32 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



evaporation was notably greater in the lowest type, least in the cen- 

 tral, and intermediate in the highest. 



The high evaporation in the oak-brush type is clue chiefly to high 

 temperatures and low relative humidity. In the spruce-fir type the 

 high evaporation is accounted for chiefly by the high wind velocity, 

 as will be shown later. Owing to the aspen-fir type being inter- 

 mediate in elevation between the other two, and having a notably 

 heavier stand of vegetation, especially tree growth, the factors in- 

 fluencing evaporation are in no instance extreme. 



FIG. 15. Monthly evaporation from spherical atmometcrs and corresponding precipitation 



in type stations, 1916. 



WIND VELOCITY. 



Largely because of the physiographical features, the velocity of 

 the wind is notably greater in the spruce-fir type than in the lower 

 associations. The comparative intensity of this factor may be ap- 

 preciated readily by summing the daily wind movement by monthly 

 periods. Since the wind velocity during the growing season is prob- 

 ably an influential factor in the development of the vegetation, 

 data are presented in Table 10 showing the wind movement during 

 the growing seasons of 1915 and 1916. 



