A Föhn is a weather phenomenon that influences the climate throughout Europe and in Switzerland makes it possible to grow grape varieties in regions that would otherwise be inhospitable.
The Föhn is a dry wind that blows down the leeward slopes of mountains. The word Föhn is a generic German term originally just applied to winds in the European Alps, but Föhn is now used for all similar winds such as the Chinook in the U.S. Rockies.
A Föhn can last less than an hour or for several days and is reputed to cause headaches and make people act odder than usual. This is called "Föhnkrankheit" (Föhn-sickness). Causation is as yet unconfirmed. However, a study at the Ludwig-Maximilians-Universität München has found suicide and accident rates increased by 10 percent during Föhn periods in Central Europe.
The Föhn is caused when moist air flows up the windward side of a mountain. As it gains altitude the air pressure decreases, the temperature lowers, clouds form, and finally it rains. It reaches its lowest temperature as it crests the peak.
On the leeward side of the mountain the now dry wind drops and warms as it compresses because of the increasing air pressure. The temperature can be significantly higher than at the same altitude on the other side of the mountain, and the change can be dramatic: the Föhn can raise temperatures by as much as 30C (54F) in just hours. Central Europe's climate is warmer, thanks to the effects of the Föhn.