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A foehn wind or föhn wind is a type of dry down-slope 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. Föhn winds can raise temperatures by as much as 30 °C (54 °F)^[1] in just a matter of hours. 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 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.

These winds are often associated with illnesses ranging from migraines to psychosis. The first clinical review of these effects was published by the Austrian physician, Anton Czermak in the Nineteenth 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.^{[citation needed]} 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.

Why the air is hotter on the Lee Side

If wind pushes air over a mountain range the air travels up the windward side and down the leeward side of the mountain. As the air travels up the mountain the atmospheric pressure is less and the air will expand. When air expands it will also cool per the gas law relation shown below. If there is no precipitation in the air flow over the mountain range then the temperature at any given elevation on both sides of the mountain will be equal and the wind velocity on both sides of the mountain range will be about the same.

P_w, P_le is the Pressure atmospheric: windward (w), leeward (le), and is proportional to altitude.

V_w, V_le is the Volume: windward (w), leeward (le)

T_w, T_le is the Temperature: windward (w), leeward (le)

P_wV_w/T_w = P_leV_le/T_le

Solving for T_le yields.

T_le = (P_leV_le/P_wV_w) *T_w

For dry-air the above shows that at the same altitude (pressure) on both sides of the mountain are the same P_leV_le/P_wV_w = 1 then the temperatures will be equal.

Now if the air flow starts off with humid-air (water air mix) flowing over a mountain the temperatures will be very different.

When humid-air flows up the mountain side the air expands and cools. The humid-air will lose temperature with elevation, but the temperature loss is at a much slower than with dry-air. Heat will transfer from the water to the air keeping the humid-air 'warm'. The water can hold more energy than air. As the humid-air continues to move up it will continue to cool and eventually will turn to precipitation in the form of rain or snow.

With all of the moisture rained out of the air, the air flow is cold and dry as it crests the mountain top. As the air flow descends the leeward side of the mountain and is compressed by atmospheric pressure and the act of compressing the air heats the air-flow up - following the gas laws. Because the compression is heating the dry-air (and not humid-air) the air temperature is hotter. In addition to the air-flow being heated by compression the sun also adds heat to the air flow. Because the air flow on the leeward side is dry, skies are typically clear and the sun contributes to the heating of the air as it flows down the mountain.

Etymology

The name föhn (German: Föhn, pronounced [ˈføːn]) originated in the alpine region. From Latin (ventus) favonius, a mild west wind of which Favonius was the Roman personification.^[3] The German word "Fön" (without the "H") is also used to mean "hairdryer".

Local examples

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

Lyvas wind in Elefsina, Athens in Greece
Zonda winds in Argentina
Chinook winds east of the Rocky Mountains, in the United States and Canada, and north, east and west of the Chugach Mountains of Alaska, United States
The Nor'wester in Hawkes Bay, Canterbury and Otago, New Zealand
Föhn in Wollongong and South Coast, NSW Australia. Often associated with heavy orographic lifiting on the windward side of the escarpment
Halny in the Carpathian Mountains, Central Europe
Fogony in the Catalan Pyrenees
Bergwind in South Africa
Viento del Sur (Southern Wind) in the Cantabrian region (northern Spain)
Terral in Málaga (southern Spain)
Föhn in Austria, southern Germany, German-speaking regions of Switzerland and Northern Italy (even non German-speaking regions)
Puelche wind in Chile.
Favonio in Ticino and Italy
The Helm wind, on the Pennines in the Eden Valley, Cumbria, England
Garmoosh, Garmesh, Garmbaad (Warm Wind): (Persian: گرمباد, Gilaki: گرموش) in Gilan region, in the south of Caspian Sea in Iran
Hnúkaþeyr in Icelandic.
Vântul Mare in the Carpathian Mountains, Romania.

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.

In popular culture

Peter Camenzind, a novel by Hermann Hesse, refers, at length, to the Alpine Föhn.
The Föhn is used for the letter F in "Crazy ABC's" from the album Snacktime! by the Barenaked Ladies.
The threat of the Föhn drives the protagonists Ayla and Jondalar in Jean M. Auel's The Plains Of Passage over a glacier before the spring melt. The pair make references to the mood altering phenomena of the wind, similar to those of the Santa Ana wind.
In Southern Germany, this wind is supposed to cause disturbed mood. Heinrich Hoffmann in his book "Hitler Was My Friend" notes that on the evening of September 18, 1931 when Adolf Hitler and Hoffmann left their Munich apartment on an election campaign tour, Hitler had complained about a bad mood and feeling. Hoffmann did try to pacify Hitler about the Austrian Föhn wind as the possible reason. Hours later, Hitler's niece, Geli Raubal was found dead in his Munich apartment. It was declared that she had committed suicide though it had conflicting testimonies from the witnesses present.
Mentioned as a surprise change in weather during the ascent of Switzerland's Eiger in the book "The Eiger Sanction" by Trevanian.

Fön trademark

AEG registered the trademark Fön in 1908 for its hairdryer. The word became a genericized trademark and is now, with varying spelling, the standard term for "hairdryer" in several languages, such as Finnish, German, Swiss German, Danish, Italian, Dutch, Norwegian, Czech, Croatian, Latvian, Romanian, Hebrew, Slovak, Slovenian, Swedish, Russian, Ukrainian, Turkish and Swiss French.

References

McKnight, TL & Hess, Darrel (2000). Foehn/Chinoonk Winds. In , Physical Geography: A Landscape Appreciation, pp. 132. Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-020263-0

Footnotes

^ [1]
^ 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.
^ Concise Oxford Dictionary, 10th edition, Oxford University Press, entry föhn.

External links

Photo of Föhnmauer The strong clouds at the mountain ridges where the Foehn winds form are called Föhnmauer (Foehn wall).
Illustration
Movie of a Foehn situation in the Swiss Alps

[SD_Weather_History_and_Trivia_for_January-1] [1]

[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] Concise Oxford Dictionary, 10th edition, Oxford University Press, entry föhn.

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[2]

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@@ Line 31: / Line 31: @@
 Now if the air flow starts off with '''humid-air''' (water air mix) flowing over a mountain the temperatures will be very different.
-When humid-air flows up the mountain side the air expands and cools.  The humid-air will lose temperature with elevation, but the temperature loss is at a much slower than with dry-air.  Heat will transfer from the water to the air keeping the humid-air 'warm'.  The  water can hold more energy than air.  As the humid-air continues to move up it will continue to cool and eventually will turn to perception in the form of rain or snow.
+When humid-air flows up the mountain side the air expands and cools.  The humid-air will lose temperature with elevation, but the temperature loss is at a much slower than with dry-air.  Heat will transfer from the water to the air keeping the humid-air 'warm'.  The  water can hold more energy than air.  As the humid-air continues to move up it will continue to cool and eventually will turn to precipitation in the form of rain or snow.
 With all of the moisture rained out of the air, the air flow is cold and dry as it crests the mountain top.  As the air flow descends the leeward side of the mountain and is compressed by atmospheric  pressure and the act of compressing the air heats the air-flow up - following the gas laws.  Because the compression is heating the dry-air (and not humid-air) the air temperature is hotter.  In addition to the air-flow being heated by compression the sun also adds heat to the air flow.  Because the air flow on the leeward side is dry, skies are typically clear and the sun contributes to the heating of the air as it flows down the mountain.