30 ELEMENTARY FORESTRY. 



our prairie sections know that great damage is often done to 

 wheat, corn and othercrops by the hot south-west winds which 

 we are likely to have during the growing months. In Kansas 

 and Nebraska during the summer of 1894 immense tracts of 

 corn, fully tasselled out, were killed by such winds. At the 

 same time it was noticed that where corn was protected by 

 trees or slopes of land or where the humidity of the wind was 

 increased by passing over bodies of water or clover fields, 

 the injury was greatly lessened. 



"What the possibilities of evaporation from hot and dry 

 winds may be can be learned from statements regarding the 

 "Foehn," which is the hot wind of Switzerland, correspond- 

 ing to the "chinook" of our western country. 



"The change in temperature from the normal, experienced 

 under the influence of the Foehn has been noted as from 28 

 degrees to 31 degrees Fahr., and a reduction of relative 

 humidity of 58 per cent. A Foshn of twelve hours' duration 

 has been known to "eat up" entirely a snow cover $ feet 

 deep. 



"In Denver a chinook has been known to induce a rise in 

 temperature of 57 degrees Fahr. in twenty-four hours (of 

 which 36 degrees in five minutes) while the relative humidity 

 sank from 100 to 21 per cent. 



"The degree of forest influence upon rate of evaporation 

 by breaking the force of winds is dependent upon the extent 

 and density of the forest, and especially on the height of the 

 trees. For according to an elementary law of mechanics the 

 influence which breaks the force of the wind is felt at a con- 

 siderable elevation above the trees. This can be practically 

 demonstrated by passing along a timber plantation on the 

 wind-swept plains. Even a thin stand of young trees not 

 higher than five feet will absolutely calm the air within a con- 

 siderable distance and height beyond the shelter." 



Prof. King found that an oak grove 12 to 15 feet high 

 exerted an appreciable effect in a gentle breeze at a distance 

 of 300 feet. In a strong wind the effect of such a grove would 

 be felt at a much greater distance to the leeward. It may 

 probably be laid down as a rule that a wind break will afford 

 some protection for one rod to the leeward for every foot in 

 height. 



