Katabatic wind
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A katabatic wind, from the Greek word katabatikos meaning "going downhill", is a wind that blows down a topographic incline such as a hill, mountain, or glacier. Such winds, particularly when they occur over a wide area, are sometimes called fall winds.
A distinction is drawn between winds that feel warmer than their surroundings (generally called Föhn or regionally Chinook or Bergwind, and those that are cooler (for instance the Mistral in the Mediterranean, the Bora (or Bura) in the Adriatic or the Oroshi in Japan). In more recent times, however, the term katabatic wind usually refers to the cold variant.
The cold form of katabatic wind originates in a cooling, either radiatively or through vertical motion, of air at the top of the mountain, glacier, or hill. Since the density of air increases with lower temperature, the air will flow downwards, warming adiabatically as it descends, but still remaining relatively cold.
Cold katabatic winds are frequently found in the early hours of the night when the solar heating has ceased and the ground cools by emitting infrared radiation. Cold air from extratropical cyclones may contribute to this effect.
Over Antarctica prominent (although unnamed) cold katabatic winds exist, blowing for most of the year. In Greenland these winds are called Pitaraq and happen mostly in the fall, but are rare. In the Fuegian Archipelego (or Tierra del Fuego ) in South America, a wind known as a williwaw is a particular danger to harbouring vessels. With weather being predominantly westerly, the cold air will build up on the western side of the mountainous islands until this large bubble of cold, heavy air is forced over the mountaintop. The subsequent rush of wind blows down the mountainside into the otherwise sheltered eastern side of the islands. Williwaws commonly blow as high as 100 knots, and 200 knot williwaws have been reported. [1] The term originates in the Alaska Panhandle region, where what are also known as outflow winds pour from the heads of the coastal inlets to the many channels of the offshore archipelago.
Katabatic winds are sometimes experienced by yachts at anchor. They often appear after a windless evening and arrive an hour or two after dark. They can blow very hard (up to gale force) for about an hour before dying away. This can be a frightening experience as the anchor may have been set with only the expectation of light winds and the yacht can easily have its anchor dragged.
Winds which blow up a slope are called anabatic winds.
[edit] References
- ^ Dallas Murphy, "Rounding the Horn" (New York: Phoenix Books, 2004), p. 221.
- McKnight, TL & Hess, Darrel (2000). Katabatic Winds. In , Physical Geography: A Landscape Appreciation, pp. 131-2. Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-020263-0
[edit] See also
[edit] External links
es:Viento catabático fr:Vent catabatique it:Vento catabatico he:רוח קטבטית lt:Katabatinis vėjas no:Katabatisk vind nn:Katabatisk vind pl:Wiatr katabatyczny fi:Katabaattinen tuuli sv:Katabatisk vind
