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Overview for the week and 5-day outlook to Wednesday 24 October 2018

Overview for the week and 5-day outlook to Wednesday 24 October 2018

Visual: Solar radio flux as measured on earth’s surface from 2000 to September 2018. This visual shows the cycles of sun activity which has an impact on earth’s magnetosphere, in turn amplified by high pressure conditions like this week.

Source: National Oceanic and Atmospheric Administration (NOAA) of the US government as published at www.spaceweathernews.com/

What Happened

A weak cold front ahead of the South Atlantic high pressure cell crossed the southern Cape from Monday to Wednesday. Although the cell’s core remained offshore, the 1020 mB isobar moved overland with an extension into the Northern Cape and southern Botswana.

This brought anomalously cooler nights to Namibia’s interior during Monday, Tuesday and Wednesday night with a weather pattern that still reflects a winter stance. The cooler air first came from the south, then backed east coming into Namibian airspace across Botswana. This was also accompanied by extensive ridging which derives from high pressure control in the middle and upper levels of the atmosphere.

The extent of ridging must be appreciated to understand its local influence. During Wednesday and Thursday, the high pressure control was so extensive it covered most of South Africa, Zimbabwe, Botswana, Namibia above the escarpment, southern Angola and Western Zambia. It was the reason for the clear skies totally empty of even the faintest whiff of cloud. It was further confirmed by the extremely low humidity at surface level with not a single area in Namibia registering humidity above 15%. In most place above the escarpment it was less than 10%.

Such bone-dry conditions are easy to notice. It leads to skin, eye and airway irritation in many people, causing all sorts of irritation. It may also have much to do with solar conditions as shown in the accompanying visual. Solar impact on earth’s magnetosphere is at an unprecedented low leading to all sorts of weather anomalies of which this week’s conditions are probably one of the better examples.

Locally, ridging also produces excessive heat during the late afternoons. The high pressure control stretches over many thousands of kilometres as can be seen from the list of countries mentioned above. When it reaches above 30,000 feet, it constitutes an enormous volume of air. This air functions like one, huge, contiguous “bubble” covering almost the entire southern Africa.

When the sun rises, the atmosphere starts absorbing energy and tends to rise, even if ever so slightly. This reduces temperature but during summer, it is rapidly replenished by the sun. Once the sun has moved past the mid-day mark, the system very gradually sheds energy, and the whole air column starts sinking, again ever so slightly. This downward movement compresses the air but because the volume is so massive, even a 10 metre descent causes relatively large fluctuations in temperature. As the air compresses, it released energy and the ambient temperature rises, sometimes by as much as 5 or 6°C. This causes the excessively hot afternoons.

Both these spill-over effects of high pressure systems were present and very noticeable this week. First the cooler nights, and then the very hot days on Wednesday, Thursday and Friday.

What’s Coming

High pressure control between 18,000 and 30,000 feet continues on Saturday with extensive ridging over the interior as far north as Etosha leading to another very hot day followed by a cooler night.

The continental high remains fairly prominent covering South Africa, Zimbabwe and Botswana, almost to the Namibian border. There is a substantial pressure differential between the Namibian and the Botswana interior but the transition zone touches only the Karasburg district and the eastern-most area of the Hardap region bringing unpleasant windy conditions, and again a cooler night.

For the rest of Namibia, conditions start to change during Sunday with lower barometric pressures over the interior. The continental high remains in situ over the eastern half of the subcontinent during Monday and Sunday, but the lower pressure over Namibia indicates a mid-level trough from Angola through Namibia into South Africa.

It is noteworthy that this trough is expected to run in an almost straight North to South zone from the Angolan border through the interior to the Orange River, not in the usual skew pattern.

With the trough comes a strong airflow from the North (tropical air) and moisture. Much cloudiness is expected over the entire Namibia except the coastal plain. This is a broad intrusion and it has rainfall potential from north to south.

Monday and Tuesday show the strongest possibility for widespread light rain, depending on how long the continental high to the east remains in place, and how far west the low pressure system develops.

In any case, as from Sunday, the excessive heat should abate for at least three days.

By Wednesday the rainbearing potential has receded to the northern half of Namibia despite much cloudiness over the rest of the country.

In contrast to the interior where it will be cooler, the coastal plain may experience two very hot days on Monday and Tuesday due to the prominent easterly windflow, almost like a late Oosweer.


 

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