Chinook wind
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Chinook winds are a wind pattern observed in midwestern North America, named for the Chinook Indians. Chinook winds have been observed to elevate midwestern winter temperatures, often below −20°C (−4°F) to as high as 10°C to 20°C (50°F to 68°F), for a few hours or days, at the end of which, the temperatures plummet to their base levels.
The most dramatic example of the Chinook winds occurred on January 15, 1972 in Loma, Montana. The temperature rose from −47°C (−54°F) to 9°C (49°F); the greatest temperature change ever recorded during a 24-hour period.
The Chinook phenomenon results from the movement of high and low pressure systems over the Rocky Mountains. As the wind moves over and through the mountains, the moisture in the air condenses, warming the air by releasing latent heat. Then, as it moves downhill, the cold air above presses down upon it, resulting in further warming through adiabatic compression.
A similar phenomenon (also called Chinook winds) occurs in the Cook Inlet region in Alaska as air moves over the Chugach mountains between Prince William Sound and Portage Glacier. Anchorage residents commonly (but wrongly) believe the warm winds to be a midwinter gift from Hawaii, melting snow and leaving streets slushy and muddy.