John Olszewski
29 July 2011

Impressive Namibian Sandstorm from

What happened?

Weather surprises in July are scarce. Last week though, saw a harbinger that the unusual was on the doorstep and events this past week enhanced this.

First was the run of three cold mornings. Weather patterns moving south of the Cape ensured that cold air was being carried quickly away repeating the pattern of not only the recent past but of 2011 so far. The same pattern was forecast for this week but again “the weather” saw things differently. An active vortex present in mid-Atlantic deepened, intensifying to heights throughout the troposphere and tracked eastward, closing off the pursuing anticyclone from its “round-the-Cape” passage. So the cold inland push of Monday was maintained by the northward shift of the Atlantic anticyclone back to the 30oS latitude: where, before climate change got to grips, it were found on a regular basis. This saw a weak intrusion into the sub-continent with a southwesterly inflow that stayed for the rest of this week. Cold nights and mild days are the norm for our previously normal winter pattern.

As the trough moved further east, so an invasion of moist air advanced into the country. This push lay beneath the upper trough, which also brought in a alto-cumulus layer in the upper middle levels, perhaps closer to the 500hPa levels (about 19000 feet) than the more normal alto-cumulus levels (14000 feet or so). Even higher still, a layer of Cirrus had also appeared. This cloud seemed to be some 1000 feet thick in parts and perhaps below the 30,000 feet level. Some 60 years ago, when cloud analysis was practised well, it was stated that there is much more to learn about Cirrus clouds. Then came satellite imagery and the “need-to-know” motivation more-or-less disappeared. But the gap in knowledge has not been resolved.

The recent events have brought some light rain to our far south: both on Sunday and again Wednesday.

Cold air invading from the east brought unusually low minimum temperatures to the Zambezi valley, at Katima for instance, and towards the Maize Triangle areas.

The major vortex core advances along the 45 to 50oS range into the weekend by which time its influence has departed the western sub-continent. Because its upper extensions penetrate the high atmosphere, above 45,000 feet even, there is a measure of influence persisting, but by mid-week, a return towards more frequent patterns begins to take shape. But there does appear to be an overall northward extension of the Temperate weather patterns for the next few days at least around the Southern Hemisphere.

As the lower pressure area crosses South Africa, the cloud bands depart similarly and colder, drier air spreads behind it. But the departure picks up a southeasterly trend enabling the anticyclone pushing along some 35oS to also extend round the land on the surface but across the land in the lower upper levels: 850 and 700hPa (5000 and 10,000 feet).