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Wed, 19 Jun 2013 | |
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You are in: Weather > SA regional > Ecucation  BERGWIND Bergwind is the South African name for fohn wind or mountain wind. It is a hot, dry wind that blows from the interior of South Africa to the coast and usually is blustery. Most people find it rather unpleasant. It can be mild at times blowing at about 10km/h, but sometimes it can be really strong and may gust up to 100km/h causing some structural damage to buildings and uprooting trees. FORMATION Bergwinds usually occur when a strong high pressure exists south or south-east of the country and when a high pressure is also situated over the country. These conditions usually only occur in winter, but sometimes in summer too, hence bergwinds are mainly a autumn-winter-spring phenomemon. Since air rotates anti-clockwise around a high pressure in the southern hemisphere the wind direction to the north of the high pressure will be easterly or north-easterly, especially along the west coast of southern Africa. The bergwinds will usually start blowing along the Namibian coast and the first indication of bergwinds is a rise in temperature, sometimes this rise can be rapid. In winter of 1985 in CT, the temperature rose from 3°C at 07:00 in the morning to 27°C by 07:35 that SAME morning. This is not a common occurrence, but rather an extreme case. Since air in a high pressure descends and warms up as it descends it stands to reason that the off-shore winds will be warm to hot and the temperature will usually rise about 10 degrees Centigrade from the interior of SA to the coast. So the temperate at Upington may be 20°C, while at Alexander Bay it may be over 30°C. At the same time a coastal low will develop along the coast. Off-shore flow ahead of the coastal low is usually easterly to north-easterly in direction along the west coast and north-westerly along the east coast. Humidities are usually very low, sometimes as low as 5%, usually between 10-40%. The temperatures vary from 25°C to 35°C in winter and over 40°C in summer. Behind the coastal low the wind is on-shore and usually north-westerly to south-westerly in direction. It is cool and moist and usually associated with fog. The coastal low moves down the west coast and around the Cape Point and then up the east coast of South Africa until it fills up near Maputo in Mozambique. The bergwinds obviously follow the same pattern. The highest official temperatures recorded in South Africa (51.5° C) was recorded one summer during a bergwind occurring along the Eastern Cape coastline. The bergwinds are usually followed by a cold front in winter. Click on the minimized diagram below to view an enlarged image of the typical coastal low and bergwinds.
 Click to view a larger image. [ back ]
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ALEXANDER BAY 22/11 ALLDAYS 23/07 BEAUFORT WEST 19/06 BELA-BELA 21/05 BETHLEHEM 17/-1 BISHO 24/10 BLOEMFONTEIN 17/05 CALVINIA 20/05 CAPE ST LUCIA 23/17 CAPE TOWN 16/11 CLANWILLIAM 18/08 DE AAR 18/06 DURBAN 23/14 EAST LONDON 24/15 ERMELO 18/01 FAURESMITH 19/05 FICKSBURG 16/-1 GEORGE 19/07 GRAAF-REINETT 19/06 GRAHAMSTOWN 22/13 HERMANUS 18/11 JOHANNESBURG 19/02 JOUBERTINA 19/08 KIMBERLEY 19/07 KLERKSDORP 18/03 KOMATIPOORT 24/08 KROONSTAD 18/01 LADYSMITH 18/01 LANGEBAAN 17/09 LEPHALALE 21/05 LYDENBURG 20/03 MAFIKENG 19/05 MUSINA 23/08 NELSPRUIT 21/03 NEWCASTLE 21/02 OUDTSHOORN 19/07 PAARL 17/05 PHALABORWA 25/10 PIETERMARITZBURG 22/05 PLETTENBERG BAY 19/11 POLOKWANE 21/05 PORT ALFRED 21/14 PORT ELIZABETH 21/13 PORT ST JOHNS 24/14 POTCHEFSTROOM 18/02 PRETORIA 20/03 QUEENSTOWN 19/06 RICHARDS BAY 25/18 ROBERTSON 18/06 RUSTENBURG 20/04 SASOLBURG 19/02 SKUKUZA 24/07 SOWETO 19/02 SPRINGBOK 23/06 STILBAAI 18/10 SUTHERLAND 17/05 TZANEEN 21/06 ULUNDI 24/08 UMTATA 23/08 UPINGTON 23/09 VEREENIGING 18/03 VREDENDAL 19/09 VRYBURG 20/07 WELKOM 19/03 WITBANK 20/02 WORCESTER 18/06 |
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