Coen Welsh | Aug 9, 2017 | 0
1 August 2014
Towards the beginning of August, the southern African synoptic chart starts resembling scrambled eggs. Although all the familiar patterns are there, the pressure differentials of deep winter are lacking, hence barometric pressure change only slightly during the day, fresh winds are rare, and the general weather picture is controlled more by lower pressures than by high pressure cells.
An area of low pressure, by definition, describes a large surface area where air is in motion. The faster the motion, the lower the pressure. This is standard physics but it finds expression in barometric low pressure when air moves both lateral (wind) and vertical (convection). The signature high pressure presence over the eastern part of the sub-continent, the co-called Highveld, is still there, but its readings are much down, this week registering only 1022 mB early mornings. West of this high pressure cell is what can be described as a trough, a band of lower pressure extending all the way from Angola to the southern Cape with similar features to a summer trough, but lacking the intensity. Barometric pressure this week hovered around 1017 mB. The rather slight difference between the high pressure east (cold) and the low pressure west (warmth), prevents the intense early morning cold of midwinter with mild to warm, or even hot days, especially in Owambo and Kavango. The week started very much in control of the high pressure cell over South Africa with very cold mornings on Monday and Tuesday. Again the common rule applies, the further south, the colder, but the high pressure effect must not be underestimated and while the cold came from the south at first, it quickly turned east later in the week. This is a very typical late winter pattern. Meanwhile, the other late winter signature, the creeping trough advanced from the Kunene Mouth towards the south, hugging the coastline, brining foggy wet conditions further south as it moved towards the Orange River. In a nutshell, a high pressure cell was present over the South African Highveld while another high pressure cell sat some 2000 km south of Madagascar. Interspersed was a succession of weak low pressure areas driving weak cold fronts, as always moving from west to east. Conditions changed slightly on Thursday when a major low pressure vortex developed about 1000 km south-west of Cape Town.
But this low pressure area was not followed by the successive South Atlantic high pressure cell as one would expect. Rather, a weaker but very extensive area of lower pressure lay both north and west of the main vortex. The result on the surface was a prominent airflow from north to south, bringing in the fog at the coast, and some patchy clouds over the interior. The same low pressure system caused a marked zonal (west to east) flow in the upper air, (45,000 feet and above), depressing the cold effect of the high pressure cell, leading to pleasant warm days.
The winter tries hard to make its receding impact last just that little bit longer, but the overall low pressure control on the surface as well as at the alto levels, prevent frost of any kind.
Two leading forecasts expect a warm pleasant weekend with only mild wind, if at all. These conditions continue into Monday, when the South Atlantic high pressure cell ridges around the prominent low pressure vortex but the general windy conditions associated with the low pressure area opposes the cold from the south.
One noticeable low pressure effect is the mild windy conditions. These will be most prominent early morning but should subside during the day, every day from Saturday through to Wednesday. It is however difficult to pinpoint wind direction. In the Kavango, it will be north-east, in Owamboland, north, in the Kaokoveld, north-west, on the northern coastline, north-west, at Walvis Bay, west, at Lüderitz, standard south-west, and for the rest of the south and the interior, mostly south-east. The weather waits in anticipation of more forceful low pressure intrusions which are the hallmark of August. The three Ice Angels are not here, yet.