Bergwind
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Berg wind (from
Afrikaans Afrikaans (, ) is a West Germanic language that evolved in the Dutch Cape Colony from the Dutch vernacular of Holland proper (i.e., the Hollandic dialect) used by Dutch, French, and German settlers and their enslaved people. Afrikaans gra ...
''berg'' "mountain" + ''wind'' "wind", i.e. a mountain wind) is the
South African __NOTOC__ South African may relate to: * The nation of South Africa * South African Airways * South African English * South African people * Languages of South Africa * Southern Africa Southern Africa is the southernmost subregion of the Afric ...
name for a katabatic wind: a hot dry wind blowing down the
Great Escarpment The Great Escarpment is a major topographical feature in Africa that consists of steep slopes from the high central Southern African plateauAtlas of Southern Africa. (1984). p. 13. Reader's Digest Association, Cape Town downward in the directio ...
from the high central plateau to the coast.


Overview

When the air that has been heated on the extensive central plateau flows down the escarpment to the coast it undergoes further warming by
adiabatic process In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, ...
es. This accounts for the hot and dry properties of these off-shore winds, wherever they occur along South Africa's coastline.Danielson, Levin, and Abrams (2003). ''Meteorology'', McGraw Hill Although berg winds are often called a Föhn winds, this is probably a misnomer, as Föhn winds are
rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carrie ...
winds that result from air moving over a mountain range, resulting in precipitation on the windward side. This releases latent heat into the atmosphere which is then warmed still further as the air descends on the leeward side (e.g., the Chinook or the original
Föhn A Foehn or Föhn (, , ), is a type of dry, relatively warm, downslope wind that occurs in the leeward, lee (downwind side) of a mountain range. It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped m ...
). Berg winds do not originate in precipitation, but in the mostly dry, often arid central plateau of
Southern Africa Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number of ...
. On the other hand,
katabatic winds A katabatic wind (named from the Greek word κατάβασις ''katabasis'', meaning "descending") is a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity. Such winds are sometim ...
are technically drainage winds, that carry high density, usually cold air from a high elevation down a slope under the force of gravity. These are thus "fall winds", which occur most typically down the coastal ice slopes of
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
and
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
. Berg winds blow off the African escarpment in response to large scale weather systems in the South Atlantic Ocean, the African interior, and the
Southern Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
.


Coastal lows and berg winds

Berg winds are usually accompanied by coastal lows. These coastal lows owe their existence to the configuration of the plateau, escarpment and coastal plain (see diagram on the right, above), in that they are confined to the coastal areas, always below the escarpment. Though they can arise almost anywhere along the coast, they often first appear on the west coast, or even on the
Namibian Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...
coast. They are then always propagated counter-clockwise along South Africa's coastline at between 30 and 60 km/h, from the west coast southwards to the
Cape Peninsula The Cape Peninsula ( af, Kaapse Skiereiland) is a generally mountainous peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At the southern end of the peninsula are Cape Point and the Cape of ...
and then eastward along the south coast, and finally north-eastward along the
KwaZulu-Natal KwaZulu-Natal (, also referred to as KZN and known as "the garden province") is a province of South Africa that was created in 1994 when the Zulu bantustan of KwaZulu ("Place of the Zulu" in Zulu) and Natal Province were merged. It is locate ...
coastline, to finally dissipate north of Durban, due to the divergence of the coastline from the plateau which disappears altogether in the vicinity of the Limpopo valley.Carter, T.J. (2005). The evolution of coa
Masters Thesis, University of Zululand.
retrieved 3 June 2015.
There is always a hot off-shore berg wind ahead of a coastal low, which can blow for several days or for only for a few hours. This is then followed by cool onshore winds which bring low cloud, fog or drizzle to the region, but may, on occasions, produce substantial precipitation when coupled to an approaching cold front. Coastal lows are a common feature of the coastal weather in South Africa with an average of about 5 lows of varying intensities passing through Port Elizabeth per month. They are shallow (not more than 1000–1500 m deep), mesoscale (medium-sized) systems that are generally not more than 100–200 km across, trapped on the coastal plain by the escarpment on the inland side, Coriolis effects on the oceanic side, and an inversion layer above. The pressure minima of these systems lie just off-shore. In the south-west corner of the country the coastal lows are bounded on the inland side by the
Cape Fold Mountains The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Mount ...
, which tend to have a higher elevation than the escarpment, and form an almost continuous 1000 km mountain barrier running parallel to the coast from the
Cederberg The Cederberg mountains are located near Clanwilliam, approximately 300 km north of Cape Town, South Africa at about . The mountain range is named after the endangered Clanwilliam cedar (''Widdringtonia wallichii''), which is a tree end ...
, 300 km to the north of
Cape Town Cape Town ( af, Kaapstad; , xh, iKapa) is one of South Africa's three capital cities, serving as the seat of the Parliament of South Africa. It is the legislative capital of the country, the oldest city in the country, and the second largest ...
, to
Cape Hangklip Pringle Bay ( af, Pringlebaai) is a small, affluent coastal village in the Overberg region of the Western Cape, in South Africa. It is situated at the foot of Hangklip, on the opposite side of False Bay from Cape Point. The town and surrounds are p ...
on the east side of
False Bay False Bay (Afrikaans ''Valsbaai'') is a body of water in the Atlantic Ocean between the mountainous Cape Peninsula and the Hottentots Holland Mountains in the extreme south-west of South Africa. The mouth of the bay faces south and is demarcat ...
and then eastwards for 700 km to
Port Elizabeth Gqeberha (), formerly Port Elizabeth and colloquially often referred to as P.E., is a major seaport and the most populous city in the Eastern Cape province of South Africa. It is the seat of the Nelson Mandela Bay Metropolitan Municipality, Sou ...
, where they eventually peter out (see the map above).


Origin of coastal lows

Coastal lows are initiated by the interaction of large scale weather systems such as the quasi-permanent
South Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...
and South Indian Ocean
Anticyclone An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from abov ...
s (high-pressure systems), the cold fronts that approach the subcontinent from the South Atlantic Ocean, as well as the pressure systems on the plateau, causing air that has been warmed on the plateau by 2–3 days of sunny weather to flow down the Great Escarpment on to the coastal plain either on the west or south coasts of the country (i.e. causing a berg wind). The descending air warms up adiabatically, heating up the coastal plain, while, at the same time, causing an off-shore wind which blows the surface water away from the land to be replaced by cold water which wells up from the depths. This upwelling of cold subsurface water from the ocean increases the ocean-land temperature difference, causing an on-shore wind. The on-shore airflow is strengthened by the fact that the berg wind is not only hot, but it is also “stretched” vertically due to the sudden lowering of the floor over which it moves below the escarpment. Its low density, therefore, lowers the atmospheric pressure on the coast. This low-pressure area caused by the berg wind draws the dense moist maritime air onshore to the right of the off-shore berg wind. Shear forces between these on- and off-shore winds on the right-hand side of the berg wind tend to cause clockwise (or
cyclonic In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
) rotation of the air in this region. In addition, on reaching the escarpment the maritime air curves to the right round the low-pressure zone due to Coriolis forces (in the southern hemisphere) accentuating the cyclonic circulation of the "coastal low". The entire system is capped by an inversion consisting of a layer of warm air that has moved horizontally off the plateau at the level of the upper edge of the escarpment. This inversion layer prevents the upwardly spiraling cyclonic air of the coastal low from rising above 1000–1500 m, thus preventing it from causing significant precipitation.


The weather associated with a coastal low

Along the south coast the passage of a coastal low is typically preceded by a north-easterly wind driven by the South Indian Ocean Anticyclone. The wind then backs quickly through northerly to north-westerly as its temperature rises. This is the berg wind phase of the coastal low. The wind then changes abruptly to a strong, cold, south or south-westerly wind (called a “buster” if the change in wind speed is greater than 35 km/h). The buster coincides with the passage of the pressure minimum. The onshore wind gradually diminishes in intensity during the course of about a day, and is associated with cloudy, misty or drizzly weather. Because of the often abrupt changes in horizontal and vertical wind speeds and direction that can occur within these small weather systems they represent a significant hazard to aircraft on landing and taking off. During the climb-out and approach phases of flight, aircraft airspeed and height are near critical values, thus rendering the aircraft especially susceptible to the adverse effects of these wind shears. The Atlantic cold fronts that move into and across the subcontinent, especially during the cooler months of the year, are frequently associated, the day before, by a coastal low that moves ahead of the front. Under these circumstances the southerly or south-westerly onshore wind of the coastal low gradually diminishes in intensity over the course of 12–20 hours, when it is replaced by a westerly wind (which may temporarily reach buster proportions) and a further drop in temperature accompanied by rain, indicative of the passage of the cold front. Thus, particularly in
Cape Town Cape Town ( af, Kaapstad; , xh, iKapa) is one of South Africa's three capital cities, serving as the seat of the Parliament of South Africa. It is the legislative capital of the country, the oldest city in the country, and the second largest ...
, an obvious berg wind is generally regarded as a harbinger of cold, wet weather.


Other orographically trapped weather systems

Coastal lows are
orographic Orography is the study of the topographic relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as ''oreography'', ''orology'' or ''oreology'') falls within the broader discipl ...
ally trapped weather systems that also occur in other parts of the world, where there are mountain ranges between 1000 – 4000 km in length. Thus they occur along the coast of
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
, eastern
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
and the west coast of North America, as well as on the eastern side of the
Appalachian mountains The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. They ...
of the United States. In each of these cases the weather systems are trapped vertically by stable stratifications, and laterally by Coriolis effects against the mountains. However, only the South African and the South American coastal disturbances are “coastal lows”; the remainder are generally produced by coastal ridging.


See also

* Foehn wind *
Chinook wind Chinook winds, or simply Chinooks, are two types of prevailing warm, generally westerly winds in western North America: Coastal Chinooks and interior Chinooks. The coastal Chinooks are persistent seasonal, wet, southwesterly winds blowing in from ...
* Sirocco * Bora (wind) *
Diablo wind Diablo wind is a name that has been occasionally used for the hot, dry wind from the northeast that typically occurs in the San Francisco Bay Area of Northern California, during the spring and fall. The same wind pattern also affects other parts ...
* Norte (wind) * Katabatic wind *
Khamsin Khamsin, chamsin or hamsin ( ar, خمسين , meaning "fifty"), more commonly known in Egypt as khamaseen ( arz, خماسين , ), is a dry, hot, sandy local wind affecting Egypt and the Levant; similar winds, blowing in other parts of North ...
* Oroshi *
Australian foehn winds The southeast Australian foehn is a westerly foehn wind and a rain shadow effect that usually occurs on the coastal plain of southern New South Wales, and as well as in eastern Victoria and eastern Tasmania, on the leeward side of the Great Divid ...


References

{{reflist Winds Geography of South Africa Afrikaans words and phrases