258 FOREST PROTECTION 



rial for carrying fire down a slope. Where slopes become 

 precipitous and barren of vegetation they form effective bar- 

 riers to the spread of fire. 



A stream or narrow strip of moist ground at the bottom of 

 a ravine may serve to stop a fire. The latter has burned 

 slowly down the slope and on approaching the stream has not 

 sufficient momentum to leap this obstacle. 



Slopes with northern aspect do not suffer so severely as 

 those with southern exposure. This is explained by the greater 

 dryness of the fuel due to the heat of the sun on the more 

 exposed southern slopes. 



A smooth surface on which the inflammable material oc- 

 curs uniformly distributed tends to make the fire burn more 

 evenly and intensely. Bare rocks tend to delay and break up 

 the front of the fire. 



Atmospheric Conditions. — Atmospheric conditions in in- 

 fluencing forest fires act either through affecting the dryness 

 and inflammability of the fuel or by fanning the flames and 

 increasing the speed of the fire. 



Wind. — The wind is a factor of primary importance in 

 determining the spread and severity of fires. By its powerful 

 drying and evaporating action it increases the inflammability 

 of fuel and also furnishes the draft necessary for the rapid 

 spread of the flames. 



The velocity of the wind is the most important single factor 

 determining the rate at which a fire advances. 



The steadiness of the wind has great influence particularly 

 in crown fires. For such fires a wind of high velocity is 

 required to carry the flames from tree to tree. It must also 

 be steady because when the wind slackens the fire dies down 

 in the tops and burns along the surface, from which it may 

 jump to the crowns when the wind rises. 



The occurrence of a high wind may be inferred as an ac- 



