Foehn wind

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Foehn
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The causes of the Foehn effect in the lee of mountains. Adapted from:[1]
Cumbre Nueva (1400 m), La Palma

A Foehn or Föhn (

lee
(downwind side) of a mountain range. It is a
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 (

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

Maurienne 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.

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.

Rotor cloud revealing overturning and turbulence above the lee slopes of the Antarctic Peninsula during a westerly Foehn event

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. 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

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 remain 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]

Local examples

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

in Africa
  • Bergwind
    in South Africa
in the Americas
in Antarctica
  • Föhn wall on
    South Orkneys
in Asia
in Europe
Karawanken mountain range, Carinthia
, Austria
Foehn clouds over La Palma, Spain
in Oceania

In popular culture

Gallery

  • Foehn over Carretera Punta de Jandía in Morro Jable, Pájara, Fuerteventura, Canary Islands
    Foehn over Carretera Punta de Jandía in Morro Jable, Pájara, Fuerteventura, Canary Islands
  • Dissolving clouds from Foehn wind over the Cumbre Nueva in El Paso, La Palma, Canary Island
    Dissolving clouds from Foehn wind over the Cumbre Nueva in El Paso, La Palma, Canary Island
  • Foehn over Llano del Jable
    Foehn over Llano del Jable
  • Foehn can be initiated when deep low-pressure systems move into Europe, drawing moist Mediterranean air over the Alps.
    Foehn can be initiated when deep low-pressure systems move into Europe, drawing moist
    Mediterranean air over the Alps
    .

See also

References

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

Footnotes

  1. ^ .
  2. .
  3. .
  4. .
  5. ^ "foehn". Merriam-Webster.com Dictionary. Retrieved 2022-03-17.
  6. ^ "South Dakota Weather History and Trivia for January". National Weather Service Weather Forecast Office. February 8, 2006. See January 22 entry.
  7. ^ Concise Oxford Dictionary, 10th edition, Oxford University Press, entry föhn.
  8. ^ "22. Januar 2009 – Vor 100 Jahren: "Fön" ins Markenregister eingetragen". WDR.de. 2009-01-22. Retrieved 2019-12-21.
  9. .
  10. .
  11. ^ See the documentary: Snow Eater (the English translation of Canadian First Nations word phonetically pronounced chinook). telefilm.ca Archived 2013-10-17 at the Wayback Machine.
  12. ^ Brook, K., "Grumpy nor'west winds," University of Canterbury, 2 December 2014. Retrieved 22 November 2023.
  13. ^ .
  14. .
  15. .
  16. ^ "Wind Names". ggweather.com.
  17. S2CID 129669216
    .
  18. .
  19. ^ 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.
  20. ^ Relph, D. "The Canterbury nor'wester," New Zealand Geographic. Retrieved 17 February 2018.
  21. ^ "Brian News". brianmay.com. September 2012.
  22. .
  23. ^ John Ashbery (1983). "The Art of Poetry No. 33". theparisreview.org. Vol. Winter 1983, no. 90.
  24. ^ "Place names, northern East Greenland". data.geus.dk.

External links

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