Rikus Grobler | Oct 18, 2017 | 0
Weather 23 October 2015
This week was marked by very hot day temperatures in the north-east and the south-west. Basically every day, Owamboland, Etosha, Khorixas, Outjo, the Kavangos and the Zambezi, reached the upper thirties in the late afternoon.
The same happened in the southern Namib where temperatures also reached for forty. The hot temperatures indicate a thick atmosphere with considerable depth. Over the southern Namib this is exacerbated by high pressure conditions on the surface, by the daily duration of solar irradiation, and by bergwind conditions during the afternoon. Yet, almost everyday saw the formation of not inconsiderable cloud over a wide swathe from the Kunene River across the interior in a south-easterly direction up to the Orange River Valley in the south, and the Botswana border in the east. From a visual perspective, daily satellite images clearly show open skies in the Caprivi area of the Zambezi region, over Kavango West, and over the western sections the Hardap and Karas regions. Then later in the afternoon, these same satellite observations show a gradual built-up of cloudiness, starting usually at the western boundary of Etosha, and slowly developing towards the south and the south-east. It may be difficult to believe, but this is mostly due to the influence of the South Atlantic high pressure cell. At the beginning of the week, it made landfall between Lüderitz and Oranjemund. As it migrates from west to east, it encounters the daily heat of the interior and dissipates. But the South Atlantic high is the most important local weather element and it must be remembered that it stretches over the ocean for at least another 5000km. What we see and experience is only the very thin leading edge. The massive animal behind it, we never see, but it is so huge and so powerful, it determines the weather for basically the entire southern African sub-continent. It is the origin and the auctor of the Namib Desert, and the number one reason for the general aridity of the western half of southern Africa. As a volume of air that descends, the South Atlantic high has its biggest impact on the surface and up to around 15,000 feet. This is however a flexible ceiling and some days it may lie at 12,000 feet, the next day at 18,000 or even 20,000 feet. But it is still the South Atlantic high at work, and it determines what happens on the ground. As it crosses the continent, it wraps around the southern Cape and then moves up the Mozambican Channel, with an equally important impact on eastern Africa. From there it moves further east to become the next southern Indian high. This process describes exactly the weather of this week. The South Atlantic high dominated the south with cooler nights but it also cleared the entire sub-continent of moisture. The airflow was first south-west, then due south, then south-east and finally by Friday, due east. This is a very typical pattern for southern Africa, but it is somewhat out of sync. The current synoptic pattern shows clear late-winter traits, and a summer pattern has not emerged fully yet. But flowing from the north, and moving over the high on the surface, is warmer, moister, lighter air. This is driven by the anti-cyclonic circulation over the continent. It is the intrusion of this layer of air above the surface high pressure cell, which forms the cloudband at around 15,000 feet. And when this intrusion moves further south, it tends to rise, enhancing the modest convection that takes place internally. This slipping of the northern system over the southern system creates the conveyor that takes the moisture across Namibia into the Northern Cape Province and from there further south even into the Eastern Cape.
The South Atlantic high remains the dominant feature. The days will continue to be hot but cloud built-up will improve as the weekend progresses. Since it is the northern system that brings the cloud, it will still be elevated with the base at around 15,000 feet.
Moisture continues to enter Namibian airspace from the Kunene at the mid-level and moves across the interior along the familiar route towards the Kalahari in the south-eastern quadrant. By Monday, a so-called cut-off low develops just offshore from Saldanha Bay followed by a very strong vortex driven by the next South Atlantic high which is still many thousands of kilometres away. This interplay enhanced the mid-level trough, same as this week, but only much stronger. There is actually, for the first time this season, a reasonable expectation for country-wide rainfall, excepting only the southern Namib and the Caprivi.