A Foehn or Föhn (, , ), is a type of dry, relatively warm, downslope 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 Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and cr ...

). 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 Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...

slopes. Foehn winds can raise

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

s by as much as 14 °C (25 °F) in just a matter of hours. Switzerland, southern Germany and Austria have a warmer climate due to the Foehn, as moist winds off the Mediterranean Sea blow over the

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...

Etymology

The name ''Foehn'' (german: Föhn, ) arose in the

Alpine Alpine may refer to any mountainous region. It may also refer to: Places Europe * Alps, a European mountain range ** Alpine states, which overlap with the European range Australia * Alpine, New South Wales, a Northern Village * Alpine National Pa ...

region. Originating from Latin ''(ventus) favonius'', a mild west wind of which Favonius was the Roman personification and probably transmitted by rm, favuogn or just ''fuogn'', the term was adopted as goh, phōnno. In the Southern Alps, the phenomenon is known as ''Föhn'' but also it, favonio and ''fen'' in

Serbo-Croatian Serbo-Croatian () – also called Serbo-Croat (), Serbo-Croat-Bosnian (SCB), Bosnian-Croatian-Serbian (BCS), and Bosnian-Croatian-Montenegrin-Serbian (BCMS) – is a South Slavic language and the primary language of Serbia, Croatia, Bosnia an ...

and Slovene. The German word ''Föhn'' (pronounced the same way) also means "hairdryer", while the word ''Fön'' is a

genericized trademark A generic trademark, also known as a genericized trademark or proprietary eponym, is a trademark or brand name that, because of its popularity or significance, has become the generic term for, or synonymous with, a general class of products ...

today owned by AEG. The form ''phon'' is used in French-speaking parts of Switzerland as well as in

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

. The name "Föhn" was originally used to refer to the south wind which blows during the winter months and brings thaw conditions to the northern side of the

. Because Föhn later became a generic term that was extended to other mountain ranges around the world that experience similar phenomena, the name "

Alpine föhn The Alpine föhn (german: Alpenföhn) is the name given to the föhn wind in the Alps, Alpine region. The name ''föhn'' was originally used to refer to the south wind which blows during the winter months and brings thaw conditions to the northern ...

" (''Alpenföhn'') was coined for the Föhns of the Alpine region.''Der Brockhaus. Wetter und Klima.'' Seite 101, Brockhaus, Leipzig/Mannheim, 2009,

Causes

There are four known causes of the Foehn warming and drying effect. These mechanisms often act together, with their contributions varying depending on the size and shape of the mountain barrier and on the meteorological conditions, such as the upstream wind speed, temperature and humidity.

Condensation and precipitation

When winds blow over elevated terrain, air forced upwards expands and cools due to the decrease in pressure with height. Since colder air can hold less water vapour, moisture condenses to form clouds and precipitates as rain or snow on the mountain's upwind slopes. The change of state from vapour to liquid water releases

latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be underst ...

energy which heats the air, partially countering the cooling that occurs as the air rises. The subsequent removal of moisture as precipitation renders this heat gain by the air irreversible, leading to the warm, dry, Foehn conditions as the air descends in the mountain's lee. This mechanism has become a popular textbook example of atmospheric thermodynamics. However, the common occurrence of 'dry' Foehn events, where there is no precipitation, implies there must be other mechanisms. Foehn rotor cloud

Isentropic draw-down

Isentropic draw-down is the draw-down of warmer, drier air from aloft. When the approaching winds are insufficiently strong to propel the low-level air up and over the mountain barrier, the airflow is said to be 'blocked' by the mountain and only air higher up near mountain-top level is able to pass over and down the lee slopes as Foehn winds. These higher source regions provide Foehn air that becomes warmer and drier on the leeside after it is compressed with descent due to the increase in pressure towards the surface.

Mechanical mixing

When river water passes over rocks, turbulence is generated in the form of rapids, and white water reveals the turbulent mixing of the water with the air above. Similarly, as air passes over mountains, turbulence occurs and the atmosphere is mixed in the vertical. This mixing generally leads to a downward warming and upward moistening of the cross-mountain airflow, and consequently to warmer, drier Foehn winds in the valleys downwind.

Radiative warming

Dry Foehn conditions are responsible for the occurrence of rain shadows in the lee of mountains, where clear, sunny conditions prevail. This often leads to greater daytime radiative (solar) warming under Foehn conditions. This type of warming is particularly important in cold regions where snow or ice melt is a concern or where avalanches are a risk.

Effects

Winds of this type are also called "snow-eaters" for their ability to make snow and ice melt or sublimate rapidly. This is a result not only of the warmth of Foehn air, but also its low relative

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...

. Accordingly, Foehn winds are known to contribute to the disintegration of ice shelves in the polar regions. Foehn 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

wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...

s, making some regions which experience these winds particularly fire-prone.

Purported physiological effects

Anecdotally, residents in areas of frequent Foehn winds have reported experiencing a variety of illnesses ranging from migraines to

psychosis Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real. Symptoms may include delusions and hallucinations, among other features. Additional symptoms are incoherent speech and behavior ...

. 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 The Ludwig Maximilian University of Munich (simply University of Munich or LMU; german: Ludwig-Maximilians-Universität München) is a public research university in Munich, Germany. It is Germany's sixth-oldest university in continuous operatio ...

found that suicide and accidents increased by 10 percent during Foehn winds in Central Europe. The causation of '' Föhnkrankheit'' (English: Foehn-sickness) is unproven. Labels for preparations of aspirin combined with

caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is mainly used recreationally as a cognitive enhancer, increasing alertness and attentional performance. Caffeine acts by blocking binding of adenosine to ...

, codeine and the like will sometimes include ''Föhnkrankheit'' amongst the indications. Evidence for effects from

Chinook wind Chinook winds, or simply Chinooks, are two types of prevailing warm, generally westerly winds in western North America: Coastal Chinooks and interior Chinooks. The coastal Chinooks are persistent seasonal, wet, southwesterly winds blowing in from ...

s remain anecdotal. In some regions, Foehn winds are associated with causing circulatory problems, headaches, or similar ailments. Researchers have found, however, the Foehn wind's warm temperature to be beneficial to humans in most situations, and have theorised that the reported negative effects may be a result of secondary factors, such as changes in the electrical field or in the ion state of the atmosphere, the wind's relatively low humidity, or the generally unpleasant sensation of being in an environment with strong and gusty winds.

Local examples

Regionally, these winds are known by many different names. These include: ;in Africa * Bergwind in South Africa ;in the Americas *The

Brookings Effect Brookings is a city in Curry County, Oregon, United States. It was named after John E. Brookings, president of the Brookings Lumber and Box Company, which founded the city in 1908. As of the 2020 census, the population was 6,744. History F ...

on the southwestern coast of Oregon, also known as the Chetco Effect. *

s east of the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...

and the Cascade Range in the United States and Canada, and north, east and west of the

Chugach Mountains The Chugach Mountains of southern Alaska are the northernmost of the several mountain ranges that make up the Pacific Coast Ranges of the western edge of North America. The range is about long and wide, and extends from the Knik and Turnagai ...

of Alaska, United States * Foehn winds in the foothills of the southern Appalachian mountains, which can be unusual compared to other Foehn winds in that the relative humidity typically changes little due to the increased moisture in the source air mass *The

Santa Ana winds The Santa Ana winds (sometimes devil winds) "Scholars who have looked into the name's origins generally agree that it derives from Santa Ana Canyon, the portal where the Santa Ana River -- as well as a congested Riverside (CA-91) Freeway -- leav ...

of southern California, including the Sundowner (wind), Sundowner winds of Santa Barbara, are in some ways similar to the Föhn, but originate in dry deserts as a katabatic wind. *Puelche (wind), Puelche wind in Chile *Suêtes on the west coast of Cape Breton Island, Nova Scotia, Canada *Zonda winds in Argentina ;in Antarctica * Föhn wall on Signy Island, South Orkneys ;in Asia * Garmesh, Garmij, Garmbaad (Warm Wind): ( fa, گرمباد, glk, گرمش) in Gilan region (near the Alborz) in the south west of Caspian Sea in Iran. * In winter, a Foehn effect occurs in the West Azerbaijan province, Iran (around Lake Urmia) as manifested by the province's dry winters relative to those in the windward part of the region (Northern Iraq or Kurdistan Region and Hakkâri Province in Turkey). For example, the winter rainfall of Urmia and Salmas in Iranian Azerbaijan is much lower than Batifa and Soran, Iraq, Soran in Iraqi Kurdistan, and Hakkâri (city), Hakkâri in the Hakkâri Province, which are roughly on the same latitude but are on the windward side of the Zagros mountains. * Loo (wind), Loo in Indo-Gangetic Plain *Warm Braw in the Schouten Islands north of West Papua (province), West Papua, Indonesia. *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. *Laos wind ( vi, gió Lào), hot-dry west wind ( vi, gió tây khô nóng) in northern and central Vietnam. ;in Europe Mittagskogel Foehnwolken 15032008 01

* Favonio in Ticino and north-western

due to western and northern winds crossing the

(mostly in winter) *Garbino in the Adriatic Sea, Adriatic coast of

due to south-western winds crossing the Apennine Mountains (mostly in fall and winter) * Fen in northwest Slovenia * Fønvind in South Norway, in particular Central Norway, resulting in extreme winter warming, including Scandinavia's warmest winter temperature in Sunndalsøra. * Fogony in the Catalonia, Catalan Pyrenees * Föhn or Foehn in Austria, southern Germany, Switzerland, France and Liechtenstein *Föhn in Ostrobothnia (region), Ostrobothnia and Western Lapland (Finland), Lapland in Finland as moist air crosses Scandinavian Mountains and dries up. * Halny in the Carpathian Mountains, Poland (Central Europe) *The Helm Wind, on the Pennines in the River Eden, Cumbria, Eden Valley, Cumbria, England *Hnjúkaþeyr in Icelandic *Livas (wind), Livas wind in the Thessalian plain, Boeotia plain, Plain of Thessaloniki, Elefsina and Athens in Greece *Košava (wind), Košava (Koshava) wind in Serbia that blows along the Danube River *Nortada in Cascais, and most notoriously in Guincho Beach, making it one of the best windsurfing spots in Europe *Ponentà in Valencia (eastern Spain) *Terral in Málaga (southern Spain) *Vântul Mare in the Carpathian Mountains, Romania *Viento del Sur (Southern Wind) or Hego haizea in Basque in the Green Spain, Cantabrian region (northern Spain) ;in Oceania *The Australian foehn winds, Great Dividing foehn in southeast Australia, leeward of the Great Dividing Range, observed in the coastal plains of New South Wales, and also in eastern Victoria (state), Victoria and eastern Tasmania. *The Nor'west arch, Nor'wester in Hawkes Bay, Canterbury, New Zealand, Canterbury, and Otago, New Zealand

In popular culture

* The Foehn was mentioned by Queen's lead guitarist Brian May while talking about the band's grim Munich recording studio experience in 1982. * The foehn is attributed by the narrator of Jens Bjørneboe's 1966 novel ''Frihetens øyeblikk'' (''Moment of Freedom'') as the traditional cause of occasional unprovoked murders in a small Alpine town. * "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." *Fønfjord, meaning "Foehn Fjord," was named by Arctic explorer Carl Ryder after the powerful Foehn wind gusts blowing during the first exploration of the fjord in August 1891.

Gallery

File:Pájara Morro Jable - Carretera Punta de Jandía - cemetery + foehn 01 ies.jpg, Foehn over Carretera Punta de Jandía in Morro Jable, Pájara, Fuerteventura, Canary Islands File:La Palma - El Paso - Cumbre Nueva+Foehn (Mirador Llano del Jable) 01 ies.jpg, Dissolving clouds from Foehn wind over the Cumbre Nueva in El Paso, La Palma, Canary Island File:Wolkenwasserfall ms1408101.jpg, Foehn over Llano del Jable File:Storm Oratia 30 Oct 2000.jpg, Foehn can be initiated when deep low-pressure systems move into Europe, drawing moist Mediterranean air over the

References

* McKnight, TL & Hess, Darrel (2000). "Foehn/Chinook Winds". In ''Physical Geography: A Landscape Appreciation'', p. 132. Upper Saddle River, NJ: Prentice Hall. .

Footnotes

External links

Photo of Föhnmauer
The strong clouds at the mountain ridges where the Föhn winds form are called ''Föhnmauer'' (Föhn wall).
Illustration

Movie of a Föhn situation in the Swiss Alps

East Scotland warmth due to Foehn Effect

{{Authority control Föhn effect Mountain meteorology Weather and health Wind Winds cs:Místní názvy větrů#Fén