Meteorological Physical Background

The basic meteorological processes behind this Conceptual Model are related to orographically forced rising and sinking, and associated dry and wet adiabatic cooling and warming.

The basic condition leading to a Foehn process is a flow perpendicular to a mountain range. Regardless of the orientation of the mountain chain (W-E or N-S) an intensive low to the W, NW of this mountain chain is the basic synoptic-scale pre-condition for such a perpendicular stream. Reaching the mountain chains the air is forced to rise. Initially the cooling of the air mass is dry-adiabatic (1°C/100 m) until saturation is reached. Condensation begins and from then on the cooling is at the moist adiabatic lapse rate which is 0.65°C/100 m. At the height of the mountain tops the air mass will have lost much of its humidity content and can now be regarded as relatively dry. The sinking on the other side of the mountains and associated warming therefore takes place with the dry-adiabatic lapse rate. Consequently the air warms up during descent at a rate higher than the cooling during ascent and reaches the plains as a very dry, very warm and very strong wind - the Foehn.

The principals of this process are true for every mountain barrier but there are some conditions which must combine for more notable Foehn effects. The air initially must have the possibility to gain enough humidity and the mountain chain must be high enough so that the whole water content can be condensed while rising.

A mountain range for which these conditions are very well fulfilled are the Alps: if there is an intensive low in the W to NW of the Alps, a southerly stream reaches the Alpine mountain chain after it has overflow the Mediterranean Sea and therefore has the possibility to gain high values of humidity. Stau Cloud and rain develops to the south of the Alps; taking into account the height of mountains - up to about 3500 - 4000m - there is enough ascent to condense out the humidity content and to produce a very dry, warm Föhn-storm north of the Alps. This Foehn is called the S-Foehn (or Süd Foehn e.g. South Foehn) and usually reaches strengths of 100 km/h.

28 February 2000/12.00 UTC - Meteosat IR image

In November 1982, the Alps were affected by an exceptionally marked Foehn event that resulted in strong winds in Austria, Germany and Switzerland. The observed pressure gradient across the Alps reached a record of 20 hPa causing winds of up to 190 km/h across the Gottard Pass. While Locarno observed an air temperature of +1°C with mixed rain and snow, skies above Zürich were clear with a temperature of 25°C.

27 January 2008/00.00 UTC - Meteosat 9 IR10.8 image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa

There is a similar effect for flow from the opposite direction - the "North Foehn" when an air mass crosses the Alps from N to S (like the example of 27 January 2008, above). But such an air mass is usually colder than the one coming from the S and as it is streaming over the continent it does not have so much possibility to gain humidity. Consequently the North Foehn which blows S of the Alps is by far not as dramatic as the S-Foehn.

Whether the Foehn-storm is noticed as far down as in the valleys on the lee side of the mountains also depends upon the structure of the local boundary layer. If there is a mixed boundary layer, the Foehn will penetrate down to the lowest levels. If there is a boundary layer with a strong inversion it will not be able to reach the ground. In the satellite images a small black (cloud free) area can be seen moving eastward; it can be regarded as sign of such a breakthrough of the Foehn.

26 January 2008/20.00 UTC - Meteosat 9 IR10.8 image; 20.00 - 06.00 UTC half-hourly image Loop