foehn

foehn (fān, Ger. fön), warm, dry wind that occurs on the leeward slopes of a ridge of mountains. The term was originally applied to a wind of the Alps but is now used as a generic term for all winds of this type. In other parts of the world the various foehn winds have often been given local names, e.g., the chinook over the eastern slopes of the Rocky Mts., the "sky sweeper" over Majorca, and the aspre over the Garonne plain of France. A foehn originates as follows: Air is first forced upward over the windward mountain slopes, cooling as it encounters the lower pressures of higher altitudes. If, however, it reaches its condensation temperature, the cooling is somewhat reduced owing to the release of latent heat that results from water vapor condensing into liquid water. As the air flows downward over the leeward slopes, it is warmed as it encounters the greater pressures of lower altitudes. This warming, however, is greater than the cooling that occurred during the ascent if heat was added to the air as a result of condensation, so that the air is both warmer and drier than originally. The foehn occurs when the circulation is strong enough to force air over the mountains in a relatively short period of time. The nature of the foehn in a particular locale depends on the topography, the strength and direction of circulation, and the moisture supply on the windward side of the mountains. The chinook, for example, generally blows from the southwest and sometimes raises temperatures by as much as 20°F; (7°C;) in 15 min.