087 

A field sample was taken on a 1110-acre plot 

near the confluence of Young's Creek and 

Pack River in a stand of white pine and 

Douglas-fir appearing typical of timber in that 

area. All trees were tallied by species and 

1-inch d.b.h. classes. Crown weights were 

calculated from relationships between tree 

d.b.h. and crown weight (4). Dry-weight 

estimates per acre are as follows: 

Foliage plus 

Species Foliage branches < 118" 

Total 14,256 17,115 (lbs./acre) (lbs./acre) 

White pine 3,620 3,700 A study by Fahnestock (3), relating timber Douglas-fir volume to slash weight, provided estimates of 5,360 6,800 

the crown fuel. First, average timber volumes Total 8,980 10,500 
This plot contained a heavier loading than 

determined for the fire area as a whole and 

was used to represent near maximum conditions. The figure used for average loading did 

not contain crown fuel for trees under 5 

inches d.b.h. and probably should be somewhat higher. 

A 114-milacre plot near Brown's Mill at the 

northeast end of the fire provided an estimate 

of the brush level. The ovendry weight was 

5,400 lbs./acre of subordinate vegetation. 

Brush conditions were probably quite variable 

over the entire area but this plot, appearing 

typical of the area, was used as an average. 

Ground litter was sampled at two locations; 

one near Brown's Mill and the other at the 

southwest end of the fire area near Coolin. 

These 114-milacre plots yielded the following 

loadings (ovendry basis) : 

Brown's Mill Coolin 

(Ibs./acre) (lbs./acre) 

Branches 10,040 

Duff 17,000 23,880 

Buried wood 23,040 

Humus 7,680 

Total 40,040 41,600 

These forest floor plots should be representative of much of the Sundance area although 

considerably lower loadings existed in some 

areas. Moisture content was determined by 

ovendrying as follows: 

- Fuel 

Green foliage 

White pine 

Hemlock 

Brush 

Green and some dead brush 

Sno wberry* 

Forest Floor 

Entire floor (Coolin area) 

Ponderosa pine litter layer* 

Surface duff under spruce* 

Dead branches on spruce* 

Moisture 

content 

(Percent) 

*Samples collected and handled on the Priest River 

Experimental Forest by C. Carpenter. 

The moisture content of logs in the 

Hellroaring Creek area was sampled. Logs 3, 

6, 12, and 18 inches in diameter were sampled 

for center and outer moisture contents. These 

measurements provided some indication of 

the effect of the moisture depletion for the 

season. 

Outer Center 

Log moisture moisture 

Species diameter content content 

(Inches) (Percent) (Percent) 

White pine 3 8.6 8.6 

Douglas-fir 6 32.4 37.4 

White pine 6 15.2 14.9 

Douglas-fir 12 12.7 29.7 

Cedar 12 12.2 26.0 

Cedar 18 11.6 27.3 
METEOROLOGICAL FACTORS 

IN THE FIRE RUN 

The account of the Sundance Fire in the 

text of this paper has emphasized the influence of the weather conditions, particularly 

those that produced the unusually strong 

winds that were a determining factor in the 

fire's phenomenal run. In this section we will 

first look at the general weather situation that 

existed over North America and the eastern 

Pacific Ocean, and then pay special attention 

to the relation of upper-air and surface winds 

in the vicinity of the fire. Comments on the 

relative humidity are also included. 

The Largescale Weather Situation 

For the general weather picture, we will refer to a series of maps depicting atmospheric 

conditions at about 10,000 ft. m.s.1. (figs. 18 

through 22). Maps at this level represent the 

general airflow and temperature pattern in the 

lower troposphere, largely free of irregularities 

induced by local terrain effects in this area. 

The solid lines on these maps are height 

contours; these connect points of equal height 

(m.s.1.) at which an atmospheric pressure of 

700 millibars (mb.) is reached, in analogy to 

contours on topographic maps (which give 

heights of the ground surface). The patterns 

shown by this form of depiction are nearly 

identical to the isobaric (equal-pressure line) 

patterns that would be found at a fixed height 

of 10,000 ft. m.s.1. The contours are drawn 

for 30-meter intervals and are labeled in tens 

of meters. The dashed lines are isotherms, 

drawn at intervals of 5 degrees Celsius. In addition, the winds observed at several individual stations are plotted in conventional 

symbolic form. That is, the windspeed is proportional to the number of barbs on the tail 

of an arrow (head not shown). Each full barb 

represents 10 knots (or 11.5 m.p.h.); a half 

barb indicates 5 knots. The tail of the arrow 

points in the direction from which the wind is 

blowing. 

From the wind arrows in these maps, the 

airflow at 700 mb. is seen to be approximately parallel to the adjacent height contours. The windspeed, at a given latitude, is 

approximately inversely proportional to the 

contour spacing. Thus, the steeper the height 

gradient, the greater the windspeed in that 

area. In the present discussion, we will substitute the term "pressure gradient," which may 

be more dynamically descriptive. 

The first map (fig. 18) depicts conditions 

on the morning of August 30 and illustrates a 

rather abnormal pattern, characteristic of 

much of the summer of 1967. The dominant 

pressure features seen here are (a) a strong, 

extensive warm ridge over the Western United 

States and Canada and (b) a deep trough and 

low center in the eastern Pacific area. This 

trough has been stationary for 3 days. Between the ridge and trough there is a strong 

pressure gradient. Over the interior Northwestern United States, winds are very light; 

the map shows a tendency for slow northward 

movement of warmer air. 

AUBSO.IOB7 

Figure 18. - Analysis of 7Wmb. conditions at 0500 

ms.t. August 30, 1967. For explanation of 

symbols in figures 18-22, see text. Small circle 

in northern Idaho is approximate location of 

Sundance Fire. 
Figure 19. - Analysis of conditions at 0500 m.s.t. 

September 1, with concurrent surface position 

of cold front shown by sharp-toothed line. 

In comparison, the "normal" 700-mb. flow 

over the Pacific Northwest at this time of year 

is rather zonal (west-to-east), with a slight 

trough along the Washington-Oregon coast 

and only a slight ridge over the Canadian 

Yukon. The "normal" refers to conditions 

averaged, and thus smoothed, over the period of a month. Nevertheless, the anomaly in 

the pattern on August 30, 1967, is outstanding. 

Twenty-four hours later (map not shown), 

eastward movement of both the upper pressure trough and ridge had resumed but the 

area of highest temperature remained stationary over the West. The pressure gradient 

and observed winds near the British 

Columbia-Alaskan coast weakened somewhat, 

while a slight increase appeared over the 

Washington-Oregon area. A tendency for slow 

northward movement of warmer air continued over the interior Northwest. Resumption of the trough's movement was in response to a sequence of larger scale, Northern 

Hemispheric circulation features, whose complexity is beyond the scope of this paper. 

The second map (fig. 19), an additional 24 

hours later at 0500 m.s.t. (P.d.t.) September 

1, shows continued eastward movement of 

the upper trough. At this time, a portion of 

the western ridge persisted over the Great 

Basin area. These two factors brought an increased pressure gradient in the WashingtonOregon area; there were southwesterly winds 

of about 40 knots near the coast. The speeds 

approached 25 knots in northern Idaho. 

Warm air persisted over the interior Northwest, but colder air associated with the trough 

had increased the temperature gradient across 

western Washington and Oregon. 

Superimposed on this map as the standard 

sharp-toothed line, is the current (0500) surface map position of a cold front (or possibly 

an occluded front) that had been moving 

slowly eastward toward the Pacific Coast. 

This front is weak, and no pronounced temperature contrast corresponding to it can be 

found at the 700-mb. level. 

By 1700 on September 1 (fig. 20), we see 

that the strongest pressure gradient has 

shifted further southeastward. This has been 

$EPT.l.lO(R 

I? YST 1 I' 

3U) 

Figure 20. - Analysis of conditions at 1700 m.s.t. 

September 1, with surface cold fronts shown as 

in figure 19, except that dissipating Pacifii front 

is shown in dashed form. The frontal position 6 

hours earlier is shown by dotted line. 
Figure 21. - Analysis of conditions at 0500 m.s.t. 

September 2, with surface fronts shown. Warm 

front is shown by round-toothed line. 

brought about by (a) further erosion of the 

ridge over western Canada as the trough 

pushed inland, and (b) a slight diurnal increase in the strength of the ridge in the Great 

Basin area. Southwesterly winds of about 35 

knots are now indicated in northern Idaho, 

while 45 to 50 knots are observed upwind in 

northwestern Oregon. 

At the surface, the weak Pacific front has 

moved inland across British Columbia and 

western Washington. (Position 6 hours earlier 

is given by dotted line.) However, by this 

time, a new cold front has apparently developed east of the Cascades in WashingtonOregon; this is related to increased juxtaposition of the cool and warm air masses (as indicated by the increased temperature gradient 

in figure 20). Also, a separate, more pronounced, cold front has apparently developed 

in the southeastern British Columbia area. 

Twelve hours later, at 0500 on September 

2 (fig. 21), we find the trough has progressed 

further eastward across western Canada and 

has dug into the northern portion of the 

western United States ridge. The offshore porFigure 22. - Analysis of conditions at 1700 m.s.t. 

September 2, with surface fronts - cold, warm, 

and occluded - shown. 

tion of the trough has flattened. Airflow over 

the northwest has thus become more westerly, and the zone of strongest pressure gradient 

and related wind has moved southward of the 

northern Idaho area. At the surface, the original Pacific front has dissipated, while the two 

newly defined cold fronts in Canada and the 

United States continue to move eastward, 

generally steered by the upper flow, retaining 

separate identities. The southern portion of 

the Canadian front has lagged behind that in 

the United States. Some light showers and 

thunderstorms have occurred with the Canadian front, but there have been none thus far 

with the United States front. Part of northern 

Idaho appears to be in a region between these 

fronts. Such frontal detail, possibly induced 

by topography, is not reflected in the upper 

air analysis, which is based on observations at 

stations about 250 to 300 miles apart. 

The final map in this 700-mb. series (fig. 

22) shows conditions 12 hours later at 1700 

on September 2. The pressure gradient over 

the northwestern United States has decreased 

further, and flow is now westerly. Little re- 
L - mains of the trough in the west, while a tendency for ridging appears again. The surface 

fronts, distorted by topographic barriers and 

perhaps other factors, have moved well to the 

east. Light thundershowers occurred during 

*, the late morning or afternoon of September 2 

in eastern Montana, with only showers to the 

north in Canada. 

1 Using the indications of the preceding 

charts (and several not shown here), the areas 

of strongest wind aloft, preceding and during 

the fire run, are summarized in figure 23. A 

speed of 40 knots (46 m.p.h.) has been 

chosen as the lower limit for inclusion. The 

outlined areas were defined on the basis of 

the observed winds and the analyzed pressure gradients. A gradual southeastward to 

east-southeastward progression of strongest 

wind is evident. The zone of such wind 

Figure 23. - Areas of strongest winds (40 knots or Passes through the northern ldaho area at 

greater) at 700 mb., August 29 to September 2, the time of the 2300 P.d.t. observations on 

1967. September 1. 

WINDSPEED 

(KNOTS) TIME 

SPOKANE - . - . -. . . - - 

235 DEGREES 

45 KNOTS 1" 

EGREES A 1 200 DEGREES 

LNOTS 38 KNOTS 40 

17 m.s.t. 05 17 05 17 05 17 05 17 

SEPT. 3 SEPT. 2 SEPT. 1 AUG. 31 AUG. 30 

Figure 24. - Tie graph of mean windspeed in the 5,000 to 7,000-ft. m.J. layer at Spokane and two adjacent 

stations, August 30 to September 3, 1967. (Note that time proceeds from right to left in this and the 

following three figures.) Wind direction in degrees is also shown for the maximum speed for each station. 
Upper-Air and Surface Winds 

in Northern Idaho Vicinity 

We may view the winds aloft from another 

standpoint, in the form of a time graph for an 

individual station. Figure 24 shows such a 

graph of the mean windspeed in the 5,000 to 

7,000-ft. m.s.1. layer for Spokane, 70 statute 

miles southwest of Sundance Mountain. For 

comparison, the corresponding windspeeds 

are given for two adjacent upper-air stations 

(to the west and east). These stations are 

Quillayute, on the northern Washington coast, 

and Great Falls, Montana. A pronounced peak 

in speed in the 5,000- to 7,000-ft. layer is seen 

to have occurred at all three stations. The 

peak at the coastal station occurred approximately 12 hours before that at Spokane, 

whereas the peak at Great Falls occurred 

about 12 hours after that at Spokane. The 

peak speed in this layer, 45 knots (from the 

southwest), is highest at Spokane; however, 

nearly as high a speed, 43 knots, was observed 

in a lower layer (2,000-4,000 ft. m.s.1.) at the 

coastal station. This diagram gives further 

evidence that the strong lower level winds in 

the Sundance Fire vicinity were a more or less 

sustained, broad-scale, moving feature which, 

at least in retrospect, was trackable. 

Sustained windspeeds of 40 to 45 knots at 

5,000 to 7,000 ft. m.s.1. appear to be exceptional in the northern Idaho area during the 

fire season. This is the impression gained from 

an examination of Spokane winds-aloft reports for the period June 15 to September 15 

of the years 1957 through 1967. Speeds of 38 

knots or greater at 5,000 feet appear to occur 

on an average of only 2 days per season 

(based on four wind soundings per day). In all 

but one of 12 actually observed cases in this 

11-year period (wind soundings were made 

only three times per day during five of the 

years), the days with such windspeeds occurred in June or September. The wind direcFigure 25. - Time graph comparing free-atmosphere windspeeds at Spokane and surface or tower windspeeds 

at several observation points, September 1-2,1967. 
tion in all but one case was southwesterly or 

west-southwesterly, with some sort of upper 

trough located near the Washington coast. 

We have thus far looked at wind indications from upper-air observations. An obvious 

question is how do these winds aloft relate to 

those actually observed or expected at 

ground- or tree-level locations. Winds in 

mountain areas may, of course, vary considerably according to exposure to the general airflow. In addition, surface friction acts to reduce the windspeed near the ground, but 

terrain-induced turbulence (proportional to 

gradient windspeed) may bring strong gusts. 

Locally, winds may be increased by terrainenforced convergence of airflow. Figure 25 

compares the free-atmosphere winds observed 

above Spokane with the winds at several surface stations. 

In the Sundance Fire vicinity, hourly wind 

data were available, except for several nighttime hours, from Lunch Peak, at 6,400 ft. 

m.s.1. 20 miles to the southeast. A mobile 

fire-weather station, sent from Boise, was in 

operation here for the Plume Creek Fire. A 

rather abrupt rise in the Lunch Peak windspeed is noted between 1600 and 1700 P.d.t. 

on September 1. Winds from the southwest 

soon reached an average of approximately 35 

m.p.h. at 1800 and 1900 P.d.t.; gusts (not 

plotted in the diagram) were up to 50 to 55 

m.p.h. for several hours. 

Roman Nose Lookout, at 7,264 ft. m.s.1. 

on the northern edge of the fire, had a 12 

m.p.h. southerly wind at the regular 1600 

P.d.t. fire-weather observation time; a steady 

speed of 35 to 40 m.p.h. from the southwest 

was reported at 2015 P.d.t. (note point 

plotted in figure 25). A lookout atop Mt. 

Henry (7,235 ft. m.s.l.), in extreme northwestern Montana, reported winds averaging 

40 m.p.h. and gusting to 60 m.p.h. (time not 

given). 

Among the mountaintop fire-weather stations reporting once daily, four of the seven 

in northeastern Washington (at elevations 

ranging between 3,800 and 5,900 ft. m.s.1.) 

had windspeeds between 27 and 33 m.p.h. at 

1600 P.d.t. I Speeds at five such stations in the 

Idaho panhandle (at elevations between 5,200 

and 6,400 ft. m.s.1.) were as yet only between 

12 and 16 m.p.h. To illustrate possible 

terrain-exposure effects, Squaw Peak, 

Montana (at 6,200 feet), nearby to the southeast, had 29 m.p.h. winds at this time. 

The Lunch Peak (and Roman Nose Lookout) windspeeds fit in reasonably well with 

the 5,000- to 7,000-ft. wind trend at Spokane 

during the late afternoon and early evening of 

September 1. The same can be said for the 

winds atop a 400-ft. tower at the Hanford 

A.E.C. Works, ground elevation 700 ft. m.s.l., 

about 120 miles southwest of Spokane. The 

available Lunch Peak and Hanford speeds differ considerably, however, from those of the 

free-atmosphere winds at Spokane later in the 

evening. This difference is probably associated 

with variations in the vertical, turbulent transfer of momentum through the "friction 

layer." This layer would extend mainly to 

near 5,000 ft. m.s.1. at Spokane and to correspondingly higher altitudes above the mountainous area in northern Idaho. Although reference is made in this paper to freeatmosphere winds at 5,000 ft. m.s.1. in the Sundance area, such winds are largely hypothetical. 

It may be well to clarify here the relationship between the strong winds in the Sundance area and the movement of a cold front. 

The strong, or relatively strong, and gusty surface winds observed in eastern Washington 

and nqrthern Idaho on September 1 began by 

middle or late afternoon. This onset took 

place, at high and low elevations, approximately 100 to 150 miles in advance of the 

cold front that had developed east of the Cascades. Cooler-air movement, representing a 

gradual change of airmass, was, however, already accompanying these prefrontal winds. 

The front in this case thus appears to represent a secondary steepening of horizontal 

temperature gradient imbedded within a 

broader gradient. The peak in 5,000-ft. m.s.1. 

f ree-atmosphere windspeed, which was 

reached in the late evening, occurred near or 

slightly behind the surface map projection of 

this eastward-moving front. But, as indicated 

earlier, the front was rather diffuse or undetectable in northern Idaho and extreme 

eastern Washington. 

In the absence of more detailed data coverage, the actual effect of this diffuse front (or 

frontal zone) on the Sundance surface winds 

remains a matter for speculation. It is, however, apparent that the front alone could not 

have accounted for the unusually strong, 
gusty winds which persisted for several hours. 

These winds were ultimately dependent upon 

the exceptional summertime pressure gradient 

that existed in the lower levels of the troposphere. The extent to which winds of gradient 

strength or greater were experienced near the 

surface would, of course, vary, as previously 

discussed. Turbulence commonly attending a 

cold front may or may not have been an important triggering factor in this case. 

The strong pressure gradient and the cold 

front, both associated with the approaching 

trough, may be regarded as individual but 

related parts of a weather system. Basic to 

such a system is the juxtaposition of two different (warm and cool) airmasses. The upper 

tropospheric jet stream, whose core in this 

case was at only 25,000 to 30,000 ft. m.s.l., 

would also be part of this system. Moving 

eastward, its axis at 2300 P.d.t. September 1 

was about 100 miles northwest of Spokane 

and the Sundance area. The windspeed over 

Spokane was approximately 80 knots (interpolated), having increased from 60 knots at 

1700 P.d.t. Such a jet could, through dynamic 

processes, possibly effect a vertical transfer of 

momentum (and other properties) to the 

lower troposphere. This would occur within a 

frontal zone sloping below the jet's core, in the 

form of a dry-adiabatic sinking of air from the 

upper tropospheric portion of the zone (located many miles upwind of its lower portion) 

(6). Some evidence was found supporting the 

presence of such an effect in the Sundance 

case, but was regarded as inconclusive. 

The estimated hourly free-atmosphere (gradient) windspeeds at 5,000 ft. m.s.1. in the 

Sundance area, used in this fire report, are 

given in figure 26. These speeds are based 

upon the 6-hourly upperwind observations at 

Spokane. Observed surface winds, referred to 

earlier, were used as a guide in allowing for a 

slight timelag in adapting a vertical crosssection analysis of Spokane upper winds (not 

shown) to the Sundance area. The lag, adjudged also according to spatial differences in 

pressure gradient on the 700-mb. maps, did 

not exceed 1 hour and was adjudged to be 

zero after the time of maximum wind. 

Humidity 

Also plotted in figure 26 is a curve of estimated "modified" free-atmo sphere relative 

humidity at 5,000 ft. These humidity values 

are a compromise between those in the free 

atmosphere and those observed near ground 

c TIME 1 80 

20 16 12 08 04 24 20 16 12 08 04p.d.t. 

SEPT. 2 SEPT. 1 

Figure 26. - Time graph of estimated free-atmosphere windspeed and "modified'' free-atmosphere relative 

humidity at 5,000 ft. m.s.l., Sundance Fire area, September 1-2, 1967. 
level on a ridge or mountaintop. Relative 

humidity is, of course, a function of both 

temperature and actual water-vapor content 

of the air. The values given here were derived 

by use of Spokane surface and upper-air observations and the hygrothermograph charts 

from both Gisbome Mountain Lookout 

(about 10 miles south of the Sundance Fire 

area) and Big Spot mobile weather station 

(about 25 miles north of Sundance). Good 

consistency was found among the temperature and vapor pressure tendencies from these 

data sources. The Lunch Peak data, however, 

were not used here; the observed temperature 

values and behavior showed discrepancy with 

those at the above stations and at 6,424-ft. 

Sunset Peak, Idaho (70 miles SSE of the Sundance Fire), and were considered unrepresentative of the general area. 

A relative humidity threshold of about 30 

percent has been found empirically by some 

researchers to be critical in the preconditioning of fine fuels. The estimated relative 

humidity for the Sundance Fire environment 

is seen to have remained below 30 percent on 

September 1 until after 2200 P.d.t., about the 

time of maximum estimated gradient wind. A 

steady increase then occurred, raising the 

humidity to 75 percent the following moming. For comparison, the low relative humidity that persisted for several preceding days 

and nights is shown in figure 27. The relative 

humidity increase (September 1-2) was related primarily to the decrease in "modified" 

free-atmosphere temperature (to 51" F. at 

5,000 ft. m.s.l.), rather than to any large increase in vapor content of the air as compared 

with preceding nights. 

Concluding Remarks 

In summary, we have described the general 

weather situation associated with the Sundance Fire run and have examined more 

specifically the related wind and relative 

humidity. Additional research, not reported 

here, examined other considerations stated in 

the literature to be possibly important in 

rapid or "explosive" fire spread. Primarily 

these are (a) vertical wind profile (change of 

speed and direction with height); (b) airmass 

instability; (c) subsidence; and (d) downdrafts 

from adjacent thunderstorms. The last-named 

factor may be safely regarded as absent in the 

Sundance Fire case. Findings with respect to 

the first three, not particularly crucial, will 

appear in a forthcoming separate, more complete meteorological paper. 

The above-presented weather factors must 

be considered together with other factors, 

such as fuels and topography, in accounting 

for the fire behavior. With this in mind, it is 

nevertheless quite apparent that unusually 

strong winds occurred in the Sundance 

vicinity at a most unfortunate time. 

70 r C TIME 

0 I I I I 

v I I I I 

00 P.d.1. FRI. THURS. WED. TUES. 

91 1 8/31 8/30 8/29 

Figure 27. - Time graph of estimated "med7' free-atmosphere relative humidity at 5,000ft. m.s.l., Sundance 

Fire area, for several days prior to and including September 1,1967. 
THE PACK RIVER BRIDGE AS AN AREA 

FIRE INTENSITY INDICATOR 

The Sundance Fire burned with extreme 

intensity in many areas, as exhibited by rock 

spalling and complete byrnout of ground 

fuels. The area where the fire crossed the Pack 

River is a notable example. Here, the visitor is 

awed by the coupled effects of high winds 

and intense fire activity. Nearly all the large 

trees - cedars and hemlocks - have been uprooted or broken and are lying charred among 

the grey ash remains of ground fuels. Also, 

the remains of the Pack River Bridge are mute 

evidence of the fire's passing. There has been 

much speculation about the intensity necessary to result in the complete burnout of the 

approach spans and distortion of the carbon 

steel plates covering the deck runway. This 

short study is intended to establish the relationship, if any, between fire intensity at the 

bridge and in the surrounding area. 

Extent of Fire Damage 

to the Bridge 

The bridge spans the Pack River in a nearly 

north-south direction near the McCormick 

Creek inlet as 'shown on the foldout map. 

Three connected segments make up the 

bridge: the center span resting on two concrete piers in the stream bed and two approach spans resting on concrete footing on 

the streambank. The design is shown in figure 

28. The center span is 65 ft. long by 14 ft. 

wide. Each approach span is approximately 

24 ft. long by 14 ft. wide. The decks of the 

approach spans are 6 inches thick, and are 

made up of laminated 2- by 6-inch boards. 

Strength is given to the deck by nine traverse 

beams 8- by 20 inches by 24 feet, spaced 

along the length of the span. Carbon steel 

plates 5116-inch thick were also placed on the 

deck runways to provide traction and reduce 

wear. The primary structural material used 

was creosote-treated Douglas-fir. 

Fire damage to the bridge was confined 

essentially to the approach spans; although 

there was evidence of burning at each end of 

the central span, the major portion of it was 

unscorched by the surrounding fire activity. 

Figure 17 (main text) shows the burnout of 

the north approach span. Figures 29 and 30 

show other views of the bridge. Notice the 

unburned paint on the central span shown in 

figure 29. Figure 30 shows the complete burnAbut. No. 1 Pier No. 1 Pier No. 2 Abut. No. 2 

Figure 28. - Side elevation of Pack River Bridge. 
Figure 29. - View of Pack River Bridge looking southeast. 

out of one of the approach spans and the deformed steel plates. The other span is identical in appearance. It is apparent from the 

photograph that the steel plates had been sufficiently softened by the heat to conform to 

the shape of the surface they fell upon when 

the span gave way. 

Hypotheses of Ignition 

and Burning 

The probability of ignition of the bridge is 

of course dependent on the residence time of 

the fire in the surrounding area. Anderson3 

gives us a relationship between 8, the residence time in minutes, and D, the particle 

diameter in inches. 

8=8D 

From evidence of burned limb stumps, 2 

inches in diameter, we estimate a residence 

time in the surrounding fuels area of 16 

minutes. Limbs of larger diameter would help 

sustain the residence time past 16 minutes; 

this would certainly be enough time to ignite 

the brush near the approach spans (the remains of this brush may be seen in figure 17). 

The burning brush would then act as an ignition source for the bridge. 

The importance of closely positioned kindling fuels is demonstrated by the fact that 

fire damage to the central span was limited, 

owing to the absence of a nearby source of 

ignition. Except for occasional hot flames the 

region just above the creek bottom would be 

relatively cool compared to the surrounding 

area. We conclude that fire activity in the 

central span near the concrete piers was 

simply an extension of the fire in the approach spans. During the period of burning, 

winds near the bridge were predominantly upstream or downstream, that is, normal to the 

bridge, as evidenced by the creosote smoke 

deposited on the steel beams on either side of 

the central span. Consequently, we believe the 

fire traveled crosswise to the wind and simply 

burned out near the concrete piers. If there 

had been ignition on the windward side of the 

central span, more extensive damage would 

have occurred in that section. 

Figure 30. - Burnout of north approach span, Pack 3 Anderson, Hal E. Heat transfer and fire spread. U.S.D.A. 

River Bridge, showing of surface Forest Sen., Intermountain Forest and Range Exp. Sta., plates. Ogden, Utah. (In preparation.} 
Decking 1- Steel surface plate 

- 

Wind 

/ - Eddy 

cavity 

Figure 31. - Cross section of approach span of bridge, showing supposed pattern of wind eddy formation 

within cavities. 

Once the bridge had been ignited, enhancement of the burning can also be attributed to the long rectangular cavities between 

the beams located below the bridge. It is hypothesized that wind blowing over these cavities perpendicularly to the beam axis or 

bridge direction produces eddies on the leeward side of the beams within each cavity, 

as illustrated in figure 31. As the windward 

side burns, flames are entrained into the 

eddies on the leeward side. Thus, combustion is propagated into the adjacent cavity. 

In this situation, only a limited amount of 

air is entrained into the cavity with the 

flames so that combustion of only a portion 

of the volatilized gases from the inside surface can be completed within the cavity. As 

the heat builds up, an overabundance of the 

volatilized gases is produced within the cavity. 

Thus, the proper fuel air mixture is not attained until the gases are swept out of the 

cavity into the outside air. However, just as 

these gases leave the cavity and begin flaming combustion they are entrained into the 

next cavity and so the cycle is repeated until 

all the cavities are burning. Notice also, in 

figure 31, that the two walls formed by the 

beams are advantageously located for enhancement of burning by reradiation between the 

walls. 

There is no direct evidence to support the 

hypothesis that eddies were formed in the 

cavities below the deck. However, a reconstruction of the bridge fire under laboratory 

conditions gives us a qualitative examination 

of eddy production that could lead to support 

or rejection of the hypothesis. For our purpose a cross-sectional slice of the beam axis of 

the bridge-approach span is sufficient. Since a 

qualitative examination is all that is possible, 

there is no strict adherence to scaling. The 

deck is made up of 1%- by %-inch white pine 

lumber and the beams are replaced by 2- by 

4-inch Douglas-fir. Two coats of diesel oil 

were applied to the model to simulate the 

creosote treatment. 

The simulated bridge section was placed in 

the wind tunnel with the wind flowing at 

right angles to the beam axis. To eliminate 

burning on the exposed cross-sectional faces it 

is necessary to cover both faces with sheet 

metal. Two bridge sizes were burned in the 

wind tunnel under identical conditions, the 

only difference being in the width. In the first 

test the bridge was 42 inches wide and in the 

second 17 inches. In both cases ignition by 

simulated ground fuels (excelsior) was on the 

windward side. Because of the size'difference 

ignition was confined to the first few cavities 

in case 1 and extended over the entire bridge 

section in case 2. 
Figure 32. - Model of bridge approach span (42-inch size) burning in wind tunnel. 

These two cases simulated two possible the creekbed acted as a relatively unoblevels of ignition. Although the ignition was structed trough for the flow of air. 

different, the circulation patterns in the cavities are identical. In the photograph, figure 32, 

a similarity to the expected eddy patterns 

shown in figure 31 is evident. There is great 

likelihood then that eddy patterns were created in the cavities below the deck, enhancing 

the fire activity at the bridge. 

Creosote preservative on the wood structures of the bridge and vegetation near the 

bridge approaches certainly contributed to 

the ignition process; however, the wind and 

the associated eddies produced within the 

cavities under the bridge are of greater irnportance. If there had been no cavities, it is likely 

that burning would have been greatly reduced 

except at the edges, resulting in much slower 

burning and possibly more unburned residue, 

rather than the complete burnout of the approach spans that actually occurred. 

Winds at the bridge site were primarily fire 

induced and generated in the drainage in an 

area remote from the bridge. They were "indraft" winds resulting from strong convective 

upward flow at the generating source. These 

winds were sustained as other areas in the 

drainage burned, creating additional generators. Furthermore, there were enough other 

areas to sustain winds for the equivalent of at 

least four residence times, thus accounting for 

1 hour of intense fire activity at the bridge 

site. This was certainly enough time for the 

bridge to have achieved sustained combustion 

without the necessity of continued sources of 

wind and heat. It is very likely that these indraft winds were present at the bridge because 

Damage to Steel Runways 

Heat transfer to the steel runways was sufficient to cause a large reduction in the tensile 

strength as evidenced by the distortion of the 

plates shown in figure 30. Considering the density of steel we may assume that it would deform appreciably under its own weight if it lost 

three-fourths of its peak strength. Examination 

of the variation in steel strength with temperature indicates that a reduction of strength of 

that extent would be achieved at 1200" F. This 

is in no way indicative of the temperature outside the immediate bridge area. Contrary to 

news reports, there is no evidence that the steel 

had reached the melting point, 2,800" F. 

Conclusions 

An examination of the bridge and the 

simulated bridge fire suggests that: 

1. The bridge approaches were ignited by 

ground fuels. 

2. Once ignited, the bridge continued to 

burn independent of the heat supplied by surrounding forest fire. 

3. The bridge design contributed to the 

fire damage, since eddies formed by the wind 

normal to the bridge produced and captured 

pockets of burning gas on the underside of 

the bridge. 

4. The distortion of the steel plates merely 

indicates that a fire was present long enough 

to heat the plates to 1,200" F. or more (but 

not up to the melting point, 2,800" F.) 

5. The bridge fire is not an indicator of 

the intensity of the surrounding forest fire. 
1. Berlad, A. L., and R. S-L. Lee. 

1967. Long range spotting. Combustion and Flame 12:127. 

(Also presented at the Nov. 1967 meeting of the Eastern 

States Section of the Combustion Institute.) 

2. Davis, Kenneth P. 

1959. Forest fire: control and use. 584 pp. New York: 

McGraw-Hill Book Co. 

3. Fahnestock, G. R. 

1960. Logging slash flammability. U. S. Forest Serv., Intermountain Forest and Range Exp. Sta. Res. Pap. 58, 67 

PP* 

4. -, and J. Kittredge. 

1944. Estimation of the amount of foliage of trees and stands. 

J. Forest. 42: 905-912. 

5. Lommasson, T. E., and J. A. Keller. 

1967. A macroscopic view of fire phenomenology and mortality prediction. DASA Pub. 1949, pp. 228-239. 

6. Reiter, E. R. 

1963. Jet stream meteorology. 51 5 pp. Chicago: Univ. Chicago 

Press. 

7. Rothermel, Richard C., and Hal E. Anderson. 

1966. Fire spread characteristics determined in the laboratory. 

U. S. Forest Serv. Res. Pap. INT-30, 34 pp. 

8. Tarifa, C. S. 

1967. Transport and combustion of firebrands. U. S. Forest 

Serv., Final Report of Grants FG-SP-114 and FG-SP-146, 

Vol. 11, 90 pp. 

CANADA 

Chimney 

AROC~ 
SCALE I" = 1 MILE 

OUTER SOLID LINE SHOWS TOTAL EXTENT OF 

BURNED AREA ON SEPT. 3. LABELED INNER LINES 

SHOW HOURLY PROGRESS OF 'FIRE ON SEPT. 1. 

EXCEPT WHERE NOTED. ISOLATED SYMBOLS CORRESPONDING TO FIRE EDGE LINES INDICATE 

SPOT FIRES BURNING IN THE DIRECTION OF 

THE ARROWS. 

P 

BRYAN OWEN-66 
Anderson, Hal E. 

1968. Sundance Fire: an analysis of fire phenomena. U. S. Dep. Agr., Forest 

Serv., Intermountain Forest and Range Exp. Sta., Ogden, Utah 

84401. 39 pp., illus. (U. S. Forest Serv. Res. Pap. INT-56) 

The Sundance Fire on September 1, 1967, made a spectacular run of 16 miles in 9 

hours and destroyed more than 50,000 acres. This run became the subject of a 

detailed research analysis of the environmental, topographic, and vegetation variables aimed at reconstructing and describing fire phenomena. This report details the 

fire's progress; discusses the fire's buildup in intensity, the fuel complex through 

which it traveled, the wind and other atmospheric variables affecting the fire's 

behavior; and describes the processes that probably account for the tree breakage 

and blowdown, the long-range spotting, and the subsidence of the fire's run. 
The Forest Service of the U. S. Department of Agriculture 

is dedicated to the principle of multiple use management 

of the Nation's forest resources for sustained yields of 

wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with th e States and private 

forest owners, and management of the National Forests 

and National Grasslands, it strives - as directed by Congress - to provide increasingly greater service to a growing 

nation.