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.
A foehn originates as follows: Air is first forced upward over the windward mountain slopes, cooling as it encounters the lower pressures of higher altitudes.
The foehn occurs when the circulation is strong enough to force air over the mountains in a relatively short period of time.
The nature of the foehn in a particular locale depends on the topography, the strength and direction of circulation, and the moisture supply on the windward side of the mountains.
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