Guest Contributor | Sep 14, 2018 | 0
To understand the interplay between high pressure cells and low pressure areas, it is convenient to slice the atmosphere into three vertical layers. The surface level reaches up to about 15,000 feet altitude, the middle layer from 15,000 feet to about 30,000 feet, and the alto level above that to some 45,000 feet. By meteorological convention, the actual atmosphere is regarded to extend way up to 56,000 feet, but that is just between the tropics. Closer to the polar regions, the atmosphere is much thinner, between 18,000 feet and 24,000 feet, but also considerably more volatile. How the season and in particular, the rain season develops in Namibia, depends largely on the dynamic between the South Atlantic high pressure cell which enters from the south-west, and controls the surface, and the lower pressure area induced by heat developing over the central plateau of the sub-continent, and entering our airspace from the north-east and north. This area of lower pressure is most prominent in the middle layers. It is also the most important conveyor of moisture from Angola, Zambia and the Kongo. In the larger picture, the local stance is further influenced much by the strength of both high and low pressure systems, south and east of Madagascar. This week provided another textbook example of the low pressure high pressure dynamic. The South Atlantic high pressure cell remains situated further north than usual. Its impact is felt along the southern coastline, but during the night, it controls the local weather across the entire southern half. It also causes the advancing lower pressure area which comes from the north, to be elevated to levels above 16,000 feet. The area of lower pressure controls most of the weather over the northern half of the country. The week started with low pressure control north of Grootfontein, and high pressure control south of Windhoek. In between, due to daily solar heating, a weak trough develops that flows from the north-west to the south-east (from Kunene to the Kalahari). The western edge of this trough is called the convergence line where the two systems meet. When moisture is very low, it is not visible, but as moisture penetrates in the middle layer from the north, it becomes visible as a broad band of cloud.This has developed every day this week, more pronounced in the north, but with an observable drift over the central parts, accumulating in the eastern half of Karas. Yet, the cloud base remains elevated meaning whatever precipitation occurs, is limited in intensity. This can also be seen readily in the recorded rainfall for the week with very few showers actually exceeding 10mm. Furthermore, the rainfall occurs only sporadically leading to brief, scattered showers. This is a daily occurrence for the interior east of the convergence line.
The southern Namib and the southern interior remains hot to very hot. Conditions for rainfall are not conducive and remain so throughout the weekend and into next week.The northern regions all have good prospects for daily rain covering the areas from the Kunene river mouth along the northern border to Kavango West. However, high pressure control in the upper air over Kavango East, Bwabwata and the Zambezi, will inhibited precipitation leading to a dry weekend over these areas. It is only by about Wednesday next week, that rainfall conditions over the north-eastern districts improve. Rainfall prospects for the northern half of the country are good during Saturday, Sunday, Monday and Tuesday. However, the South Atlantic high continues to control the south-western third, pushing up the cloud base and limiting precipitation. The high pressure control over eastern Africa is still in situ limiting the advection of moisture from the Indian Ocean, so while scattered showers are expected for the areas east and north of the convergence line, intensities will be limited. The so-called heat low is present everyday with its core over western Botswana. Therefore the areas bordering Botswana, from the Kaudom in the north to the Kalahari in the south, will be very hot, daily reaching or exceeding 36oC.