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 the divide the climate is described as modified northern Pacific coast climate, with milder winters, cooler summers, less wind, and a longer growing season.^[1] 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.^[2]

Average daytime temperatures vary from 28 degrees in January to 84.5 degrees in July.^[3] 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 4000 ft. Snowfall is not unknown any month of the year in the central part of Montana, but is quite rare in July and August.^[1]

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 to the southeast had a low of only -36°F.^[1] Winter cold spells last a week or so. They are the result of cold continental coming south from Canada. The front is often well defined, causing a large temperature drop in a 24 hour period. Conversly, 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 east of the mountains where temperatures sometimes rise into the 50's and 60's.^[1]

Average annual precipitation is 15 inches, but great variations are seen. The mountain ranges block the moist Pacific air, holding moisture in the western valleys, and creating rain shadows to the east. Heron in the west receives the most precipitation, 34.70 inches. On the east side of a mountain range the valleys are much drier; Lonepine averages 11.45, and Deerlodge 11.00 inches of precipitation. The mountains themselves can get over 100 inches, for example the Grinnell Glacier in Glacier National Park gets 105 inches.^[2] Perhaps the driest is an area southwest of Belfry that averaged only 6.59 inches over a 16 year period. Most of the larger cities get 30 to 50 inches of snow each year. Mountain ranges themselves can accumulate 300 inches of snow during a winter. Heavy snowstorms can occur as early as September or as Late as May, but most snow falls from November to March.^[1]

The climate has become warmer in Montana and continues to do so.^[4] The glaciers in Glacier National Park have receded and are predicted to melt away completely in a few decades.^[5] Many Montana cities set heat records during July 2007, the hottest month ever recorded in Montana.^[4][6] Winters are warmer, too, and have fewer cold spells. Previously these cold spells had killed off bark beetles which are now attacking the forests of Western Montana.^[7][8] The combination of warmer weather, attack by beetles, mismanagement during past years has led to an substantial increase in the severity of forest fires in Montana.^{[4] [8]} Sadly one no longer hears the saying, "It just isn't Halloween without snow!"