The mean annual temperature is 42.60. The warmest areas are in the south-central section traversed by the upper Yellowstone Valley, while the coldest habitable areas are in the northern prairie counties. The highest mean temperature (46.30) in the State is recorded in the Billings region; Bowen, Beaverhead County, because of its elevation, has the lowest mean, 32-3°. The highest recorded temperature, 117°, occurred on July 20, 1893, at Glendive, Dawson County; the lowest, --65°, at Fort Keogh, Custer County, on January 15, 1888. Usually July is the warmest month and January the coldest. In the eastern section hot winds sometimes cause rapid deterioration of grain crops and range grasses, but in late summer and early autumn the winds become beneficial, curing the grasses to provide excellent fodder. Autumn, dry and temperate, usually lasts until December.
The average annual precipitation is 15.48 inches. In the west, rainfall is distributed through the year, but in the east it is heaviest in late spring and early summer. Mineral County and Bull River Valley in Sanders County have the heaviest precipitation, 30 to 34 inches. Regions of lightest precipitation are Dell, Beaverhead County, with 8.7 inches, and (curiously, because of its nearness to the place of heaviest precipitation) Lonepine, Sanders County, with 10.3. In 1909 Snowshoe, in Lincoln County, established an all-time record with 79.75 inches; and in 1894 Fort Shaw, Cascade County, parched with 4.24 inches. Averages in eastern Montana range from 12 to 16 inches, but the peculiar topography causes wide variations within small areas. June is usually the wettest month, February the driest.
Snowfall is heaviest in the mountains. Saltese, Mineral County, averages 160 inches; Fallon, Prairie County, 13. Snow on the lower levels has less water content, and diminishes rapidly by melting and evaporation. Strong winds sweep it into drifts, leaving ranges uncovered for grazing. Fallplanted grains, lacking cover, often winter-kill.
The frost-free growing season is longest in low altitudes. The southeast has 125 frostless days yearly, the northeast 123, the southwest 105. In high mountain districts freezing occurs every month. Average number of clear days is 161; partly cloudy, 107; cloudy, 97. The long daylight hours of this latitude stimulate crops.