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Weather 18 September 2015

What Happened
A number of local weather phenomena are all based on the same physics principle – when air is compressed, it releases energy and the ambient temperature rises.

We call them by various names: Oosweer, bergwind conditions, heatwaves, katabatic winds, diabatic compression, but they all refer to one or other form of descending air. One of the perceptible results, an increase in temperature is the most convenient way for the observer on or close to the surface, to detect the effect of descending air. When air descends, it compresses and this releases energy raising the temperature. Whether it is localised and fractional, like katabatic winds, or covers very large areas over thousands of kilometres with a significant rise in temperature (diabatic compression), the same physics is at work.
This week we had the ideal opportunity to witness the effect of diabatic compression over Owamboland and the Kavango. While those areas south of the 25oS latitude, i.e. south of Windhoek were still subject to a typical winter synoptic pattern, the northern half of the country experienced the first intrusion of more typical summer conditions. These went hand in hand with a significant improvement in the so-called “thickness” of the atmosphere. This is a measurement made in metres and it expresses the vertical height between the 1000 mB and the 500mB isobars, more or less from sea level to 18,000 feet or 5760 metres aloft. A thicker atmosphere is denoted by generally lower barometric pressure but with much higher vertical reach. It is this body of air with considerable depth that pushes down and results in a heatwave. A thinner atmosphere is shallower, denser, colder and more stable with higher pressure only at sea level. In the previous rain season, two characteristics stood out. The atmosphere did not develop to its full summer thickness, and there was persistent upper level high pressure control over eastern Africa up to the equator. Therefore, this week’s heatwave conditions over Owambo and Kavango were a very welcome sign. It indicated an early increase in the thickness of the atmosphere over the western half of the subcontinent. Unfortunately, the upper level high pressure control over eastern Africa persists, now exceeding far beyond the equator even covering the highlands of Ethiopia.
The synoptic progression for the week showed two weather system, in effect splitting Namibia into a northern half and a southern half. South of the Windhoek latitude, the pattern was typical late winter. The South Atlantic high pressure cell of normal strength (1024 mB) and the southern Indian high of the same strength, sat on either side of southern Africa. In between a mild vortex formed. As the week progressed, the outer rim of the South Atlantic high slowly migrated across the Karas and Hardap regions from west to east, while the core slipped past Cape Agulhas. In the mid levels between 18,000 and 30,000 feet a weak trough developed running from the Angolan border through Namibia’s north-eastern quadrant, through the interior of Botswana to the interior of South Africa.
While nothing spectacular came from this trough, scattered clouds were visible on almost every day with the cloud base at around 18,000 feet. On the surface, the southern half was still very much under control of the South Atlantic high while the northern half displayed all the characteristics of a turning point in the season.
What’s Coming
On Thursday, the number of sunshine hours per day, reached 12 hours at the Windhoek latitude. North of Windhoek, this state has been reached earlier while south of Windhoek it will only be reached after 21 September. Coupled with the thicker atmosphere over northern Namibia, it sets the stage for hot conditions to continue over the northern half. During the weekend, it will slowly spread with very hot conditions on Sunday and Monday over the northern Namib. It will also spread to the Zambezi and the eastern areas adjacent to the Botswana border.
The trough in the mid levels re-establishes itself and by early next week, some unstable air should be present over the north-eastern quadrant covering the Kavango, Bushmanland and Hereroland East. Vestiges of this system will remain in place until at least Wednesday.
Windy and dusty conditions will prevail over the southern Namib on both Saturday and Sunday.

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