A föhn wind or foehn wind is a type of dry down-slope wind that occurs in the lee (downwind side) of a mountain range. It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of its moisture on windward slopes (see orographic lift). As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes. Föhn winds can raise temperatures by as much as 30 °C (54 °F)[1] in just a matter of hours. Central Europe enjoys a warmer climate due to the Föhn, as moist winds off the Mediterranean Sea blow over the Alps.
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Winds of this type are called "snow-eaters" for their ability to make snow melt or sublimate rapidly. This snow-removing ability is caused not only by warmer temperatures, but also the low relative humidity of the air mass having been stripped of moisture by orographic precipitation coming over the mountain(s).
Föhn winds are notorious among mountaineers in the Alps, especially those climbing the Eiger, for whom the winds add further difficulty in ascending an already difficult peak.
They are also associated with the rapid spread of wildfires, making some regions which experience these winds particularly fire-prone.
These winds are often associated with illnesses ranging from migraines to psychosis. The first clinical review of these effects was published by the Austrian physician, Anton Czermak in the 19th century.[2] A study by the Ludwig-Maximilians-Universität München found that suicide and accidents increased by 10 percent during föhn winds in Central Europe. The causation of Föhnkrankheit (English: Föhn-sickness) is yet unproven. Labeling for preparations of aspirin combined with caffeine, codeine and the like will sometimes include Föhnkrankheit amongst the indications.
The condition exists because warm moist air rises through "orographic lifting" up and over the top of a mountain range or large mountain. Because of decreasing atmospheric pressure with increasing altitude, the air expanded and adiabatically cooled at the dry adiabatic lapse rate to the point that the air reaches its adiabatic dew point (which is not the same as its constant pressure dew point commonly reported in weather forecasts). Upon reaching the adiabatic dew point, water vapor in the air begins to condense, with the release of latent heat from condensation slowing the overall rate of adiabatic cooling of the air to the saturated adiabatic lapse rate as the air continues to rise. Condensation is also commonly followed by precipitation on the top and windward sides of the mountain. As the air descends on the leeward side, it is warmed by adiabatic compression at the dry adiabatic lapse rate. Because the air has lost much of its original water vapor content, the descending air creates an arid region on the leeward side of the mountain.[3]
The name föhn (German: Föhn, pronounced [ˈføːn]) originated in the alpine region. From Latin (ventus) favonius, a mild west wind of which Favonius was the Roman personification.[4] The German word "Fön" (without the "H", but pronounced the same way), a genericized trademark, is also used to mean "hairdryer".
Regionally, these winds are known by many different names. These include:
The Santa Ana winds of southern California, including the Sundowner winds of Santa Barbara, are in some ways similar to the Föhn, but originate in dry deserts as a katabatic wind.
AEG registered the trademark Fön in 1908 for its hairdryer. The word became a genericized trademark and is now, with varying spelling, the standard term for "hairdryer" in several languages, such as Finnish, German, Swiss German, Danish, Italian, Dutch, Norwegian, Czech, Croatian, Latvian, Romanian, Hebrew, Slovak, Slovenian, Swedish, Serbian, Russian, Ukrainian, Turkish and Swiss French.