Locally described as a "Northwest arch" in Canterbury, New Zealand
A foehn or föhn wind is a type of dry, warm, 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 (seeorographic 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 32 °C (58 °F) 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.
Winds of this type are also 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.
Anecdotally, residents in areas of frequent föhn winds report 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. 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. Evidence for effects from Chinook winds remain anecdotal.
The cause of warm, dry conditions on the lee side
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.
The name foehn (German: Föhn, pronounced [ˈføːn]) arose in the Alpine region. Originating from Latin (ventus) favonius, a mild west wind of which Favonius was the Roman personification and probably transmitted by Romansh: favuogn or just fuogn, the term was adopted as Old High German: phōnno. In the Southern Alps, the phenomenon is known as Italian: favonio and Slovene: fen. The German word "Fön" (without the "H", but pronounced the same way), a genericized trademark, is also used to mean "hairdryer," and the form "foehn" is used in Suisse Romande to mean "hairdryer" as well.
North-East Scotland, south-westerly winds create a Föhn effect bringing relatively warm temperatures on the lee side of the Grampians and Cairngorm mountain ranges. The reverse occurs when south-easterly winds create a Föhn effect to the North-West of Scotland, with the air drying out and warming up as it crosses the Grampians and Cairngorms from east to west. With the prevailing wind direction in the UK being from the west or south west, the Föhn effect in Scotland is more common in the North-East of the country, with the west of Scotland being much wetter.
The threat of the Föhn drives the protagonists Ayla and Jondalar in Jean M. Auel's The Plains of Passage over a glacier before the spring melt. The pair make references to the mood altering phenomena of the wind, similar to those of the Santa Ana wind.
In Southern Germany, this wind is supposed to cause disturbed mood. Heinrich Hoffmann notes in his book Hitler Was My Friend that on the evening of September 18, 1931, when Adolf Hitler and Hoffmann left their Munich apartment on an election campaign tour, Hitler had complained about a bad mood and feeling. Hoffmann tried to pacify Hitler about the Austrian Föhn wind as the possible reason. Hours later, Hitler's niece, Geli Raubal, was found dead in his Munich apartment. It was declared that she had committed suicide though it had conflicting testimonies from the witnesses present.
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, Lithuanian, Romanian, Hebrew, Slovak, Slovenian, Swedish, Serbian, Russian, Ukrainian, Turkish and Swiss French.