Coen Welsh | Nov 14, 2017 | 0
Weather 20 February 2015
A notable synoptic feature of this summer was the moderate but persistent high pressure control off the coast of western Africa from around Gabon all the way south to the Cape of Good Hope. Air density of the upper atmosphere are mostly determined by what happens on the surface. When high pressure control is present on the surface, it has a so-called adiabatic effect as far up as 35,000 feet aloft, at which level, conditions are neutralised by the very cold temperature and the very thin atmosphere. Over the equator, the atmosphere is at its thickets, reaching to about 56,000 feet. Over the poles, it is at its shallowest, seldom reaching 20,000 feet. This “thickness” of the atmosphere over the equator is an indicator of the volume of moisture present. Meteorologists use the 500 mB altitude as a benchmark to get a grip on how the inter-tropical atmosphere is developing. Ordinarily, this pressure level sits at about 18,000 feet. When it descends, it is due to high pressure control, when it rises, it is an indication of lower pressure and an upward-extending atmosphere. This whole summer, the atmosphere over the Atlantic west of Africa south of the equator, was depressed because of the influence of Atlantic high pressure. The line that indicates the separation between continental low pressure and oceanic high pressure, hugged the coastline closely. This week it changed. Lower pressure control can now be seen on the synoptic map from around the Orange River in the south, all the way up to West Africa and around the bulge of Africa. This indicates a lesser influence of the South Atlantic high pressure cell, which by default, allows more space for the low pressure area over the continent, to develop to its full potential. These changes have also become visible recently. As the battle between high pressure from the south and low pressure from the north continued over Namibia, a prominent convergence line formed by Thursday but slightly further south than usual. This is a good sign. By the end of the week, the synoptic map has reverted to its typical pattern of two high pressure cells, one on either side of the sub-continent, and a low pressure area in the middle, associated with the eastward migration of the cold front.
The weekend starts with a cool Friday morning for the Karas region south of the Lüderitz Keetmanshoop latitude. This is the result of a mild, but well-defined, cold front passing over the southern half of South Africa at a fairly rapid rate. The northward push of this cold front will be felt up to Windhoek but it plays itself out during Friday as the low pressure control on the surface exerts itself and repels the associated high pressure. As the South Atlantic high slips across the southern tip of the continent, it leads to cool, clear conditions south of the Orange River latitude. The local impact, other than the Karas region, is fleeting, and by Monday, low pressure reigns again over the entire southern Africa. But the shifting high pressure cell now finds itself at the southern end of the Mozambican channel and from there, it acts as a powerful northward driver, bringing some high pressure control over eastern Africa. However, another vortex starts developing some 2000 km east of Madagascar over warmer water, conveying moisture into the middle layers. With a east to west zonal flow over the Indian Ocean, this may lead to considerable advection of moisture. What we are waiting for is to see how far south of the Namibia Angola border, the so-called inter-tropical convergence zone (ITCZ) will penetrate. It may bring some rainfall relief during March. For next week, limited rain is indicated only for the Kalahari adjacent to the Botswana and South African border (Gemsbok Park area).