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Montana Interagency Plant Materials Handbook
Chapter 1

Temperature | Precipitation | Cloudiness-Sunshine | Wind | Humidity | Storm Types
Freeze-Free Season | Beginning of Vegetative Growth

Climate of Montana

By J. M. Caprio and P. Farnes

 

The 146,318 square mile area of Montana is characterized by a complex climatic pattern. This complexity is attributed to the highly variable topography within the state and the air masses which flow over the state. The principal topographic feature is the Continental Divide, which extends south-southeast from the Canadian border north of Kalispell to just west of Helena, then southwest to the Idaho border near Gibbons Pass, where it coincides with the Idaho border to Yellowstone Park. The area west of the Divide

is mountainous with many deep valleys and high ridges. The ridges and valleys decrease from approximately southeast to northwest. The topography east of the Divide varies greatly. Mountains extend from the Divide to a line from Cut Bank to the south east of Billings; the country east of this line consists largely of rolling plains and hills. Scattered mountains occur even in this so-called plains area; those include Highwood Mountains, Sweet Grass Hills, Bears Paw, Snowy and Little Rocky Mountains.

Temperature

Mean temperatures for selected weather stations in Montana are shown in Table 1. The Continental Divide and great differences in elevation have a marked influence on the temperature distribution within the state. Elevation ranges from about 1,800 feet above sea level where the Kootenai River enters Idaho, to 12,850 feet at the summit of Granite Peak, just north of Yellowstone Park. Perhaps the most important and dramatic effect that the Continental Divide has on the climate is its shielding of the Columbia Basin portion of the state from most of the winter cold waves that affect central and eastern Montana each year. The climate of eastern Montana is classified as continental, while that west of the Divide is a modified North Pacific Coast type.

On the west side of the Divide, winters are not so cold, summers are usually cooler, and winds are generally lighter than on the east side. The lack of wind and the narrow, deep valleys on the west side result in cool nights during the summer.

The lowest temperature ever recorded in Montana is -70 F at Rogers Pass on January 20, 1954. The highest temperature ever recorded is 117 F at Medicine Lake on July 5, 1937, and at Glendive on July 20, 1893. (Top)

Precipitation

Mean precipitation amounts for selected weather stations in Montana are shown in Table 1. The mountainous country along both sides of the Continental Divide supply enough orographic lift to make this divide area the wettest part of the state. Some of the state's driest areas are located in sheltered mountain valleys, because of the rain-shadow effects on the leeside of some ranges. Annual precipitation in the state varies from an average of 9.07 inches per year at Dillon airport, to about 105 inches at Grinnell Glacier. The wet season occurs during late spring and early summer, except in narrow strips along the Bitterroot Mountains and the Continental Divide, where a large portion of the precipitation occurs during the winter months. In nearly all areas, almost half of the yearly precipitation falls during three months--May through July. Winter precipitation is usually in the form of snow. (Top)

Cloudiness-Sunshine

Montana's topographical complex of valleys and mountains has important effect son the amount of cloudiness. In general, the western and south-central mountain areas experience more cloudiness and correspondingly less sunshine, than the eastern slopes and plains section. Valley fog and low clouds often form during fall and winter in western Montana valleys. This phenomenon occurs in the Clark Fork of the Columbia, Kootenai and Flathead Valleys several times each season and can persist for several days while surrounding mountain ridges and passes are bright and clear. Mountains, however, are usually more covered with clouds than are valleys. Cloudy weather is most frequent during the period from late fall until early spring. Seasonal difference in cloudiness is less east of the Continental Divide. (Top)

Wind

Average wind speed is considerably lower on the west side of the Continental Divide than on the east side. Wind speed is often high along the eastern slope of the Divide. The middle part of the Missouri, Milk, Sun, St. Mary and Yellowstone River drainages are known as windy areas. Generally, most wind occurs during the day. Chinook winds are a fairly common phenomenon along the eastern slope during the winter. The winter warmth of this wind is largely accounted for by its effectiveness in replacing the cold air mass, its dynamic warming as it moves downslope from the west, and its inhibiting effect on the normal tendency toward formation of inversions. (Top)

Humidity

Relative humidity tends to vary inversely with temperature changes throughout the day. Highest humidity occurs just before sunrise when the temperatures are lowest, and lowest humidity usually occurs in the afternoon when the temperatures are highest. Generally, relative humidity in the early morning averages about 75 percent throughout the year. Relative humidity during the afternoon, however, differs markedly between summer and winter. Afternoon relative humidity over most of the state tends to run about 70 percent during the winter and less than 35 percent during mid-summer. (Top)

Storm Types

1. Hail and Severe Thunder Storms:
During the warm season, thunderstorms often develop which produce hail, damaging lightning and gusty winds. Once or twice a year a small tornado may occur; these are usually observed in the eastern third of the state. Hail storms tend to be spotty and rarely cover large areas.

2.  High Winds:
High winds often accompany thunderstorms. In addition, certain locations along the eastern slope of the Continental Divide often experience winds of high speed. Chinook-type winds occasionally reach speeds greater than 70 mph.

3.  Heavy Rain:
June is normally the wettest month in much of Montana. The only exceptions to this are at the higher elevations and in a few northwestern counties where winter precipitation is heavier and usually falls as snow. Flooding occurs primarily in June when the effects of rains are multiplied by runoff from snow-melt in the mountains. Flooding is most frequent along rivers that have large drainage basins such as near the confluence of the Missouri and Sun River, and along the Milk River below Fresno reservoir. Flooding is sometimes caused by ice jam blockage in the winter. Small areas sometimes experience heavy storms that cause flash flooding. A serious flood occurs somewhere in Montana about once in every 10 years.

4.  Cold Waves and/or Blizzards:
During a typical cold wave or blizzard, the temperature falls from above freezing to below zero. This is usually accompanied by blowing snow borne on strong north to east winds. Such storms occur about five times per year in the northeastern part of the state and about one or two times a year in the extreme southwest. Lowest temperatures are generally recorded after the skies have cleared and the wind has subsided. (Top)

Freeze-Free Season

The average length of the freeze-free season across broad areas of the state is shown in Figure 1 (see map). The length of this freeze-free season varies from more than 125 days in areas along the Missouri and Yellowstone Rivers to less than 30 days in the higher mountains of western Montana. The short, freeze-free season in some of the western mountain valleys results from frequent, low nighttime temperatures caused by strong air inversions which form in the relatively windless sheltered valleys. The average length of the freeze-free season in all areas is highly influenced by local features of the terrain. On clear, relatively windless nights, cold air from surrounding lands flow downhill under the influence of gravity. For this reason, the bottoms of valleys and basins tend to be cold-air lakes at night. Hillsides, several hundred feet above the basin or valley floor, are usually well drained of cold air and tend to be warm, thermal belts. Lakeside locations also tend to have a local climate more moderate than valley bottoms. Such local variations in climate usually cover too small an area to be included in a broad scale map. The last spring freeze in most agricultural sections of the state normally varies between May 10 and May 31; the first fall freeze generally occurs between the first and last of September. (Top)

Beginning of Vegetative Growth as Indicated by Lilac Bloom

A lilac bloom map (figure 2) shows the average date when the common purple lilac, Syringa vulgaris L., begins to bloom. This map indicates, better than any available single climatic map, how vegetation throughout the state generally develops in the spring. It shows the broad general pattern, but does not include local variations in plant development. A close relationship exists between elevation and time when lilacs begin to bloom. Lilacs bloom about one day later for every 100-foot increase in elevation. In general, lilacs do not bloom later than about mid-June. Map lines for later than mid-June, therefore, are drawn by projection assuming a constant elevational gradient. Bloom normally begins prior to mid-May in areas of the Missouri and Yellowstone drainage basins east of the Continental Divide and south of Flathead Lake, in the Bitterroot Valley and along the Clark Fork River west of the Divide. Lilacs bloom prior to May 30 in most of the important agricultural areas throughout the state. (Top)