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WEATHER AND CLIMATE 



distribution of rainfall can readily be divided into four 

 belts which, although gradually shading the one into the 

 other, are yet quite distinct. These belts may be called 

 the north Pacific slope, the south Pacific slope, the western 

 interior region, and the eastern region. 



In the north Pacific coast region the storms of the " wester- 

 lies " are common, particularly in winter, when the westerly 



winds are strong and 

 stormy. The yearly 

 rainfall here amounts 

 to about seventy 

 inches. 



From central Cali- 

 fornia south the rain- 

 fall of the Pacific 

 slope decreases until, 

 in southern Califor- 

 nia, there is almost 

 no rain in summer 

 and the entire rain- 

 fall for the year aver- 

 ages about 15 inches. 

 The high-pressure 

 area of the dry tropi- 

 cal calm belt moves 

 sufficiently far north 

 in summer to take this region out of the influence of the 

 wet westerlies and into that of the drier belt. 



The western interior region, extending from the Cas- 

 cade and Sierra Nevada mountains to about the 100th 

 meridian, is dry over the larger part of its surface, since 

 the winds have deposited most of their moisture in pass- 



SALMON RIVER DAM, IDAHO 

 A typical irrigation dam in the United States. 



