Type of dry down-slope wind occurring near mountains
"Föhn" redirects here. For the antiaircraft rocket, seeHenschel Hs 297.
The causes of the Foehn effect in the lee of mountains (adapted from:[1])DissolvingFöhn clouds overCumbre Nueva,La Palma, at an elevation of 1,400 m (4,600 ft)
Foehn winds can raisetemperatures 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 theMediterranean Sea blow over theAlps.
The nameFoehn (German:Föhn,pronounced[ˈføːn]) arose in theAlpine region. Originating from Latin(ventus) favonius, a mild west wind of whichFavonius was the Roman personification[7] and probably transmitted byRomansh:favuogn or justfuogn, the term was adopted asOld High German:phōnno. In the Southern Alps, the phenomenon is known asFöhn but alsoItalian:favonio andfen inSerbo-Croatian andSlovene. The German wordFöhn (pronounced the same way) also means 'hairdryer', while the wordFön is agenericized trademark today owned byAEG.[8] The formphon is used inFrench-speaking parts of Switzerland as well as inItaly.
The nameFöhn was originally used to refer to the south wind which blows during the winter months and brings thaw conditions to the northern side of theAlps. 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]
The warm moist air from northern Italy is blocked on thewindward side, loses much of its water vapor content, and descends on the French plateau and valley of the Mont-Cenis range in theMaurienne valley.
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.
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 vapor, moisture condenses to form clouds and precipitates as rain or snow on the mountain's upwind slopes. The change of state from vapor to liquid water releaseslatent 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.
Rotor cloud revealing overturning and turbulence above the lee slopes of the Antarctic Peninsula during a westerly Foehn event
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.
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.
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 orsublimate rapidly. This is a result not only of the warmth of Foehn air, but also its low relativehumidity. 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 theEiger, for whom the winds add further difficulty in ascending an already difficult peak.
They are also associated with the rapid spread ofwildfires, making some regions which experience these winds particularly fire-prone.
Anecdotally, residents in areas of frequent Foehn winds have reported experiencing a variety of illnesses ranging frommigraines topsychosis. The first clinical review of these effects was published by the Austrian physician Anton Czermak in the 19th century.[11] A study by theLudwig-Maximilians-Universität München found thatsuicide and accidents increased by 10 percent during Foehn winds in Central Europe.[citation needed] The causation ofFöhnkrankheit (English: Foehn-sickness) is unproven. Labels for preparations of aspirin combined withcaffeine,codeine and the like will sometimes includeFöhnkrankheit among the indications.[citation needed][12] Evidence for effects fromChinook winds remains anecdotal, as it does for New Zealand'sNor'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 theorized 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 southernAppalachian 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]
Wuhan in China is famously known as one of theThree 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 centralVietnam.
Fønvind inSouth Norway, in particularCentral Norway, resulting in extreme winter warming, including Scandinavia's warmest winter temperature inSunndalsøra.
Lodos wind, causing warm temperatures in the leeward side of mountains in the mild-winter climate of theAegean Sea,Greece and westernTurkey, as well as unusually mild temperatures in the cool or moderately cold winter climates north of theMarmara Sea, such asIstanbul,Adapazarı andZonguldak.
Košava (Koshava) wind in Serbia that blows along the Danube River[18][19]
Nortada inCascais, and most notoriously inGuincho 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 theWayback Machine.
^Sharples, J.J. Mills, G.A., McRae, R.H.D., Weber, R.O. (2010)Elevated fire danger conditions associated with foehn-like winds in southeastern Australia. Journal of Applied Meteorology and Climatology.
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