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Weather 30 April 2015

What Happened
The prominent feature of this week’s local synoptics is the marked difference in temperature between north and south. The south-western half of the country remained under high pressure control, both on the surface and in the upper levels, with a weak anti-cyclonic circulation in the middle level.

However, temperatures in the north, across the entire breadth of the borders with Angola and Zambia, remained excessively hot, reaching the upper thirties in the afternoon from the beginning of the week and throughout up to Workers Day.
When the number of sunshine hours is less than the number of nighttime hours, it is axiomatic that the cooling phase is longer than the heating phase. This means night radiation is more than diurnal irradiation, indicating a decrease in average temperatures. Yet this is not the case over the northern half. From Otjiwarongo to the north, daytime temperatures across Owamboland and the Kavangos rose every day to about 34oC. Furthere west, into Damarland and the Kaokoveld, late afternoon temperatures went even higher. In the east over the Zambezi, temperatures reached the lower thirties even before noon.
If this heat does not come from the sun, what is its origin? It comes from the presence of a weak high pressure cell over Zambia, Mozambique and Malawi. This area of higher pressure ensures that the daily airflow on the surface maintains an anti-cyclonic bias although this circulation is not very strong. This high tends to descend once the sun sets, so it compresses and the so-called anabatic effect releases energy. This is the source of the heat over the Zambezi.
In the west, the picture is slightly different. The anti-cyclonic circulation over central southern Africa means that the reigning airflow is orientated from the north-east. The air, very slowly, moves from Zambia (high) across the Zambezi and the Caprivi strip, over the Kavango regions, across Owambo and into the Kaokoveld. As its source is an area of slightly higher pressure, it also tends to descend, also releasing energy. This causes the abnormally high daytime temperatures.
When this same airflow from the north-east crosses the escarpment, descending even more to the coastal plain, the anabatic effect is enhanced, leading to even higher temperatures. Since this process happens rather slowly, the heat over the northern Namib was felt mostly in the late afternoons. This is typical Oosweer, although for this week, it only effect the northern half of the coastline.
The bigger synoptic picture shows the South Atlantic high pressure cell on the same latitude as Saldanaha Bay. This is its customary position and from there it influences conditions in Namibia over the southern Namib, reaching east across the Karas and Hardap regions. But as an extension of the weather system of the Western Cape, it reached the Orange River valley by the middle of this week, bringing in visible cloud from Oranjemund towards the east. This presence of a thin cloud cover was however restricted only to the very southern strip of the Karas Region.
South of Agulhas, a prominent vortex has formed by the middle of the week. Another vortex was present south of Madagascar, bringing the eastern half of southern Africa under low pressure control despite the presence of the weak high pressure cell over Zambia. The southern Indian high pressure cell sat thousands of kilometres away, halfway between Africa and Australia, too far removed to have a marked impact on southern African weather.
What’s Coming
By week’s end the South Atlantic high is making its presence feel in Oranjemund and the adjacent southern Namib where it will become considerably cooler. The high pushes the two vortices away to the south-east, clearing the skies over the Karas Region. Meanwhile, the weak high pressure over Zambia fades, removing the driving force of the high daytime temperatures across the north. In the north west, a reciprocal area of lower pressure develops from the Kunene River mouth southwards. During the weekend the general synoptic stance reverts to a more customary picture as the high migrates to the east and start influencing conditions up the Mozambican channel.

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