CHAPTER II. 



CLIMATE. 



THE climatic influences affecting vegetation are chiefly 

 precipitation and temperature. Precipitation as the 

 source of moisture is a matter of interest to be considered 

 in connection with its seasonal distribution, with soil structure 

 and composition, and with temperature, relative humidity and 

 other atmospheric conditions. Data upon most of these points 

 are available from records of the Weather Bureau, from over 

 90 stations fairly evenly distributed throughout Montana and 

 covering periods varying from three to thirty or more years (70). 



The prevailing winds of Montana are from the west. Fig. 6 

 indicates the direction in different sections of the State. Tables 

 published by the Weather Bureau giving the usual wind direc- 

 tion by months at the numerous stations show west, northwest 

 and southwest with few exceptions throughout the year. Easterly 

 winds at times are strong, but such are usually of short duration. 



On the western slope of the Divide the prevailing winds are 

 the bearers of rain, which comes chiefly in the cooler months of 

 the year. In this respect the climate of the western slope of 

 the Rocky Mountains is not essentially different from that of 

 the Pacific Coast west of the Cascades, except in the quantity 

 of the precipitation. The seasonal features are much the same 

 on the whole, however, with lower minimum temperatures. On 

 the eastern slope of the Rockies, however, the rainfall features 

 are diametrically opposite. Here the principal precipitation 

 comes in the warmer months of the year, a fact of distinct ad- 

 vantage to the agricultural operations as well as to -the native 

 vegetation of the Plains region, and of importance as linking the 

 climate of the eastern slope with that of the central states of 

 which it is logically a part. 



A diagramatic representation of the distribution of rainfall 

 is shown in Fig. 7. The distribution by localities and seasons 

 is shown and it will be noted that at most of the stations on the 

 western slope (Plains, Missoula, Hamilton, Kalispell and Butte) 



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