26 September 2014
The week begun with a relatively strong trough developing along its customary track from Angola’s south-western corner, stretching across Namibia towards the south-east, and covering much of South Africa’s western and northern provinces. This came in the wake of the previous South Atlantic high pressure cell having just slipped past the Cape moving towards the east, driving a massive anti-clockwise circulation in the upper air that covers most of South Africa, Mozambique, Zimbabwe, and even the southern half of Zambia.
The trough is a channel of low pressure. It typically forms between two high pressure cells, in our case the South Atlantic high, and the offspring of this high, the Southern Indian Ocean high, when its core is located over the ocean some 1000 km south of Madagascar.
Windy conditions were prevalent along the so-called convergence line, being felt most at night when the temperature differential between the trough and the ridge (high pressure extension) is at its highest. The trough advected some moisture from Angola in the middle layers between 15,000 and 25,000 feet. But the overall high pressure control in the upper air, from 25,000 to 45,000 feet proved too much and the recorded rainfall on Monday and Monday night was scanty at best.
Since the hallmark of high pressure control is clear skies, it was only to be expected that from Wednesday onwards, the interior had very little cloud development and early mornings proved to be coldish. Upper air high pressure control originating from far south of Madagascar is very typical for September. The high pressure cells are still dominant, and located relatively far north, as oceanic conditions are still determined by a late-winter scenario. It is only over a period of months, as solar energy in the southern hemisphere gradually increases, that this broad synoptic lay-out changes.
By the end of the week, typical September conditions were again prevalent with a strong high pressure cell (1023 mB) south of Madagascar, and an equally strong South Atlantic high approximately 1000 km out to sea. In between, due to solar radiation was an extensive area of lower pressure (about 1016 mB), but not yet low enough to qualify as a low pressure area or a trough i.e. below 1013 mB.
As summer draws nearer, the amount of solar energy that reaches the continental surface, increases incrementally every day. During the day this leads to rising air and a subsequent reduction in barometric pressure but as soon as the sun sets, the air pressure reverts to normal, radiation is at its highest, and it often leads to surprisingly cold nights.
Rainfall recorded on Monday and Tuesday
Eenhana 0.2, Oshikango 0.2, Bagani 0.1, Oshakati 0.2, Mariental 2.0, Omaruru 0.4, Aroab 8.4, Okaukuejo 0.3, Karasburg 5.2 and Aus 4.6. Source: Namibia Meteorological Service.
The South Atlantic high pressure cell grows in strength as it approaches the continent from the west. Making landfall on Friday evening, it leads to colder night temperature in the south but the effect is neutralised by the heat of the interior, and by the relentless push of air from the north-east and the north.
Ahead of the high is a cold front which will bring colder conditions to the south, again more or less restricted to the latitudes south of the Lüderitz Keetmanshoop line. At Oranjemund and Lüderitz its impact will be most noticeable with strong and cold south-westerly winds over the weekend. This cold intrusion is of sufficient strength to make itself felt as far north as Windhoek, during Monday. Over the interior to the north, warm to hot conditions will continue. The five-day outlook, up to Wednesday next week, indicates only clear skies, hot days and wind from the north-west. A relatively strong convergence line forms north-west to south-east by Wednesday which will be accompanied by windy conditions over the escarpment, and the adjacent interior.
Zero rain is indicated for next week, not even for the Zambezi.