05 September 2014
As the southern hemisphere moves into the spring season, not only do the sunshine hours increase, the sun’s position itself changes relative to the earth and the sunlight angle decreases. In practical terms this means the same amount of solar energy now heats a smaller area of the earth’s surface, thus transferring incrementally more energy every day. One can put it very simple: every day the sun shines a little longer, and a little hotter. The daily increase is about 34 seconds, or about 18 minutes a month.
This week was firmly in the grip of a high pressure cell. The South Atlantic high pressure cell touched the western edge of the continent late last week. Over the weekend it slipped across the sub-continent but to much surprise, retained a fair amount of its strength. It settled over the eastern highveld of South Africa where it stubbornly remained for the larger part of this week.
This high pressure cell was not particularly strong, around 1024mB, but it remained continental for longer than expected as its passage to the east was blocked by a far stronger cell some 1000km south of Madagascar. It was this bigger, stronger high, reading some 1034mB at its core, that kept the cold air stuck over South Africa. This weaker high had a typical anti-clockwise circulation leading to an easterly airflow over Namibia.
Meanwhile, the signatory lower pressure that originates in Angola, continued with its southward movement along the coastline, effectively splitting Namibia again into two weather halves, only this week it was a western half, eastern half split.
If one roughly divides the country along an imaginary line running from Eenhana due south to Ai-Ais, it gives an ideas of the two opposing systems either side of this line. In the east, higher pressures prevail and the airflow is from the east, fed by the remaining (weaker) core of the high over South Africa, extending across southern Zimbabwe and Botswana. This gave Bushmanland, Omaheke and the western sections of Hardap and Karas, their typical cooler nights.
West of this imaginary line however, the barometric pressure was substantially less, around 1016mB, the airflow was from the north, and as the effect increased during the week, the days became progressively warmer. Since there is a notable pressure differential between the eastern half and the western half, the dominant air movement was from east to west, leading to so-called anabatic compression, also contributing to rise the daytime temperatures in the west. By Thursday, Kunene Region and the inland areas of Erongo, became really hot late in the afternoon.
To understand the current inter-seasonal weather pattern, it is necessary to note the positions of the high pressure cores. Historically, the South Atlantic high pressure cell tracks a latitude around 37oSouth. This week however, a very erratic pattern emerged.
The strong high south of Madagascar was more or less where it should be, at 37oS. The weaker high over the eastern sub-continent extended far north to about 22oS while the core of next approaching South Atlantic high is way off track to the South at about 43oS.
The western half eastern half split remains in situ over the weekend. The west, particularly below the escarpment becomes progressively warmer, and even hot. This extends all the way to the Orange River. The airflow remains east over the eastern half and north over the western half. It is only by Tuesday next week as the northern fringe of the next South Atlantic high approaches Oranjemund that the windflow backs to south-west and the temperatures below the escarpment subside. For the remainder of next week, the weather picture reverts to a southern half, northern half split. Somewhat lower temperatures in the south, warm to hot conditions in the north.