CLIMATE 41 



May and June are thn months of heaviest rainfall, comprising 

 possibly as much as one-fourth the total of the year, and that 

 the months of July, August and September are comparatively 

 dry. 



At these stations the precipitation during the growing sea- 

 son, from April to September inclusive, represents about 54% 

 to 63% of the total of the year. At some places, however, as 

 at Troy, in the extreme western part of the State, the climate 

 more nearly approaches that of the Pacific Northwest,^ and of 

 the total annuai rainfall (about 23 inches) about 35% only 

 occurs during the same period. On the eastern side of the 

 Divide the rainfall from April to September varies from 62% 

 to 82% of the year's total. 



The reason for the heavier precipitation during the months 

 of 3Iay and June on the western slopes of the Rockies is doubt- 

 less due to the altitude of the land and the coolness owing to 

 accumulated snows. During these months the snows are rapidly 

 melting, the rivers are at flood, and the air is more heavily 

 loaded with moisture. By the first of July the snows have 

 mostly disappeared, except from the highest peaks, and the tem- 

 perature of the whole region becomes noticeably higher. Through- 

 out the whole summer showers occur among the higher moun- 

 tains, but the valleys and foothills are usually almost rainless 

 during July and August. Most of the observation stations are 

 at moderate altitudes (2,000 to 6,000 feet) so that almost no 

 data are available from the higher ranges, either in winter or 

 summer. 



Precipitation during the winter is usually in +he form of 

 snow. Table 1 presents precipitation data, both rain v^d snow, 

 and the stations selected from various parts of the State afford 

 representative figures. Strange as it may seem, the sno^^f«ll 

 on the main range of the Rocky Mountains is comparatively 

 light, notwithstanding their greater altitudes. This no doubt is 

 largely due to the Bitter Root and Cabinet Ranges to the west 

 where the snows fall to great depth. The figures from Saltese 

 and Snowshoe are far above anything that occurs on the main 

 Divide, being in the neighborhood of 12 feet for Saltese and 23 

 feet for Snowshoe. Butte, at an altitude of 5,716 feet, and 

 within a few miles of the Divide, has an average snowfall of 

 less than 5 feet. Philipsburg, at 5,275 feet, has S^U feet, while 



