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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 chinookchinook,
warm, dry air mass that descends the eastern slopes of the U.S. and Canadian Rocky Mts. after having lost moisture by condensation over the western slopes. Chinooks occur mainly in winter.
..... Click the link for more information. 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 heatlatent heat,
heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, or the
..... Click the link for more information. 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.
a strong, gusting wind of high temperature and low relative humidity that blows down from the mountains into the valleys in many mountain systems. The air properties of the foehn can be explained by its adiabatic heating during its descent down the mountain slopes. The foehn usually lasts less than 24 hours, although it can sometimes last up to five days. The change in temperature and relative humidity during a foehn can be extremely sudden and sharp. The foehn has been studied in the Alps, the Caucasus, the mountains of Middle Asia, and other mountainous regions.