Idaho's climate is diverse and influenced by weather patterns off the Pacific Ocean. Generally, northern Idaho receives more precipitation than southern Idaho, which has warmer summer temperatures.
|City||Elevation||Annual Precip.||Mean Snowfall||July High °F/°C||Jan. Low °F/°C|
|Boise||2,840 ft.||12.1 in.||21.3 in.||90.2/32.3||21.6/-5.7|
|Coeur d'Alene||2,158 ft.||25.9 in.||52.2 in.||85.4/29.6||23.3/-4.8|
|Idaho Falls||4,730 ft.||10.9 in.||37.5 in.||86.0/30.0||10.0/-12.2|
|Lewiston||1,440 ft.||12.4 in.||19.8 in.||89.0/31.6||27.6/-2.4|
|Pocatello||4,450 ft.||12.1 in.||47.2 in.||88.1/31.2||14.4/-9.7|
|Twin Falls||3,670 ft.||10.4 in.||31.3 in.||85.0/29.4||18.6/-7.4|
Idaho's highest annual average temperatures are found in the lower elevations of the Clearwater and Little Salmon River basins, and in the stretch of the Snake River Valley from the vicinity of Bliss north to Lewiston, including the open valleys of the Boise, Payette and Weiser rivers. At Swan Falls the annual mean is 55° F, the highest in the state. Obsidian, at an elevation of 6,780 feet in Custer County, has the lowest annual average, 35.4° F, with such places as Sun Valley, Stanley and Island Park not far behind.
In the Snake River Basin and its tributaries, between Twin Falls and Idaho Falls, monthly mean temperatures of 32°F or lower persist from December through February, while downstream from Twin Falls, at the lower elevations, monthly mean temperatures are freezing or below only in December and January. Low-level areas like Riggins and Lewiston show no month in the year with mean temperature of 32° F or lower.
Boise temperatures can range from very chilly winters to very hot summers. The coldest monthly mean minimum temperature has been -20° F, and the warmest monthly mean maximum 104° F.
To a large extent, the Pacific Ocean is the moisture source for Idaho's precipitation. In summer there are exceptions when moisture-laden air is brought in from the south at high levels to produce thunderstorm activity, particularly in eastern Idaho.
Sizeable areas in the Clearwater, Payette and Boise River basins receive an average of 40 to 50 inches per year, with a few points or small areas receiving in excess of 60 inches. Large areas including the northeastern valleys, much of the Upper Snake River plains, central plains and the lower elevations of the southwestern valleys receive less than 10 inches annually.
Idaho's snowfall distribution is affected by the availability of moisture and by elevation. Annual snowfall totals in northern Idaho have reached nearly 500 inches in the past. The greatest long-term (1942-56) seasonal average was 182 inches at Mullan Pass, while the greatest snow depth (also 182 inches) was recorded at that same station Feb. 20, 1954. Idaho's major mountain ranges accumulate a deep snow cover during the winter months and the release of water from the melting snowpack in late spring furnishes irrigation water for more than two million acres, mainly within the Snake River Basin above Weiser.
The annual average percentage of possible sunshine ranges from about 50 percent in the north to about 70 percent in the south. Winter, with its frequent periods of cloudy weather has about 40 percent of possible sunshine in the large open valleys of the south and less than 30 percent in the north. In July and August the average percentage rises to the upper 80s in the southwest and to near 80 in the east and north.
Like the average temperature, Idaho's growing season (freeze-free period) varies greatly throughout the state because of differences in elevations, soil type, topography and vegetative cover. Year-to-year variations are also considerable in many areas. The immediate vicinity of Lewiston has the longest growing season, approximately 200 days, followed by a sizeable area in the central Snake and lower Boise, Payette, and Weiser River basins with 150 days or more. Upstream along the Snake the growing season shortens but is still about 125 days in the Pocatello and Idaho Falls areas. A few high valleys have no months without freezing temperatures and the agricultural land is used entirely for grazing.
Source: Western Regional Climate Center; http://www.wrcc.dri.edu/
For more information on Idaho's weather, visit:
Interactive Weather Information Network: http://iwin.nws.noaa.gov/iwin/ID/ID.html