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Thu, 11 Mar 2010 | |
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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 27/16 ALLDAYS 37/20 BEAUFORT WEST 32/17 BELA-BELA 34/18 BETHLEHEM 28/11 BISHO 31/20 BLOEMFONTEIN 32/16 CALVINIA 30/15 CAPE ST LUCIA 29/23 CAPE TOWN 23/17 CLANWILLIAM 29/17 DE AAR 33/18 DURBAN 29/21 EAST LONDON 30/23 ERMELO 28/12 FAURESMITH 33/18 FICKSBURG 28/13 GEORGE 25/19 GRAAF-REINETT 31/19 GRAHAMSTOWN 30/22 HERMANUS 25/19 JOHANNESBURG 30/14 JOUBERTINA 25/20 KIMBERLEY 34/20 KLERKSDORP 34/17 KOMATIPOORT 30/19 KROONSTAD 33/15 LADYSMITH 27/10 LANGEBAAN 25/18 LEPHALALE 34/20 LYDENBURG 28/14 MAFIKENG 36/19 MUSINA 38/21 NELSPRUIT 29/15 NEWCASTLE 29/12 OUDTSHOORN 28/18 PAARL 28/17 PHALABORWA 35/20 PIETERMARITZBURG 28/16 PLETTENBERG BAY 23/20 POLOKWANE 33/19 PORT ALFRED 29/23 PORT ELIZABETH 28/23 PORT ST JOHNS 28/19 POTCHEFSTROOM 33/15 PRETORIA 31/17 QUEENSTOWN 30/16 RICHARDS BAY 29/24 ROBERTSON 28/17 RUSTENBURG 32/17 SASOLBURG 33/14 SKUKUZA 32/19 SOWETO 30/15 SPRINGBOK 33/15 STILBAAI 25/20 SUTHERLAND 29/14 TZANEEN 33/19 ULUNDI 30/19 UMTATA 30/15 UPINGTON 38/23 VEREENIGING 32/14 VREDENDAL 28/17 VRYBURG 35/19 WELKOM 34/16 WITBANK 30/14 WORCESTER 28/16 |
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