Foehn wind

For the antiaircraft rocket projector, see Henschel Hs 297.
How a föhn is produced

A föhn or foehn is a type of dry, warm, down-slope wind that occurs in the lee (downwind side) of a mountain range.

Föhn can be initiated when deep low pressures move into Europe drawing moist Mediterranean air over the Alps.

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 14 °C (25 °F)[1] 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.

Effects

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 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.[2] 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.[3] Evidence for effects from Chinook winds remain anecdotal.

Cause

Main article: Rain shadow
The warm moist air from northern Italy stops on the windward side, loses much of its water vapour content, and descends on the French plateau and valley of the Mont-Cenis range in the Maurienne valley

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.[4]

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[5] 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 Slovene: fen. The German word "Fön" (without the "H", but pronounced the same way), a genericized trademark, also means "hairdryer," and the form "phon" is used in French-speaking parts of Switzerland and in Italy to mean "hairdryer" as well.

Local examples

Regionally, these winds are known by many different names. These include:


The Santa Ana winds of southern California, including the Sundowner winds of Santa Barbara, are in some ways similar to the Föhn, but originate in dry deserts as a katabatic wind.

See also

References

Footnotes

  1. ↑ "South Dakota Weather History and Trivia for January". National Weather Service Weather Forecast Office. February 8, 2006. See January 22 entry.
  2. ↑ 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.
  3. ↑ See the documentary: Snow Eater (the English translation of Canadian First Nations word phonetically pronounced chinook). telefilm.ca.
  4. ↑ Whiteman, C. David (2000). Mountain Meteorology: Fundamentals and Applications. Oxford University Press. ISBN 0-19-513271-8.
  5. ↑ Concise Oxford Dictionary, 10th edition, Oxford University Press, entry föhn.
  6. ↑ "Wind Names". ggweather.com.
  7. ↑ Romanić; et al. (2015). "Contributing factors to Koshava wind characteristics". International Journal of Climatology. Bibcode:2016IJCli..36..956R. doi:10.1002/joc.4397. Retrieved 20 August 2015.
  8. ↑ Romanić; et al. (2015). "Long-term trends of the 'Koshava' wind during the period 1949–2010". International Journal of Climatology. 35 (3): 288–302. Bibcode:2015IJCli..35..288R. doi:10.1002/joc.3981. Retrieved 20 August 2015.
  9. ↑ "Brian News - September 2012". brianmay.com.
  10. ↑ "Paris Review - The Art of Poetry No. 33, John Ashbery". theparisreview.org.
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