EVAPORATION. 133 



tweeu tlie trunks, and this intlueucc npon available water supply, the 

 ivind-hrealc'mg poicer of the forest, must be considered as among the 

 most important factors of water preservation. Especially is this the 

 case on the Western plains and on those Western mountain ranges bear- 

 ing only a scattered tree growth and Avhere, therefore, the influence of 

 shade is but nominal. 



The evaporation under the influence of the wind is dependent not 

 only on its dryness, but also on its velocity, which being impeded, the 

 rate of evaporation is reduced. 



Interesting experiments for the purpose of ascertaining the changes 

 in the rate of evaporation elfected by the velocity of the wind were 

 made by the Signal Service in 1S87. The result of these experiments 

 (made with Piche's hygrometers whirled around on an arm 28 feet in 

 length, the results of which were compared with those from a tin dish 

 containing 40 cubic centimeters of water exposed under shelter) show 

 that with the temperature of the air at 84 degrees and a relative hu- 

 midity of 50 per cent., evaporation at 5 miles an hour was 2.2 times 

 greater than in a calm; at 10 miles, 3.8; at 15 miles, 4.9; at 20 miles, 

 5.7; at 25 miles, 6.1, and at 30 miles the wind would evaporate 0.3 

 times as much water as a calm atmosphere of the same temx)erature 

 and humidity. 



Now, if it is considered that the average velocity of the winds which 

 constantly sweep the Western subarid and arid plains is from 10 to 15 

 miles, not rarely attaining a maximum of 50 and more miles, the cause 

 of the aridity is not far to seek and the function of the timber-belt or 

 even simple wind-break can be readily appreciated. 



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 Foehn, has been noted as from 28° to 31"^ F. and a re- 

 duction of relative humidity of 58 per cent. A Foehn of twelve hours' 

 duration has been known to "eat up" entirely a snow cover of 2^ feet 

 deep. 



In Denver a chinook has been known to induce a rise in temperature, 

 of 57° F. in twenty-four hours (of which 36° 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 elevation above the trees. This 

 can be practically demonstrated by passing along a timber j^lantation 

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

 higher than 5 feet will absolutely calm the air to a considerable dis- 



