Fig. 1

Thermodynamic foehn theory. An air pressure difference with high pressure in the southern region of the Alps causes south to north-directed winds. Dry adiabatic cooling: During its ascent over the mountains, the air cools down proportionally to the decreasing air pressure by 1 °C per 100 m. Cool air saves less water than hot air. This results in the formation of clouds with rain or snow on the southern side of the Alps. Dry adiabatic warming: Due to its higher density, the cool air descends after passing the crest. During its descent, the air warms proportionally by 1 °C per 100 m. This results in a warm dry wind on the northern side of the Alps with a clear vision without clouds