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 
lero eels rial eee fe Mey seals, | ate | 
S)e/s/<| 3/4/48) 4| 8/6) 2) alee 
Mean daily maximum....... 35 | 36 | 47 | 56 | 63 | 72 | 83 | 83 | 70} 56| 42| 35 | 56 
Mean daily minimum....... 22 | 24| 31 | 37| 43 | 50| 55 | 54| 46 | 38 | 31 | 24 | 38 
Mean: @ SE ete eeu cie oes ac 29 | 32 | 38 | 47/53 |59| 67 | 66| 58! 40] 38132] 47 
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 
