NOTES ON THE CLIMATE OF MONTANA 



William T. Lathrop, 

 Meteorologist, U, S. Weather Bureau. 



Marked influences upon its climate are exerted by topographic features of Mon- 

 tana, especially the Rocky Mountains. The Continental Divide crosses the western part 

 of the state and in the southwest determines a part of the Montana-Idaho boundary line. 

 The south-central and western portions of the state are very largely mountainous, and 

 the central part also has several important mountain ranges. Outlying ranges and 

 groups of mountains and hills rise in other sections. The eastern portion of the state 

 belongs to the Great Plains region, but the level expanses of the plains here are 

 broken up by rolling hills and increasing roughness of surface as the Rocky Mountain 

 country is approached. 



Rivers and streams in great numbers have their sources of first supply in the 

 winter accumulations of snow in the mountain regions. The flow from the melting of 

 these snow deposits in the spring and summer months is augmented by water absorbed 

 by the soil and by the rains of those seasons. The Missouri River is formed by the 

 confluence of the Jefferson, Madison, and Gallatin rivers in a well watered section of 

 the southwestern part of the state. It proceeds in a generally northerly and then easterly 

 direction into North Dakota. Its great tributary, the Yellowstone, takes the drainage 

 of all of southern Montana from Yellowstone Park eastward, except the extreme south- 

 eastern corner, and much of eastern Montana. The Missouri and another tributary, 

 the Musselshell, receive all the drainage of the central part of the state. The Milk, 

 Marias, and Poplar rivers are the principal streams bringing into the Missouri the 

 waters of the north. To the westward of the Continental Divide the bulk of the drainage 

 is carried out of the state to the Columbia River by the Clark Fork, while the Kootenai 

 River, also of the Columbia system, drains Lincoln County, in the extreme northwest. 

 Because of the agricultural importance of its valley, the Bitter Root River, just east 

 of the Bitter Root Mountains, which flows into the Clark Fork, should also receive 

 mention. Into Flathead Lake, in the northwest, flows the drainage of a considerable 

 area of the west slope of the Main Range of the Rockies. 



The lowest parts of the state are in the northeast, where the Missouri and Yellow- 

 stone flow into North Dakota at an elevation of a little less than 2,000 feet. Numerous 

 peaks in the mountains exceed 10,000 feet in altitude, the highest named being Granite 

 Peak, in the Beartooth Mountains in the south, elevation 12,850 feet. 



Long Its northern geographical location lengthens the growing days, and 



f> . the summer sun, not masked by too great cloudiness, encourages rapid 



growth of crops and native grasses. The winter days are correspond- 



*^* ingly short. The range of temperature is rather wide, but the annual 



mean temperature in Montana as a whole is 42.2°. The warmest part of the state, 



considering the annual averages, is the southeastern, especially the Yellowstone Valley 



and the neighboring section of the Musselshell in Musselshell County. Billings and 



Huntley, Yellowstone County, have the highest average temperatures for the year 



through this section, the former being 46.2°, and the latter, 46.0°. Hamilton, in the 



Bitter Root Valley, however, equals Billings, and these two places have the highest 



mean annual temperatures in the state. 



The northern tier of counties east of the Main Range, adjoining the international 

 boundary line, taking in mountain slope sections in the west and exposed plains in the 

 east, is the coldest large area, and has an average temi)erature slightly under 40°. The 

 slope of the Main Range in Beaverhead and Madison counties, in the southwest, is 



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