The foehn or foehn wind is a warm, dry fall wind that often occurs on the lee side of larger mountains facing away from the wind . It usually develops over a large area in weather conditions with pressure gradients across the mountains. It can blow steadily, but it can also be gusty .
The name foehn comes from the German-speaking Alpine region and has established itself as a meteorological term for corresponding wind events. There are numerous regionally different names for both the Alpine Foehn and corresponding weather phenomena in other parts of the world.
The foehn arises from a wind current (or a horizontal pressure gradient) over the mountains and is connected to the windward side of the mountain with incline rain , which leads to relatively warm mountain air. Characteristic is the significant warming and drying of the air flowing down, which can lead to health problems ( foehn disease ), as well as the pronounced visibility due to the aerosol- poor (suspended particles) air masses.
In addition to this warm hair dryer due to humid adiabatic air rising in front of the mountains and dry adiabatic air descending after the mountains, there are other causes. Less warm foehn winds occur as a physical weather phenomenon, at least in the Eastern Alps, depending on the stratification of the air masses, even without the rain that generates the additional heat.
Föhn and Bora are the type-defining warm and cold winds that can be observed around the world in this or a similar way. Due to the divergent bioclimatic effects and opposing consequences that shape the landscape, a separation of foehn and boragen types is inevitable. Phenomenologically , they can be easily distinguished:
“The foehn is a warm fall wind on the lee side of a mountain range. When it blows, the temperature rises on the leeward mountain slope. In contrast, the bora is also a downwind in the mountain lair, but the temperature on the lee slope drops after it sets in. "
The definition of the World Meteorological Organization (WMO) is:
Etymology and regional names
The name foehn came from the Latin favonius "lukewarm west wind", probably via the Rhaeto-Romanic ( favuogn, dialectal also fuogn ), in the Old High German (phōnno) . The root is related to the Latin verb fovere , to warm.
In addition, names for regional foehn locations have been created:
- The more well-known foehn wind in the Alps is the southern foehn , which occurs north of the main Alpine ridge . In Carinthia , the southern foehn over the Karawanken is called Jauk , derived from the Slovenian jug 'south'. In Styria , the Jauk is a foehn wind from southwest to west over the Koralpe towards Deutschlandsberg and Graz.
- There are on the southern side of the Alps, z. B. in Ticino , also the Nordföhn . In Italy, the foreign German word foehn or favonio is mostly used for this , the term Tedesco ('the German') and generally vento di caduta ('fall wind') or Tramontana , in Slovenia fen .
- See also: Föhntal - on the spread of the alpine foehn
Further examples are:
- the Halny wiatr in Poland
- the Aspr in the French Massif Central
- the "Norway foehn" ( fønvind ), a north wind that rains down on the Norwegian mountain range and, as far as it moves overland (and not over the Baltic Sea or North Sea), leads to cloudless weather in Schleswig-Holstein and Hamburg
- the Chanduy in Ecuador
- the Chinook on the east side of the Rocky Mountains
- the Santa Ana wind in Southern California
- the Puelche in the south of Chile (wind from the east) and Zonda in Argentina (wind from the west)
- the Canterbury Northwester in the New Zealand Alps
In contrast, no warm foehn winds, but katabatic down winds are z. B.
- the Bora on the Croatian and Montenegrin Adriatic coast on the southwest side of the Dinaric Mountains
- the Mistral in the lower Rhone Valley
History of the foehn theory
The explanation of the foehn, which is most widespread in textbooks - even today - is linked to the illustration by Ficker & De Rudder from 1943, is often called thermodynamically and erroneously attributed to Julius Hann . According to today's understanding, this theory is only of historical significance, although it correctly explains important phenomena. Its characteristics are precipitation on the windward side , which is the sole explanation for the relatively high temperatures on the lee side compared to the windward side, as well as a disturbance following the slope profile on both sides. However, in many cases this is not the case.
Thermodynamic Foehn theory
According to the thermodynamic foehn theory, a foehn, like all winds, is created by the effect of a pressure gradient force with lower pressure on the lee side of a mountain range. When the relatively humid air rises on the windward side of the mountain, it first cools down dry adiabatically at 1.0 ° C per 100 m rise in altitude until the relative humidity is 100%. This is because the water vapor capacity of the air decreases as the temperature drops, so that when the dew point is reached it is saturated with steam and forms water droplets. If the air continues to rise, it only cools down moist adiabatically at around 0.6 ° C / 100 m. The relative humidity remains constant at 100%: The air can no longer retain its (invisible) water vapor, and ongoing condensation and cloud formation occur, while the evaporation heat of 2257 kJ / kg of the water vapor is transferred to the air. This lasts until the air has reached the mountain ridge and almost always leads to so-called incline rain , which can also turn into snowfall at great heights, and the heat of fusion (333.5 kJ / kg) is also given off. Both forms of energy are generally "stored solar energy"
From the ridge, the cooled air begins to sink down the slope on the other side of the mountain. In spite of the stable stratification of the atmosphere , the foehn arises first as a katabatic wind according to the thermodynamic foehn theory . The reasons for the sinking lie in both the low temperature and the slope of the terrain and are intensified when the wind on the leeward side of the mountain is "sucked in" by a low pressure area. The sinking air heats up again dry adiabatically at a continuous 1 ° C / 100 m - much faster than it cooled during the "ascent" (in the wet adiabatic phase): It lacks the amount of water rained down during the ascent, which simultaneously transfers its heat of condensation to it Gave off air. The amount of water rained down in connection with the rapid warming of the air on the leeward side is the cause of the relative dryness and high temperature of the foehn wind. Once in the lowlands, the foehn is no longer a katabatic wind, but a warm fall wind.
Problems of the thermodynamic theory of the foehn
The thermodynamic theory as an explanation of the hair dryer is based on the different temperature behavior of the air during vertical movements and is particularly widespread in textbooks because of the didactic clarity: In many textbooks the condensation effect was highlighted as "the thermodynamic hair dryer effect", as if there were no other reasons for the There was an increase in temperature with a hair dryer. For a long time this effect has been emphasized too much, probably also because of its didactic advantages. Two observations show that it is not an essential part of the hair dryer:
- There is also a hair dryer without clouds on the windward side or on the main Alpine ridge.
- The air trapped on the windward side is not always involved in the overflow; it can stagnate or even move in the opposite direction. Lammert's measurements provided examples of this as early as 1920.
The fact that descending warm air runs counter to Archimedes' principle is problematic, dynamic criteria are missing from this theory and neither the observations of the hydraulic jump nor the mountain waves or the rotors - which will be discussed below - can be explained with the theory.
Dynamic hair dryer theory
Although the atmosphere is made up of gases , in many cases it behaves like a liquid. Therefore, much of the atmospheric turbulence occurs as waves. Atmospheric interference wave resulting from the interaction of various forces, including pressure gradient force , Coriolis force , gravity and friction . For a long time, the above thermodynamic assumption was the determining theory of a foehn principle. Today general laws of flow dynamics are in the foreground in the principles of the formation of down winds, which lead to the mountain wave concept.
Hydrological-hydraulic analogy of the foehn flow
The most suitable way of explaining down winds in a three-dimensional system are hydrological models, as they are also suitable for movement patterns in a terrain with a strong relief with valleys and passes. Today the topographical conditions are still taken into account with the hypothesis called gap flow dynamic . According to this, the vertical narrowing (at the pass) and a lateral contraction (in a gap ) of the air flow are essential for down winds such as foehn and bora.
Hydraulic terms such as flowing water , shooting water , water flowing at critical speed and the Froude number (similar to the Mach number ) are used today in the Föhnt theory. Analogous to the division of gas dynamics in flows with under- and supersonic speed the hydraulics of flow with free surface in water flow with bottom and those with divided over fundamental wave speed. Water that flows with a speed that is lower than the fundamental wave speed is called flowing water in hydraulics, water with a flow speed greater than the fundamental wave speed is called shooting water. If water flows exactly at the fundamental wave speed, it is called “water flowing at critical speed”. The Froud number ultimately expresses the relationship between kinetic energy (depending on the wind speed) and potential energy (stability, mountain height).
- corresponds to critical flowing water. If the number is equal to or slightly greater than one, then the probability of mountain waves is high.
- corresponds to running water. If the number is less than one, the flow is insufficient to get over the obstacle and the circulation is blocked.
- corresponds to shooting water. If the number is much greater than one, the air flows over the obstacle without major oscillations.
The problem with the explanation is to apply the different behavior in model tests of flowing and shooting water when flowing over a ground obstacle in the same way as with the foehn. When water flows over an obstacle, there are essentially two forces acting: gravity and inertia. One can now distinguish between two regimes:
- In supercritical flow, the force of inertia is dominant. Kinetic energy is converted into potential energy at the obstacle (that is, water flows more slowly, but has potential energy at the top, which enables it to fall down and flow faster, i.e. to have more kinetic energy after the obstacle).
- In subcritical flow, gravity dominates. The water flows faster over the obstacle, potential energy is converted into kinetic energy, the water layer becomes thinner. After the obstacle, the kinetic energy is converted back into potential energy.
If a sufficiently strong acceleration is achieved over the obstacle and a sufficiently large decrease in the thickness of the water layer takes place (possible with large obstacles), a transition from subcritical to supercritical flow can occur. Now that the water on the lee slope is super critical, it accelerates and falls down the slope. Because potential energy is converted into kinetic energy over the entire distance above the obstacle, strong downwinds are produced in the lee. The liquid fits on the leeward side by a hydraulic jump (engl. Hydraulic jump ) back around and switches back to thereby subcritical flow. Here there is an analogy to gas dynamics: As there is a steady transition from a flow with subsonic speed to one with supersonic speed, while the reverse is usually discontinuous on the way over a Riemann shock wave , a flowing water flow steadily changes into a shooting one, a shooting one into a flowing one on the other hand it is mostly discontinuous on the way over a water jump. This means that the heat generated by turbulence when jumping in water is lost for the hydraulic process, but in the gas-dynamic process it is retained as internal energy , so the jump in air does not entirely correspond to the jump in water. The fact that there is an air flow with supercritical speed with the foehn (shooting air flowing) is underlined by the extraordinary turbulence of the rotors when air near the ground rises in the lee.
The gap dynamic is an essential element of the Föhn hypothesis . The basic idea is that an orthogonal flow that flows against a mountain barrier initially presents a two-dimensional problem, but that if so-called gaps (valleys, passes) are present, the dimensionality of the problem is changed. This is especially true when the Froude number of the air is lower at a mountain barrier and it takes a route through canyons, valleys and passes instead of a passage over the obstacle. The fact that many mountains have certain wind paths reinforces this idea. Examples are the “Stampede Gap” in the cascade chain in Washington ( Cascade Windstorm ), the dry valleys of the Himalayas , the Wipptal on the Brenner Pass between Inn and Adige (Foehn), the Vratnik Pass over Senj in Velebit (Bora) or the incision in the bay from Kotor in Montenegro as the corridor of the Risaner Bora.
The following picture for the mechanism of the foehn emerges today: In the initial state there is an extensive, almost horizontal temperature inversion over a mountain relief and its surrounding area , in the mountain valleys and perhaps also in the foreland a stagnant cold air layer. An approaching low begins to suck in the cold air through the channel between the earth's surface and the inversion boundary layer above the mountains. The flow velocity in this channel is constantly increasing. If the suction effect of the deep is sufficiently strong, the flow becomes critical at some point along an initially narrow section of the mountain range, preferably on a pass, because the flow speed is particularly increased there due to the nozzle effect . The maximum conveying capacity of the canal is thus achieved along this route. The inversion is pulled down in the lee of this section and continues in the direction of the basic flow, while below the flow becomes supercritical. The foehn started at the pass and continues down into the valley, including the cold air at the bottom of the canal. During this process, the air can still flow unhindered on both sides of the mountain section, as the critical speed has not yet been reached there. However, the sucking-in low demands further air supply, so that the flow velocities also have to continue to increase to the side of the route until the critical values are gradually exceeded along the entire ridge. The foehn has set in on the entire mountain range.
Various misinterpretations of the temperature increase, especially of the southern foehn, require a precise analysis. The adiabatic heating of the air basically depends on the atmosphere between the valley station and the mountain ridge being stably stratified . Especially on summer days with a deep and well-mixed boundary layer and superadiabatic gradients near the bottom, the foehn is cooler than the air it displaces. Therefore, the basic heating and drying of the foehn air due to the descent on the lee side of a mountain range is confused with the fact that foehn air is warmer and drier than the air mass it is exchanging. This is confirmed by statistics that show a clearly increased trend in temperature maxima in the summer months at Südföhn in Innsbruck. For the southern side of the Alps, however, the effect of the northern foehn is overshadowed by the cold air advection . In contrast, the southerly current in southern foehn locations for the area of the Eastern Alps in the Tyrol area with the effect of the foehn as a southerly wind is always characterized by a corresponding increase in temperature maxima.
Foehn wall, foehn window and foehn storm
Typical for the foehn location is a striking wall of clouds - the foehn wall - in front of an almost blue sky, the foehn window . The foehn wall stands as a wall of clouds above the ridge, on which the fall wind then flows down. The blow dryer window is the nice weather zone blown out by drying.
At the end of the foehn effect, there is a second foehn wall on the cold front of the triggering low pressure area. Their advance is stopped by the head wind of the foehn. If the foehn collapses, this second foehn wall advances quickly and makes the foehn window disappear.
The statistics by Fliri (1984) clearly show that implied accumulation of precipitation is not a must for the foehn. With southern foehn there is only about 70% probability of precipitation on the eastern southern edge of the Alps, 80% in the western part with maxima of 90% in Ticino , where the precipitation intensities are also greater. However , it was shown in a partial contradiction to previous results that the case is not that simple and that a thermodynamic effect with the rising of soil air from the Po basin may play a role, albeit locally limited. For parts of the Western Alps, the moisture adiabatic component can therefore play a role. The existence of a cold air pool on the southern side of the Alps was confirmed during the ALPEX program. The not entirely new theory by Hann (1866) thus prevailed over that of Ficker and De Rudder (1943). Here the air of the lower layers is trapped in the pool and does not pass over the main Alpine ridge. This air is therefore also called dead air .
Leewaves and hair dryer lenses
On the lee side of the mountains, the flowing air starts to vibrate. If there is sufficient humidity, these lee waves are visible through the formation of characteristic clouds , the foehn lenses ( Altocumulus lenticularis , Ac lent for short ). In the lee waves, gliders can climb to over 10,000 m.
The atmospheric wave disturbances, which are formed by orographic obstacles, resemble the gravity waves of the water surface. While a sea wave moves on and the water stands still, it is exactly the opposite with mountain waves : while the wave remains essentially stationary, the air moves through it. Mountain waves can occur wherever a strong current meets a barrier in a stable atmosphere.
The waves are used in practice in gliding . In the updraft area, great heights can be reached without motor power. The associated turbulence is, however, for aircraft such as B. paragliders and hanggliders pose a serious danger.
Föhne effects on terrain levels, low mountain ranges
Less known, but quite widespread in practice, are weaker Föhne effects in the lee of lower terrain and low mountain ranges. Such effects typically occur with strong warm air advection in the winter months. The warm air mass cannot penetrate into the deep layers due to the lack of solar radiation and due to the formation of fog / high fog; a strong, but only a few hundred meters flat temperature inversion occurs. If the large-scale air flow is directed from a plateau or a low mountain range in the direction of the lowlands, the cold air layer close to the ground migrates towards the lowlands and is replaced by the warmer and drier air from higher air layers. This leads to the dissolution of deep cloud layers, with significantly improved visibility and higher temperatures. These effects occur on a larger scale, are not limited to individual valleys and can still be felt at a relatively large distance from the threshold. The wind speed increases only insignificantly.
Typical regions with hair dryer effects are:
- the southern Upper Rhine Graben (Freiburg), with southern to southwest winds over the Burgundian gate
- the Eifel foothills in southerly winds
- the Lower Rhine with southeasterly winds, foehn effects from the Sauerland
- Erzgebirge foreland with southerly winds
- Northern Harz foreland ( Thale , Wernigerode ) with southerly winds
- Eastern Wiehengebirge with south wind
Optical magnification effect
A hair dryer means that there are few particles in the air and that this cleaner air then offers an improved view of the mountains. The atmosphere also acts like a magnifying glass, as the density of the air decreases with increasing altitude and thus the refractive index is also reduced. This leads to a deflection of the light so that objects appear larger or closer. With the blow dryer, this effect is reinforced by the rising temperature, which leads to a further decrease in density.
Pictures of foehn weather conditions
Foehn to the Danube valley near Regensburg
Foehn wind over Lake Constance near Nonnenhorn - view towards Bregenz Bay
- H. Tamiya: Bora in a large-scale view and its connection with Oroshi . In: MM Yoshino (Ed.): Local wind Bora . University of Tokyo Press, Tokyo 1976, ISBN 0-86008-157-5 , pp. 83-92 .
- S. Arakawa: Numerical Experiments on the Local Strong Winds: Bora and Föhn . In: MM Yoshino (Ed.): Local wind Bora . University of Tokyo Press, Tokyo 1976, ISBN 0-86008-157-5 , pp. 155-165 .
- K. Yoabuki, S. Suzuki: Water Channel Experiment on Mountain Wave: Some Aspects of Airflow over a Mountain Range . In: MM Yoshino (Ed.): Local wind Bora . University of Tokyo Press, Tokyo 1976, ISBN 0-86008-157-5 , pp. 181-190 .
- American Meteorological Society : Glossary of Meteorology. Boston 1959. (Online version: http://amsglossary.allenpress.com/glossary/ )
- Preusse Eckermann: Global Measurements of Stratospheric Mountain Waves from Space. In: Science. 286/1999, pp. 1534-1537.
- H. Ficker, B. De Rudder: Föhn and Föhn effects - the current state of the question. Akad. Verlagsg. Becker & Erler, Leipzig 1943.
- J. Hann: On the question about the origin of the foehn. In: Journal of the Austrian Society for Meteorology. 1 (1), Vienna 1866, pp. 257-263.
- H. Schweizer: Attempt to explain the foehn as an air flow with supercritical speed. In: Archives Met. Geo. Biocl. Series A5 / 1953, pp. 350-371.
- P. Seibert: South Foehn Studies Since the ALPEX Experiment. In: Meteorol. Atmos. Phys. 43/1990, pp. 91-103.
- R. Steinacker: Unstationary Aspects of Foehn in a Large Valley. ICAM-MAP Meeting, Zadar, 2005. ( web document , pdf)
- N. Tartaglione, PP Ruti: Mesoscale Idealized Gap Flows. In: MAP Newsletter. 9/2000 ( The Mesoscale Alpine Program ( Memento from January 1, 2013 in the web archive archive.today ))
- World Meteorological Organization : International meteorological vocabulary . 2nd Edition. Secretariat of the World Meteorological Organization, Geneva 1992, ISBN 92-63-02182-1 .
- Jürgen Brauerhoch: Föhn: A redeeming breviary. Langen-Müller, 2007, ISBN 978-3-7844-3093-5 .
- Fritz Kerner von Marilaun: The foehn wall, a meteorological phenomenon of the Central Alps. In: Journal of the German Alpine Association / Journal of the German and (the) Austrian Alpine Association , year 1892, (Volume XXIII), pp. 1–16. (Online at ANNO ). . (Illustration: Foehn wall. (Im Gschnitzthal.) ).
- Alpiner Föhn a new verse to an old song - Rudolf Steinacker 2005, Promet 32: 3-10 (Promet: PDF)
- Mountain waves and Downslope winds - page with animated teaching material (English, audio)
- Felix Welzenbach: Introduction to the modern thermodynamic and hydraulic Foehn theory with illustrations
- Foehn forecast Switzerland
- Foehn forecast Austria
- How is the hair dryer made? Not in the way you think ; Weather blog Frank Wettert
- Image foehn wall
- Introduction to Climatology. SII geosciences. Page 98, Ernst Klett Verlage, Stuttgart 1985, ISBN 3-12-409120-5
- Franco Slapater: Small dictionary for mountaineers. German - Italian - Slovenian. Print: Tiskarna Tone Tomšič, Ljubljana 1986.
- "Norway Föhn" ensures there is no snow. In: Hamburger Abendblatt. November 24, 2008, p. 28.
- Wolfgang Latz (Ed.): Diercke Geographie Oberstufe. Westermann Verlag, 2007, ISBN 978-3-14-151065-2 , p. 40.
- The hair dryer (PDF)
- Basics of the Föhn - An introduction at inntranetz.at
- Frank Abel: How is a hair dryer made? Not what you think. Weather blog "Frank Wettert", 2008.
- West-northwest foehn in the Vienna basin , wetteralarm.at (updated diagram on the pressure difference Innviertel – Burgenland)
- Friedrich Föst needs “correct weather” Article with a picture of a foehn wall and foehn clouds over the Wiehengebirge near Lübbecke, hallo-luebbecke.de, April 1, 2012.