Montana is a large state with considerable variation in geography, hence the climate is equally varied. The state spans from 'below' the 45th parallel (i.e. the halfway line between the equator and the north pole) to the 49th parallel, and elevations range from under 2,000 feet (610 m) to nearly 13,000 feet (4,000 m) above sea level. The western half is mountainous, interrupted by numerous large valleys. Eastern Montana comprises plains and badlands, broken by hills and isolated mountain ranges, and has a semi-arid continental climate. The Continental Divide runs north-south through the western mountainous half, and has a great 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 °F (−2 °C) in January to 84.5 °F (29.2 °C) 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 °F (47 °C) 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 rare in July and August.

The coldest temperature on record for Montana is also the coldest temperature for the entire continental U.S. On January 20, 1954, −70 °F (−56.7 °C) 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, and are usually 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 25-50 mph (or more) winds can suddenly warm parts of Montana, especially areas just to the east of the mountains, where temperatures sometimes rise up to 50°F (28°C) - 60°F (33°C). Loma, Montana is the location of the most extreme recorded temperature change in a 24-hour period in the United States. On January 15, 1972, the temperature rose from −54 °F (−47.8 °C) to 49 °F (9 °C).

Average annual precipitation is 15 inches (380 mm), 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 (881 mm). On the eastern (leeward) side of a mountain range, the valleys are much drier; Lonepine averages 11.45 inches (291 mm), and Deer Lodge 11.00 inches (279 mm) of precipitation. The mountains themselves can receive over 100 inches (2,500 mm), for example the Grinnell Glacier in Glacier National Park gets 105 inches (2,700 mm). Perhaps the driest is an area southwest of Belfry that averaged only 6.59 inches (167 mm) over a sixteen-year period. Most of the larger cities get 30 to 50 inches (760 to 1,300 mm) of snow each year. Mountain ranges themselves can accumulate 300 inches (7,600 mm) of snow during a winter. Heavy snowstorms may occur as early as September or as late as May, though most snow falls from November to March.

The climate has become warmer in Montana and continues to do so. The glaciers in Glacier National Park have receded and are predicted to melt away completely in a few decades. Many Montana cities set heat records during July 2007, the hottest month ever recorded in Montana. 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. The combination of warmer weather, attack by beetles, and mismanagement during past years has led to a substantial increase in the severity of forest fires in Montana. According to a study done for the U.S. Environmental Protection Agency‎ by the Harvard School of Engineering and Applied Science, portions of Montana will experience a 200% increase in area burned by wildland fires, and an 80% increase in air pollution from those fires.

Luciano Mende

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