Type of dry down-slope wind occurring near mountains
"Föhn" redirects here. For the antiaircraft rocket, see
Henschel Hs 297.
A Foehn, or Föhn (UK: /fɜːn/, US: /feɪn/fayn,[2][3]US also /fʌn,fɜːrn/fu(r)n[4][5]), 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). 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.
Foehn winds can raise
temperatures by as much as 14 °C (25 °F)[6] 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.
Etymology
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[7] 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 Föhn but also
Italian: favonio and fen in
Serbo-Croatian and
Slovene. The German word Föhn (pronounced the same way) also means 'hairdryer', while the word Fön is a
genericized trademark today owned by
AEG.[8] The form phon is used in
French-speaking parts of Switzerland as well as in
Italy.
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
Alps. 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" (Alpenföhn) was coined for the Föhns of the Alpine region.[9]
Causes
There are four known causes of the Foehn warming and drying effect.[1] 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 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.
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.
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. Accordingly, Foehn winds are known to contribute to the disintegration of ice shelves in the polar regions.[10]
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
wildfires, 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. The first clinical review of these effects was published by the Austrian physician Anton Czermak in the 19th century.[11] A study by the
Ludwig-Maximilians-Universität München found that
suicide and accidents increased by 10 percent during Foehn winds in Central Europe.[citation needed] The causation of Föhnkrankheit (English: Foehn-sickness) is unproven. Labels for preparations of aspirin combined with
caffeine,
codeine and the like will sometimes include Föhnkrankheit amongst the indications.[citation needed][12] Evidence for effects from
Chinook winds remains anecdotal, as it does for New Zealand's
Nor'wester.[13]
In some regions, Foehn winds are associated with causing circulatory problems, headaches, or similar ailments.[14] 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.[14]
Foehn winds in the foothills of the southern
Appalachian Mountains,[15] 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[16]
Garmesh, Garmij, Garmbaad (
lit. transl. Warm wind): (
Persian: گرمباد,
Gilaki: 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 in Iraqi Kurdistan, and
Hakkâri in the Hakkâri Province, which are roughly on the same latitude but are on the windward side of the
Zagros mountains.
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 (
Vietnamese: gió Lào), hot-dry west wind (
Vietnamese: gió tây khô nóng) in northern and central
Vietnam.
in Europe
Favonio in
Ticino and north-western
Italy due to western and northern winds crossing the
Alps (mostly in winter)
Garbino in the
Adriatic coast of
Italy 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.
Lodos wind, causing warm temperatures in the leeward side of mountains in the mild-winter climate of the
Aegean Sea,
Greece and western
Turkey, as well as unusually mild temperatures in the cool or moderately cold winter climates north of the
Marmara Sea, such as
Istanbul,
Adapazarı and
Zonguldak.
Košava (Koshava) wind in Serbia that blows along the Danube River[18][19]
Nortada in
Cascais, and most notoriously in
Guincho Beach, making it one of the best windsurfing spots in Europe
^Upton, Clive; Kretzschmar, William A. Jr. (2017). The Routledge Dictionary of Pronunciation for Current English (2nd ed.). Routledge.
ISBN978-1-138-12566-7.
^Giannini, AJ; Malone, DA; Piotrowski, TA (1986). "The serotonin irritation syndrome – a new clinical entity?". The Journal of Clinical Psychiatry. 47 (1): 22–25.
PMID2416736.
^See the documentary: Snow Eater (the English translation of Canadian First Nations word phonetically pronounced chinook).
telefilm.caArchived 2013-10-17 at the
Wayback Machine.
^
abTuller, Stanton E. (April 1980). "The Effects of a Foehn Wind on Human Thermal Exchange: The Canterbury Nor'wester". New Zealand Geographer. 36 (1): 11–19.
doi:
10.1111/j.1745-7939.1980.tb01919.x.