Berg wind
Berg wind (from Afrikaans berg "mountain" + wind "wind", i.e. a mountain wind) is the South African name for a katabatic wind: a hot dry wind blowing down the Great Escarpment 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 processes. This accounts for the hot and dry properties of these off-shore winds, wherever they occur along South Africa's coastline.[1][2] 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 processes. This accounts for the hot and dry properties of these off-shore winds, wherever they occur along South Africa's coastline.[1][2]
Although berg winds are often called a
Coastal lows and berg winds
Berg winds are usually accompanied by coastal lows.[3] 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 coast. They are then always propagated counter-clockwise along South Africa's coastline at between 30 and 60 kilometres per hour (19 and 37 mph), from the west coast southwards to the Cape Peninsula and then eastward along the south coast, and finally north-eastward along the KwaZulu-Natal 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.[4] 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.[3]
Coastal lows are a common feature of the coastal weather in South Africa with an average of about five lows of varying intensities passing through Port Elizabeth per month.
Origin of coastal lows
Coastal lows are initiated by the interaction of large scale weather systems such as the quasi-permanent
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.
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.[3][4]
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.[4]
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.[3] Thus, particularly in Cape Town, an obvious berg wind is generally regarded as a harbinger of cold, wet weather.
Other orographically trapped weather systems
Coastal lows are
See also
- Foehn wind
- Chinook wind
- Sirocco
- Bora (wind)
- Diablo wind
- Norte (wind)
- Katabatic wind
- Khamsin
- Oroshi
- Australian foehn winds
References
- ^ a b Danielson, Eric William; Levin, James; Abrams, Elliot (2003). Meteorology. McGraw-Hill.
- ^ ISBN 9780416079401.
- ^ ISBN 9780195718065.
- ^ hdl:10530/906.