Southeastern Washington and Adjacent Idaho. 13 
tween the distribution of rainfall of this region and that of the 
Great Plains east of the Rocky Mountains, Fig. 2 also includes 
the average monthly rainfall at North Platte, Nebraska. It also 
gives the precipitation at La Crosse, a station in the western part 
of the region. 
However, it is not the absolute rainfall figures alone which 
furnish a criterion of climate, for the maximum duration of the 
a i fe AGAR 
a 
Ree 
Fic. 2. Mean monthly precipitation in inches at Pullman and La Crosse, 
Washington, and North Platte, Nebraska. 
PULLITAN si talelel || VORTH Re sy TE ana 
drought period constitutes a limiting factor of the greatest im- 
portance. 
During the dry summer months the sky is usually cloudless. 
At the base station the number of clear days during the growing 
season (April—October) was 68 per cent. for the years 1914 and 
1915. Bright, warm days are followed by cool nights. In fact, 
a notable feature of the temperature of the region is the great 
variation between day and night temperatures, especially in sum- 
mer, when a variation of 20° to 40° F. is not unusual. These 
cool nights naturally indicate rather late frosts in spring, and 
early ones in autumn. On the plateau, killing frosts not infre- 
quently occur as late as the middle of May, and, of course, much 
later in the mountains. In the fall they may occur as early as 
the middle of September, although light frosts sometimes occur 
in August. The winters are mild and the prairie soils seldom 
13 
