THE WEATHER 163 



are often two months of clear summer weather (in Au- 

 gust and September) at harvest time. In eastern Oregon 

 the Blue Mountains bring some summer rainfall, as does 

 the Bitter Root Range in eastern Washington and north- 

 ern Idaho. In Montana the figure for the total rainfall 

 seems enough to render irrigation unnecessary, but as the 

 twenty to twenty-four inches fall all through the year, 

 the short growing season compels the use of irrigation 

 over most of the state. Fortunately the great Missouri 

 and Yellowstone and their tributaries afford abundance 

 of water, and magnificent crops are thus grown on the 

 rich soils. The mountain ranges cause many local 

 summer rains, thunderstorms, and occasionally " cloud- 

 bursts/ ' which do not wet the soil to any great depth. 



As the weather in autumn gets cold enough for snow 

 before the summer drought ends, grain in the Pacific 

 Northwest east of the Cascades is not sown in autumn, 

 but in early spring. The severely cold weather of win- 

 ter " blizzards " requires the selection of specially frost- 

 resistant fruit and forest trees for the northern portions 

 of Washington, Idaho, and Montana, as well as for 

 Alberta, Saskatchewan, and the adjoining country. 



Chinook wind is the name popularly given to a 

 comparatively warm, dry wind which sometimes 

 comes suddenly, following a severely cold " blizzard " 

 (especially in Montana and the Canadian Northwest, 

 Wyoming, and Colorado) and blows northeastward 

 over the Plains. These warm winds do not come from 



