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10 october 2014

What Happened?
High pressure control on the surface in the southern half, low pressure over the northern half but a continuing overall high pressure control in the upper air, were the hallmarks of this week. With layers of high pressure control, it only followed naturally that a so-called inversion layer will develop. This happened at around 18,000 to 22,000 feet and it created a visible, thin blanker of cloud that produced some precipitation in the a narrow band from Eenhanna to Aranos. Inversion layers are typically an early summer phenomenon when the lighter, warmer air from the north starts expanding across a large area of the interior of southern Africa. This starts around the end of September, and develops progressively, day by day until the low pressure area is no longer restricted to a narrow band, but covers the entire inland plateau of the southern African sub-continent. It is usually called a heat-low as it is the result of solar radiation on the surface, and not a trough that develops on the outer fringes of the south Indian high pressure cell. But with higher pressures pushing in from the south, and high pressure control in the upper air, it must be expected that any potential rainfall is limited. Whatever moisture enters our airspace remains sandwiched in this shallow inversion layer that develops between the two high pressure control strata. When it actually rains, it evaporates on its way down, and when it does not rain, it simply disappears into the dry, cold layers above it. The summer push from the tropical systems are not there yet, and the Inter-Tropical Convergence Zone (ATCZ) where the Equatorial low pressure belt meets the high pressure zone, remains at least some 1000km north of our northern borders. It will be another two months before the ATCZ should lie smack on our northern border, ready to advect moisture across Namibia at the slightest sign of weakness in the South Atlantic high pressure cell. Until then, we must be contend with the (lower pressure) through that regularly develops in the wake of the high pressure system as it slips across southern Africa from west to east. This trough does bring some precipitation but for the reasons explained, it is only the entre’ to the main rain season. From Monday to Friday this week, this succession of events, and all its features, were displayed sequentially, almost like a textbook example of the morphing period that marks the transition from a typical winter synoptic pattern to an early summer one.
Rainfall on Monday 06 October

Gobabis 13.8; Rehoboth 13.2; Mariental 6.8; Eenhana 4.2; Aranos 2.0; Otjiwarongo 1.6; Hosea Kutako Airport 1.0; Omaruru 0.1; Buitepos 0.1 and Windhoek HQ 0.0 (Source: Namibia Meteorological Service at www.meteona.com)
What’s Coming?
On Sunday, the daily sunshine hours crosses the 12 hours and 30 minutes threshold. This gradual increase in solar radiation, as well the daily shift in the sun’s angle, boost the development of the heat-low over the central part of southern Africa covering more or less eastern Angola, western Zambia, western Zimbabwe, northern Botswana, and of course, Zambezia and the Kavango. The approaching South Atlantic high pressure cell that was first felt in Oranjemund on Thursday, continue to infringe upon the heat-low area from the south, but the effect is only on the surface, i.e. below 15,000 feet asl. This interplay between higher pressure in the south-west and lower pressure in the north-east creates a very windy Saturday and Sunday up to the Lüderitz Keetmanshoop latitude. This is great news for the skiboarders currently in Lüderitz, begging for wind to try and outdo their records of last year. As the high pressure cell shifts east, it stays mostly offshore, curling around the continent, drifting slightly southward, and re-establishing itself south-east of Madagascar. This shift creates space for a vortex to develop immediately south of Madagascar, growing in intensity from Monday onwards. What this vortex does is to draw the (low pressure) trough that develops across Namibia along the convergence line, to shift further east than usual. This indicates continued mild but windy conditions across Karas and Hardap, with very hot conditions over the Kavango and Katima. This vortex also ensures the continuation of a zonal flow in the upper air, from west to east, indicating the absence of any significant precipitation.

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