Foehn effect

The Foehn effect

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.

The Foehn effect

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.

Pyrenees Foehn

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.

Aspr, the Foehn in the Cantal

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

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Viewing: 1-15 of 15

Nelson - Nov 26, 2008 9:14 am - Voted 10/10

Chinook

The name of Rocky Mountain "Chinook" wind that you mention comes from an American Indian word meaning "snow-eater", which is also appropriate. The effect is not uncommon here along the Front Range of the Colorado Rockies. It brings a few glorious warm days in the middle of winter.

StarMan - Nov 26, 2008 9:16 am - Voted 10/10

Good article.

This effect happens often in the southern Appalachian mountains of the USA. In winter, some very impressive snow totals occur on the upslope side of the northwest winds after a cold front passage... while the downslope side is dry... usually with sunshine.

Proterra - Nov 26, 2008 2:34 pm - Voted 10/10

A few things I miss...

Winds always blow up and down mountains, but Foehn only occurs (and I guess, the people living in low-latitude valleys, say 25-40 degrees on both sides of the equator are very happy about this) in areas where the mountains are higher than the saturation point of the airmasses when it rises and cools down according to the dry adiabatic lapse rate.

You posted a nice graph, which didn't explain the technical workings of it, so I'll give it a shot :-)

Say, it's 30C down on the foot, with a dewpoint of 12C and it needs to cross a 2000 metre range, there will be no Foehn on the other side. By the time the airmass reaches the summit, it will have cooled down to 10C with a dewpoint of 8C. If the same range is 3000 metres tall, it'll reach the saturation point at around 2350 metres, with a temperature and dewpoint of about 7.2C, and it'll start raining here, and the airmass will continue rising the last 650 metres by the moist adiabatic regime, to force remove moisture from the air and force the dewpoint down. When it reaches the summit, it's down to 4C for both dewpoint and temperature, resulting in 34C temps and 10C dewpoints on the leeward side, as opposed to 30/12C on the windward side.

If one goes further north into areas where the climate is more moist, and 15C temps with 10C dewpoints are common, and the airmasses need to cross the same 3000 metre range, (Say WA state for example, although they have a double foehn, and the Cascades are less pronounced - but the effect is pretty much the same.) the saturation point is reached much earlier. At 600 metres both dewpoints and temperatures are down to 9C, and everything starts rising moist adiabatic from here, resulting in -3C for both dewpoint and temperature at the summit, and 27C (81F) temps with 3C (37F) dewpoints in Yakima (= 20%RH), as opposed to 15C (59F) temps and 10C (50F) dewpoints in Quillayute (= 70%RH). Because Washington State is in the Marine west coast climate zone, these foehns here are part of the climate instead of a weather phenomenon, creating deserts outside of the Horse Latitudes.

Cheers,
Clint.

visentin - Nov 28, 2008 7:23 am - Hasn't voted

Re: A few things I miss...

Thanks Proterra. I am convinced of the coherence of this technical addition, but I am afraid most of the common people are not able to read it. If you don't mind, I'll leave it here as a comment.

Proterra - Nov 28, 2008 7:31 am - Voted 10/10

Re: A few things I miss...

Fair enough, but MoapaPk mentioned the same thing on the thread you opened in general. This is an explanation of how it works and a few examples.

Cheers,
Clint.

yatsek - Dec 15, 2008 6:00 am - Hasn't voted

Re: A few things I miss...

Hope you don't mind me butting in. I get the feeling you're both right/wrong. Clint, as a scientist, tends to overcomplicate things a little bit while Eric, who majored in a different subject, has taken too much of a shortcut. I'd personally be for squashing Clint's to approx. 2/3
Cheers,
Jacek/Yatsek

visentin - Dec 15, 2008 7:26 am - Hasn't voted

Re: A few things I miss...

You are completely right, I am not meteorologist, and despite I know few basics physics I wrote it to be read by anyone. Clint's explanation is probably right but perhaps it needs to be rewritten a bit more clearly.

bakcast - Dec 2, 2008 8:32 pm - Hasn't voted

Bridger Mountains

I have seen this phenomenon many times on the leeward slopes of the Bridger Mountains in SW Montana - the clouds appear over the ridge creating a fascinating effect where the clouds bleed over the windward side of the range. As I recall it typically happens in the Spring and Fall and isn't aways present when we get Chinook winds. In fact, I don't think the winter Chinooks usually create the effect - only in Fall and Spring. I'll pay more attention to the time of year and snap a few photos next time. Thanks for posting!

BobSmith - Dec 7, 2008 10:47 pm - Voted 10/10

I recall...

first encountering the term when I was taking German classes some years ago.

Diggler - Dec 10, 2008 11:48 am - Voted 10/10

Good article

In particular, explaining how the winds come down hotter than they started. Thanks!

"Kriegt der Knecht vom Föhn einen Wahn, schlachtet er den Wetterhahn."- 'The farmhand that goes crazy because of (literally: gets a mania from) the Föhn kills the weathercock.'

distressbark - Jan 12, 2009 3:40 am - Hasn't voted

Glacier National Park

I lived in the Many Glacier region of Glacier NP this past summer, and had the pleasure of witnessing this effect firsthand on numerous instances. About 6 miles west of my residence is the Continental Divide, and we would often see storm clouds west of the divide that broke up as they came over Mount Gould, Swiftcurrent Mountain, and Mount Wilbur. Like your article describes, we'd see those clouds precipitously hanging atop the Divide, only for it to be warm and dry on our side of the valley. As expected, the west side of Glacier is wetter and slightly colder, with conditions in certain places such that there are actually patches of temperate rain forest within the park. The east side typically is warmer and drier.

Nice article. Thanks for the information.

visentin - Dec 6, 2010 7:50 am - Hasn't voted

Re: Glacier National Park

With such a wide continent I am pretty sure you can observe enormous contrasts between both sides. Probably there is something kind of similar to our Atlantic gulf stream coming from your Pacific coast and bringing oceanic depressions on the West.
Without having been in USA, I can see on picture how much greener is the East side of the rockies, and I guess a huge foehn occurs on such high and long barrier. Perhaps it is less visible on one single ridge, as the thickness of the Rockies divide the effect in as many subranges.

ramram - Sep 5, 2013 8:53 am - Hasn't voted

Soaring

Another name associated with the Foehn effect is mountain wave. Soaring pilots use this to fly sailplanes to impressive altitudes, the record being upwards of 50,000feet! Here in the Northeast, autumn winds provide the ride to more moderate altitudes. Glider pilots from NY and Canada will be gathering in Lake Placid later this month to fly over the Adirondack Mountains. After several decades of climbing I took up the sport of soaring and hope to fly my sailplane higher than I've ever climbed. (Hint: I just bought an oxygen system.)

visentin - Sep 6, 2013 7:52 am - Hasn't voted

Re: Soaring

You are totally right, except perhaps for one detail, Foehn wind is a mountain wawe with bad weather on one side. Which in most of the cases probably does not allow safe soaring (clouds, low visibility...)
Without higher humidity on one side, the phenomenon of temperature increase on the other is less present, it becomes a simple effect is mountain wave but without Foehn :)

ramram - Sep 6, 2013 9:30 am - Hasn't voted

Re: Soaring

Right you are! That's just one of the perils of flying in wave. With high humidity the clouds show where to go and where not to go. Stay away from the Foehn! Downwind from the Foehn there will be rows of clouds with open air in between them. The upwind side of these clouds will be rising air. The downwind side will be descending air. High lenticular clouds are an indication of mountain wave.

I find soaring to be very similar to climbing in that it is very mental; problem solving and decision making from the moment you start until you're down safe and sound. Clouds make up a lot of the scenery and you have to keep a close eye on the weather. And it's basically the same people, just a little older and maybe not in as good shape as climbers.

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