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19 September 2014

What Happened?
Windhoek this week broke the equinox barrier. This is usually assumed to happen only on 21 September, but lying some 150km north of the Tropic of Capricorn, the sun’s azimuth crosses 90 degrees before the Tropic date. At the Tropic itself, on 21 September the day and the night will both be exactly 12 hours.
Quoting a study by the Montana State University in the USA, Wikipedia tells us that as of this year, the Tropic of Capricorn’s latitude is 23° 26’ 14.440” south of the Equator, but it is very gradually moving northward, currently at the rate of 0.47 arcseconds, or 15 metres, per year. So, in a hundred years from now, it will be one and a half kilometre further north. Big deal! Will somebody please remember to move the sign next to the B1 between Rehoboth and Kalkrand.
As the summer draws closer, the sun’s position becomes ever more important. It provides the energy that heats up the interior of the entire southern Africa, opposing the cold intrusion originating in the high pressure cells further south. But these cells remain the main drivers of our local weather and the resultant effects were very visible this week. Monday and Tuesday were very much under low pressure control. A marked surface trough which started developing during the weekend crossed the border into Namibia from Angola and by Tuesday has crossed the full length of the country, reaching the Orange River valley. At the same time the system migrated to the east, slowly spreading favourable conditions for light rain from Kunene to the Kalahari (Karas East).

But it must be understood that the driver behind this low pressure trough still remains the high pressure core when it is situated south of Madagascar. By Thursday, a marked low pressure system (1008 mB) covered the whole interior of southern Africa. However, the high pressure control over the surface low pressure system is witnessed by the cloud base which stubbornly remains aloft at around 16,000 to 17,000 feet. Deeper into summer, when a more dominant low pressure system moves south from the Congo, the cloud base will descend to about 7000 feet asl, and that is when the rain season starts. Then there is low pressure control from the surface right up to the 45,000 feet level. It also has much to do with the position of the so-called Inter-Tropical Convergence Zone. As it gets warmer, this zone becomes more pronounced, extending from the equator southwards first reaching northern Angola, about now, then proceeding towards the south until eventually, towards the end of December it touched the cutline between Namibia and Angola. How far further south it will move, even if only intermittently, remains to be seen, and will determine to a large extent, the success or failure of the next rain season. During the week, a very prominent convergence line developed, both on the surface, and up to about 25,000 feet. Again, this is the colder, denser airflow from the south, clashing with the warmer, more volatile airflow from the north. The colder conditions affected the southern half of the country during Thursday night but had little effect further north, where it remained hot to very hot.

What’s Coming?
The brief intrusion of colder air pushed ahead by the arriving South Atlantic high pressure cell has a notable effect in Oranjemund with cold, windy conditions during most of the weekend. This also affects the Orange River valley, but it is limited to remains south of the Lüderitz Keetmanshoop latitude.
Over the weekend the airflow in the south is first westerly, then veers to south, and eventually south -east. For most of the country, the airflow remains easterly, then backs to north-easterly, returning to a dominant northerly direction.
By Sunday night, a (low pressure) trough has again developed with sufficient strength to set up conditions for rain over Kunene, Owambo and Kavango West.
This system migrates to the south, bringing a sparse hope for rain to the interior, and, by Tuesday into Wednesday, for some precipitation over the south-western corner (Sperrgebiet). Conditions for rain also improve over the Zambezi where the airflow will be prominently north, and where it will be blisteringly hot.

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