Foehn wind

The causes of the foehn effect in the lee of mountains. Adapted from.[1]

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)[2] 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.

In some regions, föhn winds are associated with causing "circulatory problems", headaches, or similar ailments.[3] 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.[3]

Causes

The warm moist air from northern Italy is blocked 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

Explanations of the foehn warming and drying effect in popular literature or on the web often single out just one causal mechanism (#1 - Condensation and Precipitation - in the below), but there are in fact four known causes[4] (illustrated in the schematic at top right of this page). These mechanisms often act together, with their contributions varying depending on the size and shape of the mountain barrier and on the meteorological conditions, for example the upstream wind speed, temperature and humidity.

1) Condensation and precipitation: When air is forced upwards over elevated terrain, it 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 above the mountain's upwind slopes. The change of state from vapour to liquid water is accompanied by heating, and the subsequent removal of moisture as precipitation renders this heat gain irreversible, leading to the warm, dry foehn conditions in the mountain's lee. This mechanism has become a popular textbook example of atmospheric thermodynamics and it lends itself to attractive diagrams. 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.

2) Isentropic draw-down (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.

3) 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.

4) 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 and/or 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. Accordingly, foehn winds are known to contribute to the disintegration of ice shelves in the polar regions.[5]

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.

Anecdotally, residents in areas of frequent foehn 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.[6] A study by the Ludwig-Maximilians-Universität München found that suicide and accidents increased by 10 percent during foehn winds in Central Europe. The causation of Föhnkrankheit (English: Foehn-sickness) is yet unproven. Labeling for preparations of aspirin combined with caffeine, codeine and the like will sometimes include Föhnkrankheit amongst the indications.[7] Evidence for effects from Chinook winds remain anecdotal.

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[8] 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. ↑ Elvidge, Andrew D.; Renfrew, Ian A. (14 May 2015). "The Causes of Foehn Warming in the Lee of Mountains". Bulletin of the American Meteorological Society. 97 (3): 455–466. doi:10.1175/bams-d-14-00194.1. Retrieved 23 April 2017.
  2. ↑ "South Dakota Weather History and Trivia for January". National Weather Service Weather Forecast Office. February 8, 2006. See January 22 entry.
  3. 1 2 TULLER, 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. Retrieved 21 May 2017.
  4. ↑ Elvidge, Andrew D.; Renfrew, Ian A. (14 May 2015). "The Causes of Foehn Warming in the Lee of Mountains". Bulletin of the American Meteorological Society. 97 (3): 455–466. doi:10.1175/bams-d-14-00194.1. Retrieved 23 April 2017.
  5. ↑ Elvidge, Andrew D.; Renfrew, Ian A.; King, John C.; Orr, Andrew; Lachlan-Cope, Tom A. (January 2016). "Foehn warming distributions in nonlinear and linear flow regimes: a focus on the Antarctic Peninsula". Quarterly Journal of the Royal Meteorological Society. 142 (695): 618–631. doi:10.1002/qj.2489.
  6. ↑ 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.
  7. ↑ See the documentary: Snow Eater (the English translation of Canadian First Nations word phonetically pronounced chinook). telefilm.ca.
  8. ↑ Concise Oxford Dictionary, 10th edition, Oxford University Press, entry föhn.
  9. ↑ "Wind Names". ggweather.com.
  10. ↑ 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.
  11. ↑ 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.
  12. ↑ "Brian News - September 2012". brianmay.com.
  13. ↑ "Paris Review - The Art of Poetry No. 33, John Ashbery". theparisreview.org.
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