A föhn or foehn 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 (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 32 °C (58 °F) in just a matter of minutes. 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 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 condition exists because warm moist air rises through orographic lifting up and over the top of a mountain range or large mountain. Because atmospheric pressure decreases as altitude increases, the air expands and cools adiabatically at the dry adiabatic lapse rate to the point that the air reaches its adiabatic dew point. Dew point is the temperature at which condensation begins at a given pressure and humidity. (Weather forecasts sometimes indicate humidity levels by giving a dew point at ground-level pressure for a given location, which says at what temperature dew will start condensing on the ground.)
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, also means "hairdryer," and the form "foehn" is used in Suisse Romande to mean "hairdryer" as well.
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Regionally, these winds are known by many different names. These include:
- Föhn in Austria, southern Germany, Switzerland, France, Liechtenstein, and German-speaking regions of Northern Italy
- Feun in Piedmont
- Favonio in Ticino and Italy
- Fen in NW Slovenia
- Bergwind in South Africa
- Loo in Indo-Gangetic Plain
- Chinook winds east of the Rocky Mountains and the Cascade Range in the United States and Canada, and north, east and west of the Chugach Mountains of Alaska, United States
- Fogony in the Catalan Pyrenees
- Föhn in Wollongong and South Coast, NSW, Australia. Often associated with heavy orographic lifiting on the windward side of the escarpment
- Föhn in Ostrobothnia and Western Lapland in Finland as moist air crosses Scandinavian Mountains and dries up.
- Garmesh, Garmij, Garmbaad (Warm Wind): (Persian: گرمباد, Gilaki: گرمش) in Gilan region, in the south west of Caspian Sea in Iran
- Halny in the Carpathian Mountains, Poland (Central Europe)
- 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 Helm Wind, on the Pennines in the Eden Valley, Cumbria, England
- Hnúkaþeyr in Icelandic
- Lyvas wind in Thessalian plain, Voiotia plain, Plain of Thessaloniki, Elefsina and Athens in Greece
- The Nor'wester in Hawkes Bay, Canterbury, and Otago, New Zealand
- Puelche wind in Chile
- Terral in Málaga (southern Spain)
- Vântul Mare in the Carpathian Mountains, Romania
- Viento del Sur (Southern Wind) in the Cantabrian region (northern Spain)
- Zonda winds in Argentina
- Nortada in Cascais, and most notoriously in Guincho Beach, making it one of the best windsurfing spots in Europe
- Warm Braw in the Schouten Islands north of New Guinea.
- Wuhan in China is famously known as one of the Three Furnaces on account of its extremely hot weather in summer resulting from the adiabatic warming effect created by mountains further south.
- Košava (wind) (Koshava) wind in Serbia that blows along the Danube River 
In popular culture
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- Peter Camenzind, a novel by Hermann Hesse, refers, at length, to the Alpine Föhn.
- The Föhn was mentioned by Queen's lead guitarist Brian May while talking about the band's grim Munich recording studio experience in 1982.
- The Föhn is used for the letter F in "Crazy ABC's" from the album Snacktime! by the Barenaked Ladies.
- 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.
- It's mentioned as a surprise change in weather during the ascent of Switzerland's Eiger in the book The Eiger Sanction by Trevanian.
- The föhn blowing through Zurich torments the characters in Robert Anton Wilson's Masks of the Illuminati.
- Joan Didion explores the nature of various Foehn winds in her essay "The Santa Ana".
- "Foehn" is a magic spell that deals wind/heat damage in Star Ocean: The Second Story.
- "Foehn" is the last word in A Nest of Ninnies, a 1969 novel by John Ashbery and James Schuyler. Ashbery claimed that he and Schuyler chose this particular word because "people, if they bothered to, would have to open the dictionary to find out what the last word in the novel meant." 
- McKnight, TL & Hess, Darrel (2000). Foehn/Chinoonk Winds. In Physical Geography: A Landscape Appreciation, p. 132. Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-020263-0.
- "South Dakota Weather History and Trivia for January". National Weather Service Weather Forecast Office. February 8, 2006. See January 22 entry.
- Giannini, AJ; Malone, DA; Piotrowski, TA (1986). "The serotonin irritation syndrome--a new clinical entity?". The Journal of Clinical Psychiatry 47 (1): 22–5. PMID 2416736.
- See the documentary: Snow Eater (the English translation of Canadian First Nations word phonetically pronounced chinook). telefilm.ca.
- Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8.
- Concise Oxford Dictionary, 10th edition, Oxford University Press, entry föhn.
- Romanić et al. (2015). "Contributing factors to Koshava wind characteristics". International Journal of Climatology. doi:10.1002/joc.4397. Retrieved 20 August 2015.
- Romanić et al. (2015). "Long-term trends of the ‘Koshava’ wind during the period 1949–2010". International Journal of Climatology 35 (3): 288–302. doi:10.1002/joc.3981. Retrieved 20 August 2015.
|Wikimedia Commons has media related to Foehn wind.|
|Wikisource has the text of the 1911 Encyclopædia Britannica article Föhn.|
- Photo of Föhnmauer The strong clouds at the mountain ridges where the föhn winds form are called Föhnmauer (Föhn wall).
- Movie of a Föhn situation in the Swiss Alps
- East Scotland warmth due to Foehn Effect
- Foehn chart provided by meteomedia/meteocentrale.ch