14 A Study of the Vegetation of 



freeze deeper than 4-6 inches. The snowfall normally is about 

 46 inches. The mean monthly and annual temperatures at the 

 base station are given in Table I. It is interesting to note that the 

 mean annual isotherms are approximately the same as those of 

 eastern Nebraska. These are 52 F. to 47 F. These figures 

 do not include the canyon of the Snake River, which has a dis- 

 tinctly warmer and drier climate. 



TABLE I 

 TEMPERATURES AT PULLMAN, WASHINGTON 



Since it is not the mean temperature of a region but rather the 

 extremes that exert the most profound effect upon plants, Table 

 I also includes the mean daily minimum and mean daily maximum 

 temperatures. 



Another important climatic factor is the wind. It is prevail- 

 ingly from the southwest. Just as it has had such a profound 

 effect upon moulding the topography of the region, likewise it 

 has a marked effect upon the vegetation. Sweeping across the 

 many miles of open prairie and desert of northern Oregon and 

 south-central Washington, and not infrequently attaining a 

 velocity of 30-50 miles per hour, it has a marked effect upon 

 the evaporating power of the air. By increasing the latter, it 

 increases the saturation deficit and consequently accelerates 

 transpiration. During the growing seasons of 1913-1914, the 

 wind blew from the southwest, or some southerly or westerly 

 direction over 60 per cent, of the time. The southwest winds 

 often assume the form of dust storms in summer. For days the 

 air may be so filled with dust that the sun is scarcely visible. 

 Such storms may become serious enough to cause much destruc- 

 tion. 



Because of the deficient rainfall and the low humidity through- 



14 



