Formation of the foehn
Foehn winds can arise as a result of humid airflow over a mountain range. The air is drawn up on the windward side of the mountains and rises wet adiabatically (reduction in temperature around 0.6°C/100m). As a result of the cooling down of the air, part of the water condenses and this leads, in turn, to the formation of clouds and rain. On the leeward side, the dried air sinks dry abdiabatically (temperature increase around 1°C/100m) again and this causes the typical warm and dry downslope foehn winds. Due to the strong warming effect, the air on the leeward side becomes hotter and drier than it was at the same height on the windward side of the slope.
Although many people associate the foehn exclusively with the Alpine region, the phenomenon arises in many other mountain ranges. Other examples of this phenomenon include the Chinook in the Rocky Mountains and the Bora which is caused by the Dinaric Alps.