Climate of Montana
Montana, with an area of 146,316 square miles, is the fourth largest State of the Union. Climatic variations are large. The half of the State southwest of a line from the southeastern corner to the Canadian Border north of Cut Bank in Glacier County is very mountainous, while the northeastern half is very much like Great Plains country, broken occasionally by wide valleys and isolated groups of hills. The extent of the climatic variations is indicated by the range in elevation of from 1,800 feet above sea level where the Kootenai river enters Idaho to 12,850 feet at Granite Peak near Yellowstone Park. Half the State lies over 4,000 feet above sea level.
The Continental Divide traverses the western half of the State in roughly a north-south direction. To the west of the Divide, Montana is drained by the Kootenai, Clark Fork, and Flathead Rivers into the Pacific Ocean through the Columbia River. Many of the tributary streams in this region have their origin in the high western slopes of the Rockies. Most streams traverse narrow canyons, at least through parts of their length, affording many valuable waterpower sites. A relatively small area located between the Hudson Bay Divide and the Rocky Mountains is drained by the St. Mary River, which finds its way to Hudson Bay through the Saskatchewan River. The remainder of the State is drained by the Missouri River, which is formed by the confluence of the Gallatin, Madison, and Jefferson Rivers at Three Forks, and travels northward through deep canyons in the Big Belt Mountains, and flows through the lower lying northeastern portion of the State. The Yellowstone River, the principal tributary of the Missouri in Montana and which has its source in Wyoming, drains the southeastern section of the State and has its confluence with the Missouri just east of the Montana-North Dakota line.
The Continental Divide exerts a marked influence on the climate of adjacent areas. West of the Divide the climate might be termed a modified north Pacific coast type, while to the east, climatic characteristics are decidedly continental. On the west of the mountain barrier winters are milder, precipitation is more evenly distributed throughout the year, summers are cooler in general, and winds are lighter than on the eastern side. There is more cloudiness in the west in all seasons, humidity runs a bit higher, and the growing season is shorter than in the eastern plains areas.
TemperatureCold waves, which cover parts of Montana on the average of 6 to 12 times a winter, are confined mostly to the sections northeast of a Glacier Park — Miles City line. A few of these cold waves cover the entire area east of the Divide, and will cover the State all the way from the Dakotas to Idaho. These cold waves do not now hold the dangers they did years ago before transportation, roads, communications, and even heating plants developed to their present levels. However, with temperatures well below zero accompanied by strong winds with blowing snow, these cold waves can be very inconvenient and even dangerous to the careless or inexperienced. In small areas ideally situated for radiation cooling, low tempertures can fall to -50° F or lower. The coldest ever observed was -70° F at Rogers Pass, 40 miles northwest of Helena, on January 20, 1954. This is the coldest of record for the entire United States, exclusive of Alaska. In contrast, the low at Helena that morning was only -36° F.
During the summer months hot weather occurs fairly often in the eastern parts of the State. The highest ever observed was 117° at Glendive on July 20, 1893, and Medicine Lake on July 5, 1937. Temperatures of over 100° F sometimes occur in the lower elevation areas west of the Divide during the summer, but hot spells are less frequent and of shorter duration than in the plains sections. Hot spells nowhere become oppressive, however, because summer nights almost invariably are cool and pleasant. In the areas with elevations above 4,000 feet, extremely hot weather is almost unknown. Summer days, however, are usually warm enough for light summer clothing.
Winters, while usually cold, have few extended cold spells. Between cold waves there are periods, sometimes longer than 10 days, of mild but often windy weather. These warm, windy winter periods occur almost entirely along the eastern slopes of the Divide and are popularly known as "chinook" weather. The so-called "chinook" belt extends from the Browning-Shelby area southeastward to the Yellowstone Valley above Billings. Through this belt, "chinook" winds frequently reach speeds of 25 to 50 mph or more and can persist, with little interruptions, for several days. In January, the coldest month, temperature averages range from 11° F for the Northeastern Division to 22° F for the South Central (upper Yellowstone Valley) Division. In some areas east of the Continental Divide, January or February can average zero or below, but such occurrences range from infrequent to about once in 10 to 15 years in the coldest spots. Most Montana lakes freeze over every winter, but Flathead Lake between Polson and Kalispell, freezes over completely only during the coldest winters, about 1 year in 10. All rivers carry floating ice during the late winter or early spring. Few streams freeze solid; water generally continues to flow beneath the ice. During the coldest winters "anchor" ice, which builds from the bottom of shallow streams, on rare occasions causes some flooding.
In July, the warmest month, temperature averages range from 74° for the Southeastern Division to 64° F for the Southwestern Division. This mid-summer warmth is fairly steady, very seldom severe, and is tempered by normal nighttime mnima in the 50's and 60's. Miles City, one of the State's warmest places in July, has a July average minimum temperature of 60° and an average maximum of 90° F. Generally, adequate moisture permits rapid plant and crop development during most growing seasons.
PrecipitationPrecipitation varies widely and depends largely upon topographic influences. Areas adjacent to mountain ranges in general are the wettest, although there are a few exceptions where the "rain shadow" effect appears. Generally, nearly half the annual long-term average total falls from May through July. This is perhaps the main reason why Montana in consistently one of the largest producers of dryland grain crops. The Western Division of the State is the wettest and the North Central the driest. There are a few valleys in the Western Division that are relatively dry, as reflected by Deer Lodge and Lonepine averages of 11.00 and 11.46 inches respectively. Probably the driest part of the State is along the Clark Fork of the Yellowstone River in Carbon County. In this area, 8 miles south-southwest of Belfry, the average precipitation for a 16-year period is 6.59 inches. The highest average in the State is 34.70 inches at Heron.
Annual snowfall varies from quite heavy, 300 inches, in some parts of the mountains in the western half of the State, to around 20 inches at some stations in the two northern Divisions east of the Continental Divide. Most of the larger cities have annual snowfall within the 30 to 50 inch range. Most snow falls during the November-March period, but heavy snowstorms can occur as early as mid-September or as late as May 1 in the higher southwestern half of the State. In eastern sections early or late season snows are not very common. Mountain snowpacks in the wetter areas often exceed 100 inches in depth as the annual snow season approaches its end around April 1 to 15.
The greatest volume of flow of Montana's rivers occurs during the spring and early summer months with the melting of the winter snowpack. Heavy rains falling during the spring thaw constitute a serious flood threat. Ice jams, which occur during the spring breakup, usually in March, cause backwater flooding. Flash floods, although restricted in scope, are probably the most numerous and result from locally heavy rainstorms in the spring and summer. Damaging floods have occurred in 1952, 1953, and 1964.
Other Climatic FeaturesSevere storms of several types can occur, but the most troublesome are hailstorms which cause crop and property damage averaging about $5 million annually. This is not unusually large for an area of 146,000 square miles, however, and their occurrence is limited mainly to July and August, infrequently in June and September.
Tornadoes develop infrequently (about 2 per year) and occur almost entirely east of the Divide, largely in the eastern third of the State. Severe windstorms of a general nature are rare but can occur locally, mainly east of the Divide, from a few to several times a year. Drought in its most severe form is practically unknown, but dry years do occur in some sections. All parts of the State rarely suffer from dryness at the same time. The only exceptions on record occurred during the 1930 decade. Drought infrequently lasts 2 or 3 years in one or two of the State's climatic subdivisions.
Water supplies in the mountainous southwestern half are generally ample and of excellent quality. In the northeastern half of the State water supplies are generally dependable, but the water has a variable "hard" quality, particularly where wells are used. There are numerous irrigation projects for which water supplies are usually sufficient. Irrigated crops which do well are potatoes, sugar beats, sorghums, alfalfa, and many varieties of grain. Smaller quantities of other fairly hardy crops are grown under irrigation. Wide open areas of rangeland provide excellent quality grass for an extensive livestock industry. Between livestock and other agriculture, Montana has developed into an important food supply State. In spite of figures that may indicate winters on the cold side, growing seasons (freeze-free periods) are 4 months or more in length in much of the agricultural area. In parts of the middle Yellowstone Valley, in fact, the freeze-free period runs as long as the 150-day average at Miles City. Much of the State has average freeze-free periods longer than 130 days, allowing plenty of time for growing a wide variety of crops. There is no freeze-free period in many higher valleys of the western mountains, but hardy and nourishing grasses thrive in such places, producing large amounts of high-quality grazing for stock.