WEATHER MAPS 307 



Foehn winds, chinook winds, etc. When warm, moist air is 

 forced up over mountains, it precipitates some of its moisture. 

 The precipitation sets free heat, so that the rising air is cooled much 

 less than it would be otherwise. Beyond the crest of the mountains 

 it descends, and is warmed in the process. It is warmed much 

 more (often twice as much) in the descent than it was cooled in the 

 ascent. It may, therefore, descend as a hot wind. Such winds are 

 known as foehn winds in Switzerland, and as chinook winds in the 

 United States, especially just east of the Rockies. The same 

 process takes place in other regions. 



These winds may be beneficial or harmful. Thus the chinook 

 winds temper the rigorous winters of certain parts of the north- 

 western states and the Canadian provinces east of the mountains. 

 They frequently evaporate a foot or more of snow in a few hours. 

 For this reason they are sometimes called " Snow-eaters." These 

 winds make winter grazing possible over large areas. In the prov- 

 ince of Alberta the chinook has been declared to be "the grand 

 characteristic of the climate as a whole, that on which the weather 

 hinges." These winds sometimes develop with great suddenness. 

 At Fort Assiniboine, Montana, on January 19, 1892, the tempera- 

 ture rose 43 F. (from -5.5 to 37.5), in fifteen minutes, under the 

 influence of the chinook wind. In other cases the temperature has 

 been known to rise 80 in six or eight hours. 



The chinook winds of summer are sometimes so hot and drying 

 as to wither vegetation, and occasionally to destroy crops com- 

 pletely. 



