South foehn

Typische Erscheinungen bei Südföhnlage (Quelle: Stephan Bader, Vorlesung Atmosphäre und Klima, Universität Zürich)
Südföhnlage

With a typical south foehn situation, warm humid air from the Mediterranean is transported up to the southern slopes of the Alps. Because the air masses cannot get around the mountains at the side, they are forced to rise wet adiabatically along the southern flank of the Alps (temperature reduction around 0.6°C/100m). When it crosses the dew point line, the water contained in the air condensates and causes the formation of clouds followed by rain (orographic precipitation). Extreme volumes of precipitation can arise here. When the air masses flow down the leeward side of the mountains, they are heated by the dry adiabatic sinking (temperature increase around 1°C/100m) and the humidity also declines again to below 100%. Due to the difference in temperature between the wet adiabtatic rise and dry adiabatic fall of the air masses, the temperature on the northern Alpine slopes is far higher than at the same altitude on the southern slopes.

 

Effects of the south foehn

The foehn has wide-ranging effects on the environment:  the warmed-up foehn air and extended period of sunshine caused by the dissipation of the clouds changes the climate in the foehn valleys and in the rest of central and eastern Switzerland. The rapid snow melt and autumnal foehn winds extend the vegetation period and create remarkably advantageous conditions for agricultural activity as a result. However, apart from the favourable climatic conditions it brings, the foehn is feared for the considerable storm damage it can cause. A foehn storm can leave unroofed houses, ships in distress, overturned electricity pylons and trees in its wake. The drought associated with the foehn can also increase the risk of forest fires on the northern slopes of the Alps. Extreme precipitation often arises on the southern Alpine slopes which can cause landslides and floods. In addition, the foehn is also seen as a sign that bad weather is on the way and heralds the approach of a south-west front.