Chinook, the American name for a foehn—a warm, dry wind that blows down a mountainside. The American chinook blows down the eastern side of the Rocky Mountains, and its influence is felt over a large part of the western Great Plains from Colorado to Alberta. Its occurrence, which is primarily in winter, brings about a rapid rise in temperature: changes of 50 F. (28 C.) in less than an hour have been reported.
The chinook sometimes blows for several days and usually accompanies easterly-moving storms. Mild, moist Pacific air is forced to rise over the Rockies, and, in rising, undergoes cooling and condensation. Once over the crest, the dry, cool air rushes down the leeward slopes, rapidly gaining warmth. As it reaches the plains the extreme dryness and relative warmth of the chinook enable it to melt deep snow in a short time. This effect is important to ranchers because it allows them to graze their cattle during the winter months.
The chinook is named for the Chinook Indians of Washington and originally referred to a warm, moist wind that occurs on the Pacific coast.