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.
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.
Winds of this type are called "snow-eaters" for their ability to make snow melt rapidly.
Föhn winds are notorious among mountaineers in the Alps, especially those climbing the Eiger, for whom the winds add additional difficulty in ascending an already difficult peak.
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