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 



NORTH PI ffTTfffEBfl. LUCftOSS 



FIG. 2. Mean monthly precipitation in inches at Pullman and La Crosse, 

 Washington, and North Platte, Nebraska. 



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 



