Fall wind

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fall wind

A wind that accelerates as it moves downslope because of its low temperature and greater density.

A fall wind is a larger-scale phenomenon than the individual-slope scale and is produced by accumulated cold air spilling down a slope or over a mountain range. The cold air often either accumulates on a plateau or other elevated terrain, or is part of an extensive cold air mass approaching a mountain range as a cold front. Fall winds may have a hydraulic character similar to water flowing over a dam, and one of the details of this flow is that the acceleration of the cold air begins to occur before the crest of the mountain range and therefore before the down-sloping portion of the topography. Fall winds are especially well developed as strong easterly winds on the coast of Norway, and for some distance inland; here they give a narrow strip of fine weather along the shore. They are also well developed on the northern coast of the Aegean Sea. At the southeastern tip of rocky Hagion Oros Peninsula in Greece, Mt. Athos rises to 2033 m (6670 ft) and descends steeply to the sea; northerly winds are disturbed by this great mass and descend as the cold northeasterly Athos fall wind, often of gale force, extending several kilometers out to sea. On the coast of Peru the name is given to sudden heavy gusts that often come down from the high land after onset of the sea breeze. At Rio de Janeiro, Brazil, descending squalls from the northwest are termed terre altos. In the Antarctic fall winds off the inland ice form violent blizzards. Other examples of fall winds are the mistral, papagayo, and vardar. Some authors have generalized this term to refer to downslope winds forced by large meso- and synoptic-scale processes (i.e., scales larger than that of an individual slope), even if they do not represent flows of colder air. Thus, under this nonstandard definition, the foehn and chinook could be considered fall winds.

Smith, R. B. 1987. Aerial observations of the Yugoslavian bora. J. Atmos. Sci.. 44. 269–297.