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 fire- 

 brand activity was also instrumental. The ter- 

 mination of the run was influenced by the 

 decrease in windspeed and the increase in 

 humidity, and again by the firebrand activity 

 that caused bumed-out areas ahead of the 

 main front. In addition the terrain was sloping 

 downhill and prior logging activities had re- 

 moved part of the fuel continuity. Like any 

 fire, this one was controlled by nature's in- 

 finite variety of conditions and their combina- 

 tions. The best we can do is assess the influ- 

 ence 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 cer- 

 tain 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 topo- 

 graphic diagram (fig. 1), the windspeeds re- 

 ported 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 ad- 

 vance can be attributed to the slope and wind. 

 The upslope terrain, exposed to southwest air- 

 flow from across a broad valley, allowed a 

 crowning fire to become established at a 

 lower windspeed. The fire probably accel- 

 erated 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 down- 



slope. 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 maxi- 

 mum in selected areas. Work carried out by 

 Lommasson (5) on mass fires or fire storms 



1200 1400 1600 1800 2000 2200 2400 



Time of dav Ihours) 



Figure 13. — Rate of fire ^read during a 12-hour 

 period on September 1, 



17 



