314 



The Farm Woodlot 



84 and a relative humidity 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 



FIG. 62. The results of wind erosion in the bad lands. 



6.3 times as much water as a calm atmosphere of the 

 same temperature and humidity. 



When we consider that the average velocity of the winds 

 that constantly sweep the western arid and subarid 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. 



