Lenticular Cloud, Brienz, Switzerland

A foehn wind or föhn wind occurs when a deep layer of prevailing wind is forced over a mountain range (Orographic lifting). 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 temperature increases adiabatically as it comes under greater atmospheric pressure creating strong, gusty, warm and dry winds. Föhn winds can raise temperatures by as much as 30°C (54°F) in just a matter of hours.
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