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Foehn, or Föhn in German, refer to a warm southerly wind coming over the Alps. However, the word is nowadays used to describe similar meteorological effects on all mountains all over the world.
For the anecdote, some time ago, the AEG german brand used "Fön" as a name for their electrical hairdriers. Subsequently that brand name has replaced the generic word and is now used in german for all electrical hairdriers.
Coming back to to topic, many mountaineers often use the expression "effect of Foehn" without really knowing its meaning. Let's try to understand how this phenomenon is working.
We have a mass of humid air that arrives on a mountainside. Let's take an hermetic mountain range oriented from East to West; The Pyrenees make a perfect example.
Most of the time, when the Foehn occurs, this mass of humid air comes from the Spanish side and the South.
This wind finds the mountain range on its way and is forced to follow its route, rising in order to avoid the obstacle and get to the other side.
While rising along the mountain slopes, in contact with the ground and by the cooling effect (adiabatic expansion: the highest, the coldest), it gets colder.
As cold air cannot contain the same quantity of humidity than warm air, some heavy rains occur on the humid mountainside (Spain in the example of the Pyrenees) and these rains get stronger and stronger as the mass of air gains altitude.
One of the first consequences of it is a bad weather on this side. Then the air reach the top of the mountain range, where as we can expect, occur some strong rafals of wind (Venturi effect), and get down, still pushed by the coming masses, on the opposite side. This air is now dry and will compress as it will go lower and lower. While compressing, it gets warmer quicker than it got colder on the first mountainside during the ascent.
Summarizing it, let's say that change of temperature of dry air is quicker than humid air, because of the difference of mass.
When all the humidity has gone on this side of the mountain range; the air is warm and the weather is very nice.
From the human eye point of view, the most current visible effect and most spectacular, is the barrier of clouds blocked on top of the ridge and suddenly vanishing, occasionally into a spectacular "waterfall of clouds".
By an effect of "rebound", clouds reappear slowly much further from the range, while already over the plain.
As a conclusion, for our example about the Pyrenees, we can state that a Foehn effect often means a fresh wet wind on the meridional Spanish side, especially getting higher in altidude, and a much better weather once on the French side, until a certain distance. This can seem paradoxal, as Spain is renowned for its sunnier weather. However, the described effect is more likely to occur during winter. The snow cover is very vulnerable to the Foehn effect, the layers are destabilized, and terrible avalanches are to be feared.
During the Autumn, on the other hand, this often mean a wonderful indian summer and a perfect hiking weather.
In the Pyrenees, it also occurs in the opposite direction, when cold wind and weather come from the Northern Europe, or a Gulf Stream strongly influenced from the North-West to the South-East (France having a more oceanic weather than Spain). Then, the phenomenon can be observed on the opposite way, with a brilliant spanish weather, once we passed one of the many tunnels crossing from one country to the other.
In such cases, Foehn weather is also feared by firemen to be a terrible fire starter, and propagator.
This phenomenon exists in many other ranges and countries: «Tramontane» in the East-End of the Pyrenees (West-East, along the hillsides of the southern Massif Central), «Mistral» in Provence, «Balaguère» in Central Pyrenees (name nowadays more known for the famous travel agency), «Haize Hegoa» in the Basque country, «Aspr» in the Massif Central...
In the alsace, it doesn't have a specific name but makes paradoxally northern Colmar, near the Vosges, France's driest city.
«Halny» in the Northern Carpathians, «Jauk» en Carinthia, «Jug» in Slovenian (like "Jugoslavia", "Jug" meaning "south"), Jauk in Austria (from «Jug»); «Bura» in Croatian, «Bora» in Serbian (the "Bura" often occurs over the barrier of the Dinarics).
«Sonda» in the South-American Ands («Zonda» in Argentina), «Puelche» in Chili, «Chanduy » in Mexico, «Chinook» in the Rockies, «Diablo» around San Francisco, «Santa Ana» in California... «The Nor'wester» or «Canterbury Northwester» in Southern New Zealand...
We often make a connection between the wind of Foehn and some secondary effects, such as migrains, changes of behaviours, psychosa. Changes also in animals behaviours: dogs, cattle.
A study carried out by the Munich university found that 10% more suicides were commited during Foehn weather. California's Santa Ana is also called "wind of the murder". And an old german saying, "Kriegt der Knecht vom Föhn einen Wahn, schlachtet er den Wetterhahn", means that a (« A barn valet hit by the Foehn will kill the Weathercock.»)
After all, French singer Brassens wasn't he also singing « Wind that goes through the mountain will turn me mad ! »
Note: everyone is invited to attach here Foehn effect images