SPREAD AND SEVERITY OF FOREST FIRES 257 



material can be reached and consumed and all trees affected 

 are killed. 



A crown fire burning in the tree tops where the individual 

 limbs and trees are frequently not touching requires a high 

 wind to carry the flames from point to point and very dry 

 easily ignited material. It may kill all the trees in its path, 

 but yet for the heat imits produced a crown fire is not so 

 destructive as a quiet burning ground fire with its fine grained 

 material. 



Topography. — The slope, aspect and surface conditions 

 each affect spread and severity. In a rugged coimtry, due to 

 the frequent and wide variations in topography, the progress 

 of a fire is extremely irregular. Regions without distinguish- 

 ing topographic features favor a more uniform development of 

 all sections of the fire. 



A fire burning uphill advances rapidly. The heated 

 air rising vertically and radiating horizontally.' passes near 

 (particularly in the case of very steep slopes) the groimd 

 ahead of the fire and by its heating and dr5dng action 

 hastens ignition and increases the intensity of the fire. 

 Trees are injured worst on their uphill sides, both because 

 humus and litter are apt to accumulate there, and because 

 the flames protected by the tree trunks from the draft 

 bum longer in proximity to the tree. When a fire runs up a 

 slope to the summit of a narrow ridge burning brands are 

 likely to be carried over and dropped farther ahead than is 

 the case on level ground. Fire progresses down hill very 

 slowly and relatively feebly, burning as it must against the 

 upper draft of heated air. Where the available fuel is of 

 such a character as to roll down hill easily, burning sticks or 

 cones may set fires some distance below the original fires. 

 Fires so set are apt to sweep up the slope quickly until they 

 meet the other fire. Cones furnish the most dangerous mate- 



