WIND-BREAKING POWER OF FORESTS. 37 



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, corresponding to the 

 "chinook" of our western country. 



The change in temperature from the normal, experienced under 

 the influence of the Fcehn, has been noted as from 28 degrees 

 to 31 degrees Fahr.. and a reduction of relative humidity of 58 

 per cent. A Foehn of twelve hours' duration has been known 

 to "eat up" entirely a snow cover 2.^/2 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 considerable ele- 

 vation 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 considerable distance and height 

 beyond the shelter. 



Professor 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. 



At the Dominion Experiment Station in Assiniboia Dr. 

 Saunders found on one occasion that windbreaks exerted an 

 appreciable influence at from 50 to 80 feet to leeward for every 

 foot in height, but this was during a very severe wind. It may 

 probably be laid down as a general rule that windbreaks will 

 exert an appreciable influence for at least one rod for every foot 

 in height. 



It may not be necessary to state that the damage done to 

 crops by the cold, dry winter winds is mainly due to rapid 

 evaporation, and that plants are liable to suffer as much by win- 

 ter (Jrouth as by summer drouth. 



