A warm dry wind that blows down the lee side of a mountain.
In order to describe the Foehn Effect and its resultant wind, an example like the one presented below is generally used. Values for temperature and mountain range elevations may vary.
Considering the diagram below, an air mass of sufficient water content, in this example with an initial temperature of 13C and dew point temperature of 10C, is forced up and over a mountain range (Orographic Lift). Initially, the air temperature cools dry-adiabatically (3C/1000 ft), until its dew point temperature is reached. In this example, the air cools dry-adiabatically to an altitude of 1000 feet. Condensation occurs as the air is further forced up the mountain range, resulting in the air cooling saturated-adiabatically (generally considered in the mid-latitudes to be 1.5C/1000 ft). Clouds and precipitation form. When the air mass reaches the top of the mountain range it has lost a significant amount of its water content and so has a much lower dew point temperature, in this case 7C. The effective temperature at the top of the mountain, due to the orographic lift and saturated adiabatic effects, is 5.5C. As the air then begins to descend down the lee slope of the mountain the compressed air is initially heated saturated-adiabatically, and in effect the direct reverse to the cooling effect on the windward side occurs. In the example, the dew point temperature of 7C is reached at an altitude of 3000 ft, or 1000 feet below the top of the range. As the air continues leeward and downward from the mountain range, the air, now no longer saturated, is heated dry-adiabatically, resulting in an air temperature of 16C at the foot of the range on the leeward side. The resultant wind is dry and warm giving clear conditions at airfields on the lee side of the mountain range.
The Foehn Effect also may be associated with mountain wave activity.
As well as creating a warmer climate, these dry winds can increase the potential for wild fires during the summer months which may affect flying operations.