Understanding and Forecasting Alpine Foehn

Abstract

This chapter focuses on the history, physics, climatology, forecasting and the broad effects of Alpine foehn on human populations. In the European Alps, foehn winds have been studied since the mid-1800s. The main focus of the investigations was the question of why foehn winds are so warm. While it soon became clear that adiabatic processes provide an explanation, the role of wet adiabatic rising on the upwind side of the Alps continued to be strongly debated. The so-called textbook theory for foehn – heat gain by wet adiabatic, forced lifting on the upwind side followed by dry adiabatic descent in the lee – represents only an extreme situation. Foehn occurs also with partial or complete blocking of the upwind air mass, i.e., with limited or no heat gain by wet adiabatic expansion. The second focus is on processes which lead to descending air masses after passing the mountain ridge. A discussion of the most important processes shows that there seems to be no theory which is applicable in all situations. Forecasting foehn is still a challenge to meteorologists. While the general foehn situation can be predicted reliably, today’s numerical models still often poorly simulate the sudden break in of potentially devastating foehn air in the lee. Efforts to improve this must continue because foehn storms have a significant societal impact (threat to transportation systems and massive increase of fire danger) as several recent incidents show.

Appendices

Appendix A: Explanation and Definition of Foehn-Related Terms

Alpine Foothills Foehn

A warm but humid wind on the Alpine foothills, in the lee the air is mostly calm. This is not a foehn in the meteorological sense.

Anticyclonic Foehn

Foehn in an anticyclonic situation with a high potential height of the 500 hPa level. Only partly cloudy in the south, and usually dry. Temperature gain in the foehn valleys due to adiabatic sinking, flow forced by southerly pressure gradient.

Cyclonic Foehn

Foehn in connection with a cyclonic regime, i.e. low potential height of the 500 hPa level. Causes cloudiness even in the classical foehn areas, but criteria for foehn are well pronounced. Develops in extreme cases to dimmerfoehn.

Cyclonic Foehn

Foehn in connection with a cyclonic regime, i.e. low potential height of the 500 hPa level. Causes cloudiness even in the classical foehn areas, but criteria for foehn are well pronounced. Develops in extreme cases to dimmerfoehn.

Deep Foehn, High-reaching Foehn

The classical foehn situation with a high-reaching SW flow driven by a high over northern Italy and a low over southern Germany; isobars show the typical foehn knee; lee waves, lenticularis clouds, foehn wall, often also lee side rotors are present; warm, dry winds with high speed reach the valley floors (Hoinka 1990).

Dimmerfoehn

A south foehn which does not immediately follow the topography in the lee, but touches the surface further downwind. The mountain ridge is in clouds that extend downwind. The comparatively calm area right downwind of the ridge is dark due to heavy clouds, hence the name (“dimmerig” or “dimmrig” [Swiss German] means dim, obscure). In rare cases, there is no precipitation but it is very hazy due to Saharan dust.

Double Foehn

The simultaneous existence of north and south foehn. Double foehn is a short-lived, very rare phenomenon which occurs when a cold high-pressure system in the lower troposphere moves quickly over the Alpine ridge while a stormy westerly flow is present a greater height (Frey 1986).

Flat Foehn => Shallow Foehn

Foehn Air

The warm and dry air produced by foehn.

Foehn Bank => Foehn Wall

Foehn Break => Foehn Window

Foehn Clearance

At distances larger than about 50 km downwind from the Alpine ridge, foehn winds become light, while they remain warm and dry. This leads to a significant reduction of cloudiness, i.e., to a clearing (Hoinka 1985b).

Foehn Cloud

Lenticularis clouds in the lee of the mountain barrier. They are caused by the lee waves associated with the foehn flow. Their orientation is parallel to the ridge, and because the lee waves are stationary, also the foehn clouds are quasi-stationary. Note: Lenticularis clouds are neither a prerequisite for, nor proof of foehn, they can occur with any airflow over a barrier. The term “foehn cloud” is not be confused with the => foehn wall.

Foehn Cyclone

A cyclone in the lee of a mountain massif which is formed or enhanced by a foehn process.

Foehn Gap => Foehn Window

Foehn Island

The surface area where foehn touched down while the surrounding area is still covered with cold air.

Foehn Knee, Foehn Nose, Foehn Wedge

The typical deformation of the isobars (or isohypses) in a foehn situation. A high-pressure wedge lies on the windward side of the mountain range, while in the lee a trough is formed. As a consequence, the isobars (or isohypses) bulge and take a nose-like or knee-like form. In the German literature, “foehn knee” (“Föhnknie”) is more commonly used, in the English literature the term “foehn nose” (Föhnnase) is normally found.

Foehn Nose => Foehn Knee

Foehn Pause

Foehn winds do not blow constantly, but there is varying intensity and sometimes cessations. Such an interruption is called foehn pause (the German “Pause” means break, pause, intermission). Foehn pauses occur predominantly during the early morning hours before sunrise when cold air masses intrude.

Rarely, the term foehn pause is used for the region delimiting the foehn air from the ambient air, this analogous to “tropopause”, “stratopause”, etc.

Foehn Period

This term deals with the duration or simply with the occurrence of foehn at one or several stations. The use of the term by different research groups is very diverse, hence, there is no generally accepted definition.

Foehn Phase

A rather general term used by Alpine foehn researchers for classifying the different development stages of a foehn situation; the definition of the phases might differ among different authors. The systems used by Ficker and by Streiff-Becker (1933) differentiated three phase, the recent system introduced by Seibert (1990) has four:

• Phase I: cold air masses on both sides of the Alps

• Phase II: warm advection from W or SW, development of temperature gradient and, as consequence, pressure gradient across Alpine ridge, => shallow foehn develops

• Phase III: approaching trough changes flow to SW or S, speed and humidity increase, pressure gradient reaches maximum, => deep foehn develops, precipitation in the upwind region

• Phase IV: passing of cold front, breakdown of foehn, often formation of a cyclone in the N.

Foehn Storm

A potentially destructive storm as result of a foehn situation. A foehn storm is characterized by its sudden occurrence in a practically calm situation and its intermittent nature during its onset. After the initial phase, wind speed remains high but steady. Additionally, the fast moving air is, as a result of the foehn mechanism, significantly warmer than the displaced air. Foehn storms belong into the category of downslope windstorms where they are often referred to as warm downslope windstorms.

Foehn Tongue

The area covered by foehn air flowing out of foehn valleys.

Foehn Trough

The trough in the lee of the mountain range formed dynamically by the flow across the barrier.

Foehn Wall, foehn bank

In those situations where foehn occurs with precipitation on the windward side of the mountain range, the clouds can be seen from the lee side as a “wall” topping the mountain ridge. Depending on the characteristics of the foehn flow, the clouds can be dragged over the ridge where they are dissolved while sinking. Under these conditions, the foehn wall has the appearance of a cap.

Foehn Waves

The lee waves or mountain waves associated with a foehn situation. Sometimes foehn waves become visible because they produce “foehn clouds”, i.e., cumulus lenticularis. Foehn waves must not be confused with the gravity waves occurring on the top of the cold pool.

Foehn Wedge => Foehn Knee

Foehn Window, Foehn Gap, Foehn Break

A cloud-free gap or clearance in the lee of the mountain range over which a foehn flow occurs. This gap is caused by the descending foehn air that is dry-adiabatically heated and, consequently, becomes dry.

Foehnic Situation, Foehn-Like Situation

Southerly flow which does not penetrate into the valleys. Foehn wall, foehn window, and lenticularis clouds may be present, visibility is very high.

Free Foehn

The sinking of air masses in the free atmosphere on a synoptic scale, particularly in high-pressure areas. The vertical motion is often brought to a standstill by an inversion that sometimes tops a fog layer. The increase in temperature and decrease in humidity caused by the dry-adiabatic sinking leads to dissipation of clouds. This process has nothing to do with mountain winds, and the use of the term “foehn” in this context is strongly discouraged.

Gap Foehn (Gap Flow) => Pass Foehn

High-Reaching Foehn => Deep Foehn

Light Foehn

Corresponds to phase II of the Innsbruck foehn strength scale (=> foehn phase).

Pass Foehn, Gap Foehn

A pass foehn exists when the foehn criteria are met at least at one mountain or pass station, but not at a valley station. The distinction between pass foehn and => valley foehn was introduced to characterize also foehn situations in which the flow does not penetrate into the valleys. Another name for a => foehnic situation.

Sandwich Foehn

A complex and short-lived three-layer flow situation along the Brenner cross section, first described during the Mesoscale Alpine Program MAP (Vergeiner and Mayr 2000): a shallow pressure-driven south foehn event below a decoupled flow from W or even NW, below the foehn flow a very stable cold pool which cannot be removed by the southerly flow. (In other areas, e.g., over the Swiss Plateau, such a situation can persist for up to several days; here the term is not used.)

Shallow Foehn, Flat Foehn

A foehn with all its characteristics in the downwind side valley but without the dominant southerly flow at high altitude, here flow can be from W or even NW. Shallow foehn can be regarded as compensation flow between different air masses on both sides of the mountain massif; it is often found in the lee of passes which form gaps in the mountain barrier (Seibert 1990).

Strong Foehn

Corresponds to phase III of the Innsbruck foehn strength scale (=> foehn phase).

Talfoehn => Valley Foehn

Valley Foehn, Talfoehn

A valley foehn is present when the criteria for foehn are fulfilled at least for one mountain station and at least for one valley station. Used in this sense, it is not a foehn type but a foehn phase. See also pass foehn.

Also, the term “valley foehn” is a somewhat obsolete name for deep foehn used by Streiff-Becker. He used it to distinguish between deep foehn and Alpine foothills foehn (the latter not being a foehn in a meteorological sense).

Appendix B: Names of Foehn-Type Winds in Different Regions

The German term Föhn originated most probably during Roman times. When the Romans conquered the territories north of the Alps in the first century BC, they found this stormy, warm, and dry mountain wind. From home, they knew a similar wind, the “favonius” (meaning “the favorable”), a warm and dry wind originating in northern Africa. Albeit the two winds are meteorologically very different, they do have similar characteristics; hence, it was logical that the Romans gave this name also to the warm wind they found north of the Alps. In the Raeto-Roman language (still spoken today as Rumansch by a minority in the Swiss Alps) “favonius” became “favuogn”, in its dialect “fuogn”. In the Old High German language “fuogn” became “phōnno” which over time gradually developed to the German name “Föhn” (the öh spoken as in b-i-rd or l-ea-rn).

As mentioned in Sect. 4.1, foehn winds in the meteorological sense occur wherever large mountain massifs exist. In meteorology, the general term “foehn” for the warm dry wind was chosen, because research started in the Alps, hence, the local name became also the scientific label. The following list is based on Schamp (1964), on information collected through personal contacts by Gubser (2006), and on internet searches.

Aperwind (Alps)

Foehn in Spring which melts the snow (apertus [lat.]: open; apern [ger.]: melting of snow).

Aspre (Western Slope of Central Massif, France)

Dry, warm easterly winds blowing from the Central Massif over the Garonne plain, also called “Lou Cantalié”.

Appenzeller Föhn (Eastern Switzerland)

Regional foehn in the foothills of the Alps around Appenzell, St. Gallen, and Lake Constance. No southerly wind component at the Alpine ridge, but locally favorable foehn conditions. Occurs usually with westerly winds and falling pressure north of the Alps.

Austru (Walachia, Romania)

Foehn downwind of the Balkan Massif and the Transylvanian Alps from W to SW; occurs predominantly in winter, the associated clearing leading to severe radiation frost.

Autan (Southern France)

The Foehn in the lee of Cevenne Mountains and the Pyrenees from SE to E.

Autan Blanc (Southern France)

Particularly strong => Autan with cloudless sky; can reach the Atlantic coast south of the Gironde.

Autan Noir (Southern France)

An => Autan followed by rain steered by a low over the Bay of Biscay.

Berg Wind (South Africa)

Hot and dry foehn-like wind that blows from the highlands down to the coastal plateau; particularly Southwest Africa.

Bohorok, Also Bokorot (Eastern Sumatra)

Dry wind blowing from the Karo Plateau down to the plains of northeast Sumatra, a passat wind warmed by the foehn mechanism.

Boulder Windstorm (Boulder, U.S.A.)

A windstorm occurring basically with => Chinook in Boulder, CO.

Bregenzer Fallwind (Bay of Bregenz, Austria)

An easterly to northeasterly foehn-type wind blowing form the Gebhardsberg and Pfänder, also called “Ostföhn” (east foehn), “Falscher Föhn” (wrong foehn, because it comes from the unusual direction), or “Pfänderwind”.

Broebroe, Brubru (Sulwesi, Indonesia)

A gusty foehn-type easterly wind, part of the east monsoon near Makassar on the west coast of the island.

Canterbury Northwester => Northwester

Cape Doctor (Capetown, South Africa)

Maritime air masses, heated by a foehn-type process but still cold, flowing from False Bay over Capetown towards Table Bay.

Chanduy (Mexico)

Foehn-type wind in the area of Guayaquil, Mexico; occurs primarily during the dry season in the afternoon.

Chinook (Rocky Mountains, U.S.A.)

Warm and dry foehn-type west wind on the eastern slope of the Rocky Mountains. (In coastal regions of Oregon and Washington also a warm, humid sea breeze from SW passing over coastal areas, has nothing to do with foehn.) The American analogue to Alpine foehn, for details of the mechanism see Brinkmann (1973).

Chinook Arch

A striking feature of the => Chinook, a band of stationary stratus clouds caused by air undulations over the mountains due to orographic lifting.

Falscher Föhn => Bregenzer Fallwind

Favogn, Favuogn (Grisons, Switzerland)

Romansh name for foehn.

Gending (Java)

Foehn winds over the northerly plains of Java during the SE monsoons.

Ghilbi (Libya)

Basically a hot desert wind often connected with sand storms. However, when passing over coastal mountain ranges, there is an additional foehn effect.

Great Wind (Inner Asia)

Foehn-type NE wind on the SW slopes of the Alatau.

Guggifoehn (Bernese Oberland, Switzerland)

The Alpine foehn in the region of the Lauberhorn mountain.

Halny Wiatr (Poland)

A south foehn in the High Tatra.

Helm Wind (Northern England)

A strong foehn-type wind from the Cross Fell, characterized by cloud caps on the mountain peaks, hence the name.

Himmelsbesen => Sky Sweeper

Ibe (Western China)

Strong wind through a gap, the Dzungarian Gate (separating the depressions of Lake Balkhash [Kazakhstan] and Lake Ala-Kul [Kyrgyzstan]). The wind is a gap wind similar to foehn.

Jauk (Klagenfurt Basin, Austria)

South foehn over the Karawanken mountains.

Kâchan (Sri Lanka)

Foehn-type wind connected with the SW monsoon in the easterly parts of Sri Lanka. The Tamil speaking population calls it “solaha-kâchan”, i.e., dry, burning monsoon.

Kumbang, Koembang (Java)

SE wind near Tijriban and Tegal on the north coast, the foehn effect is due to the Pembarisan mountains.

Lenzbote (Alpine countries)

Name of the spring foehn melting the snow (“Lenzbote” [ger.]: spring messenger).

Levanto (Canary Islands)

Foehn-type E wind particularly in the Orotava valley on Teneriffa.

Livas (Greece)

Any foehn-type wind. The name “Livas” (plural: “Lives”) is derived from “Lips”, a warm SW wind from the direction of Libya (which is a warm wind but not a foehn-type wind).

Ljuka (Carinthia, Austria)

Local name of foehn, most likely derived from the Slovenian word “jug” meaning south wind.

Llebetg, Llebejado (Roussillon and Eastern Pyrenees, France)

Arabic or Catalan name of a foehn-type SW wind on the northern slopes of the eastern Pyrenees.

Lou Cantalié => Aspre

Megas (Greece)

A foehn-type SW wind blowing from Parnassus towards Boeotia, particularly in the plain of the dried-up Kopais lake.

Mikuni-Oroshi (Japan)

The “downwind of Mikunitoge” is a foehn-type W to NW wind in the Tone valley near Maebashi (eastern coast).

Moazagotl (Riesengebirge, Germany)

The name of a peculiar cloud showing the lee waves. It is formed in the prefrontal stage of a foehn cyclone.

Canterbury Northwester => Northwester

Montana Monsoon (Montana, U.S.A.)

Popular name for the => Chinook in the prairies of Montana.

Northwester, Canterbury Northwester (New Zeeland)

A warm and dry northwestly wind that reaches Canterbury Plain on South Island.

Norder, Norther or Nortes (Gulf of Mexico, Central America)

Actually a cold stormy wind, however, when blowing from the southeast it passes over the Isthmus of Tehuantepec, the blocking on the upwind side causes precipitation while in the lee, on the side of the Pacific foehnic clearing occurs.

Ostföhn => Bregenzer Fallwind

Pacific Wind (Colorado, U.S.A.)

A name sometimes used for the Chinook.

Pfänderwind => Bregenzer Fallwind

Puelche (Southern Chili)

Foehn wind in the Andes, in Argentina the same wind is called Zonda.

Rotenturm Wind, Talmac Wind (Transylvania, Romania)

A foehn-type south wind near Sibiu (Hermanstadt) blowing from the southern Carpatians over the Rotenturm Pass into the Transylvanian Basin; sometimes also called Talmac Wind after the town Nagy-Talmacs near Sibiu.

Santa Ana (California, U.S.A.)

Northeastern foehn wind in the basin of Los Angeles, named after the river and the pass.

Sky Sweeper (Majorca, Spain)

Foehn-type NW wind over the coastal mountain ranges of Majorca, Spain. Note that among seamen the term “Sky Sweeper” (or its German translation “Himmelsbesen”) signifies any northeasterly wind in northerly latitudes. It causes a very clear view after “sweeping the clouds from the sky.”

Solano (Spain)

Foehn-type wind from south to southeast. (The name is also used for easterly winds from the Mediterranean Sea bringing monsoon-like precipitation.)

Talmac, Talmesch Wind => Rotenturm Wind

Tenggara (Speimonde Archipel near Sulawesi, Celebes)

Föhn-type winds during easterly monsoons in the wake of the southern peninsula of Sulawesi.

Toureillo (Arriège, Southern France)

South foehn from the Pyrenees in the Arriège Valley.

Tsinias (Aegean Sea)

Foehn wind from the southern cliffs of the islands in the Aegean Sea, name is mainly used on the island of Tinos.

Türkenwind (Northern Tyrol, Austria; Rhine Valley, Switzerland)

Foehn accelerates the ripening of corn (and other crop). In this region, corn is also called Türkenkorn (Turkish grain, instead of the German “Mais”, hence the name Türkenwind [Turkish wind]).

Vent d’Espagne (Southern France)

West to southwest foehn-type wind from the Pyrenees when a depression moves in north of them.

Wambraw, Wambru (New Guinea)

Southwesterly foehn-type wind over the Geelvink Bay in northwestern New Guinea during summer monsoon.

Wasatch (Utah, U.S.A.)

Easterly Chinook in the valleys of the Wasatch mountains.

Zephyr (Colorado, U.S.A.)

Another name for Chinook.

Zonda (Argentina)

Foehn wind in the Andes, in Chili the same wind is called Puelche.