Foehn
Foehn is a wind system created by the impact of orographic obstacles like mountains on the flow, which often shows a typical cloud structure.
The term Foehn in a general sense refers to a typical wind phenomenon in mountainous areas. At least in the Alpine area, there are cases where such a wind occurs but no or almost no cloudiness develops. Since this manual deals with satellite meteorology, only Foehn cases with cloud features are treated. The basic condition leading to a Foehn process is a flow perpendicular to a mountain range. The basic synoptic-scale precondition is an intensive low to the W, NW of this mountain chain 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 (1C/100 m) until saturation is reached. Condensation begins and from then on the cooling is at the moist adiabatic lapse rate which is 0.65C/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.