Weather 02 September 2016
The season is transitioning from late winter to early summer. The diagnostic element is the continental circulation.
The week started with a remarkably strong anti-cyclonic circulation over the entire sub-continent. Its vertical reach was impressive stretching from the surface to the alto levels above 35,000 feet. For the first time since winter, it broke the pattern of strong west to east zonal flow in the upper air. This anti-cyclonic circulation is the hallmark of summer and its first appearance this week sets the stage for the stronger summer pattern that will now start to build.
However, as the South Atlantic high pressure cell approached the continent by mid-week, it opposed the continental circulation reverting the synoptic map to a more late-winter pattern.
Solar irradiation is an important ingredient in this interplay between low and high pressure. It heats the air as it travels over land energizing it to a lighter, less dense air mass. Over Namibia, this airflow typically comes from the north-east and the north. The South Atlantic high, though, comes from the west and the south-west. The latter being colder and denser, hugs the surface, bringing cool nights and quiet days. Where these two systems meet, a so-called convergence line forms usually in the mid-levels at around 18,000 feet or the 500mB isobar. This gives us the typical September style high clouds which may produce precipitation, but it never reaches the ground.
Cloudiness over the interior on Wednesday and Thursday was the result of the high on the ground, elevating and displacing the low’s intrusion from the north.
By mid-week, a frontal system crossed South Africa south of the Orange River. This did not directly impact Namibia but it did lead to cooler nights in the Karas region from the ocean up to the South African border in the Karasburg district.
This daily reciprocal movement between high and low is a typical feature of early summer. The highs, as they approach, is felt on the surface but as soon as the sun rises the next day, the solar heat lowers the barometric pressure and the low is evidenced by the wind which veers to north-east or north. This daily occurrence is experienced by the observer on the ground as cool to cold nights with the cold intruding from the south, replaced during the day by a northerly airflow and a rapid rise in temperature.
The position of the convergence line is what makes Namibia one of the most important observation areas in the southern African continent. The relative position and height of the convergence line is an empirical indication of the relative balance between high pressure and low pressure. As such it is a weather marker for what is to be expected over the rest of the sub-continent for the next week.
The week-end starts with a typical winter synoptic pattern. The core of the approaching South Atlantic high pressure cell is about 1000km offshore from Cape Town. It is fairly strong reading 1036mB but it is displaced to the south, running along the 40°S latitude. It means the cold core will miss the continent.
The outer rim is relatively weak (1020mB) but it brings cool nights to the Karas region. The rest of the country will be pleasant during the day.
As the high departs by Sunday night, a low pressure system develops along the coastal plain from the Kunene river mouth. This should bring windy conditions to the northern Namib from Monday onwards and again hot days in the north, the Kavangos and the Zambezi.
This low pressure system develops into a mid-level trough, and it links up with the low pressure system over the South African interior. Over the south-western quadrant it will be cooler by night and warm by day while the north-eastern quadrant will continue to have hot days.