Montana Climate
Montana is a large state with considerable variation in geography, and so the climate is equally varied. The western half is mountainous, interrupted by numerous large valleys. Eastern Montana is plains, badlands, broken by hills and isolated mountain ranges, and has a continental climate The Continental Divide runs north-south through the western mountainous half, and has a large effect on the climate. It restricts the flow of warmer air from the Pacific from moving east, and cooler, drier continental moving west. West of the divide the climate is described as modified northern Pacific coast climate, with milder winters, cooler summers, less wind, and a longer growing season. In the winter Valley fog and low clouds often form in the valleys west of the divide, but this is rarely seen in the east.
Average daytime temperatures vary from 28 degrees in January to 84.5 degrees in July. The variation in geography leads to great variation in temperature. Hot weather occurs in the eastern plains on occasion; the highest observed being 117° at Glendive on July 20, 1893, and Medicine Lake on July 5, 1937. Throughout the state summer nights are generally cool and pleasant. Temperatures decrease with altitude, and hot weather is unknown above 4,000 ft (1,200 m) Snowfall is not unknown any month of the year in the central part of Montana, but is quite rare in July and August.
The coldest temperature on record for Montana is the coldest temperature for the entire continental U.S. On January 20, 1954 -70 °F was recorded at a gold mining camp near Rogers Pass. Temperatures vary greatly on such cold nights, and Helena, 40 miles (64 km) to the southeast had a low of only −36 °F (−37.8 °C). Winter cold spells last a week or so. They are the result of cold continental air coming south from Canada. The front is often well defined, causing a large temperature drop in a 24 hour period. Conversely, air flow from the southwest results in “Chinooks”. These steady 25-50mph (or more) winds can suddenly warm parts of Montana, especially areas just to the east of the mountains, where temperatures sometimes rise into the 50’s and 60’s.
