Katabatic wind

Katabatic wind

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A katabatic wind, from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 word katabatikos meaning "going downhill", is the technical name for a drainage wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

, a wind that carries high density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds. Katabatic winds can rush down elevated slopes at hurricane speeds, but most are not that intense and many are on the order of 10 knots (18 km/h) or less.

Not all downslope winds are katabatic. For instance, winds such as the Foehn, Chinook
Chinook wind
Chinook winds , often called chinooks, commonly refers to foehn winds in the interior West of North America, where the Canadian Prairies and Great Plains meet various mountain ranges, although the original usage is in reference to wet, warm coastal winds in the Pacific Northwest.Chinook is claimed...

 or Bergwind
Bergwind
Bergwind is the South African name for a foehn wind, a hot dry wind blowing from the mountainous interior to the coast. It varies from mild to gusts up to 100km/h that cause structural damage...

, are rain shadow
Rain shadow
A rain shadow is a dry area on the lee side of a mountainous area. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them. As shown by the diagram to the right, the warm moist air is "pulled" by the prevailing winds over a mountain...

 winds where air driven upslope on the windward side of a mountain range drops its moisture and descends leeward drier and warmer. Examples of true katabatic winds include the Bora
Bora (wind)
Bora or Bura is a northern to north-eastern katabatic wind in the Adriatic, Bosnia and Herzegovina, Croatia, Montenegro, Italy, Greece, Slovenia, and Turkey....

 (or Bura) in the Adriatic
Adriatic Sea
The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula, and the system of the Apennine Mountains from that of the Dinaric Alps and adjacent ranges...

, the Bohemian Wind or Böhmwind in the Ore Mountains
Ore Mountains
The Ore Mountains in Central Europe have formed a natural border between Saxony and Bohemia for many centuries. Today, the border between Germany and the Czech Republic runs just north of the main crest of the mountain range...

, the Mistral
Mistral
Mistral commonly refers to the Mistral in southern France and SardiniaMistral may also refer to:* Mistral, various ships of the French Navy, including:** Mistral class amphibious assault ship of the French Navy...

, the Santa Ana
Santa Ana wind
The Santa Ana winds are strong, extremely dry offshore winds that characteristically sweep through Southern California and northern Baja California in late fall and winter. They can range from hot to cold, depending on the prevailing temperatures in the source regions, the Great Basin and upper...

 in southern California, the Tramontana
Tramontana
Tramontana or Tramontane can refer to:*Tramontane, a northern wind *the Pole Star*Tramontane, a science fiction novel by Emil Petaja...

 and the Oroshi
Oroshi
, is the Japanese term for a wind blowing strong down the slope of a mountain, occasionally as strong gusts of wind which can cause damage. Oroshi is a strong local wind across the Kanto Plain on the Japan Sea side of central Honshu...

 in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. Another example is "The Barber," an enhanced katabatic wind that blows over the town of Greymouth
Greymouth
Greymouth is the largest town in the West Coast region in the South Island of New Zealand, and the seat of the Grey District Council. The population of the whole Grey District is , which accounts for % of the West Coast's inhabitants...

 in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 when there is a southeast flow over the South Island. It is a wind that is known in the area for its coldness.

Theory



A katabatic wind originates from radiational cooling of air atop a plateau, a mountain, glacier, or even a hill. Since the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of air is inversely proportional to temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, the air will flow downwards, warming adiabatically
Adiabatic process
In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...

 as it descends. The temperature of the wind depends on the temperature in the source region and the amount of descent. In the case of the Santa Ana, for example, the wind can (but does not always) become hot by the time it reaches sea level. In the case of Antarctica, by contrast, the wind is still intensely cold.

The entire near-surface wind field over Antarctica is largely determined by the katabatic winds, particularly outside the summer season, except in coastal regions when storms may impose their own windfield.

Impacts


Katabatic winds are most commonly found blowing out from the large and elevated ice sheets of Antarctica and Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

. The buildup of high density cold air over the ice sheets and the elevation of the ice sheets brings into play enormous gravitational energy. Where these winds are concentrated into restricted areas in the coastal valleys, the winds blow well over hurricane force. In Greenland these winds are called Piteraq
Piteraq
A Piteraq is a cold Katabatic wind which originates on the Greenlandic icecap and sweeps down to the east coast. Piteraqs are most common in the fall and winter. Wind speeds typically reach 50 to 80 m/s. On February 6, 1970 the community Tasiilaq was hit by very strong Piteraq causing severe...

 and are most intense whenever a low pressure area approaches the coast.

In a few regions of continental Antarctica the snow is scoured away by the force of the katabatic winds, leading to "dry valleys" (or "Antarctic oasis
Antarctic oasis
An Antarctic oasis is a large area naturally free of snow and ice in the otherwise ice-covered continent of Antarctica.-Geology:In Antarctica there are, in addition to mountain tops and nunataks, other natural snow- and ice-free areas often referred to as "Antarctic oases" or "dry valleys"...

") such as the McMurdo Dry Valleys
McMurdo Dry Valleys
The McMurdo Dry Valleys are a row of snow-free valleys in Antarctica located within Victoria Land west of McMurdo Sound. The region is one of the world's most extreme deserts, and includes many interesting features including Lake Vida and the Onyx River, Antarctica's longest river.-Climate:The Dry...

. Since the katabatic winds are descending, they tend to have a low relative humidity which desiccates the region. Other regions may have a similar but lesser effect, leading to "blue ice" areas where the snow is removed and the surface ice evaporates, but is replenished by glacier flow from upstream.

In the Fuegian Archipelago (or Tierra del Fuego
Tierra del Fuego
Tierra del Fuego is an archipelago off the southernmost tip of the South American mainland, across the Strait of Magellan. The archipelago consists of a main island Isla Grande de Tierra del Fuego divided between Chile and Argentina with an area of , and a group of smaller islands including Cape...

) in South America as well as in Alaska, a wind known as a
williwaw
Williwaw
In meteorology, a williwaw is a sudden blast of wind descending from a mountainous coast to the sea. The word is of unknown origin, but was earliest used by British seamen in the 19th century...

is a particular danger to harbouring vessels. Williwaws originate in the snow and ice fields of the coastal mountains, and while they commonly blow as high as 100 knots, 200-knot williwaws have been reported.

Further reading

  • Giles, Bill. Weather A-Z - Katabatic Winds By Bill Giles OBE, BBC, Retrieved 2008-10-14
  • 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