From Wikipedia, the free encyclopedia.
A foehn wind occurs when a deep layer of prevailing wind is forced over a mountain range. As the wind moves upslope, it expands and cools, causing water vapor to precipitate out. This dehydrated air then passes over the crest and begins to move downslope. As the wind descends to lower levels on the leeward side of the mountains, the air heats as it comes under greater atmospheric pressure creating strong, gusty, warm and dry winds. Foehn winds can raise temperatures as much as 30°C (50°F) in just a matter of hours. Winds of this type are called "snow-eaters" for their ability to make snow vanish. This ability is based on not only the high temperature, but also the low relative humidity of the air mass.
The name "foehn" (or "föhn", pronounced as "fern") originated in the Alps. Regionally, these winds are known as "zonda winds" in Argentina, "chinook winds" in the Rocky Mountains, "Diablo winds" in the San Francisco Bay Area, "Santa Ana winds" in Southern California, and the "Nor'wester" in Christchurch, New Zealand and the Canterbury Plains.
See also: Katabatic wind.