| Understanding Weather -not predicting- 25July 2008 |
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| Written by John Olszewski | |
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While most of the country enjoyed the pleasantries of the winter season, i.e. warm by day but cool to cold overnight, further south conditions were not as mild.
South of the Orange River brought one into the northward range of a cold front sequence which dominated the winter rainfall area. This may seem normal, but there is more to come. The zone of extensive turbulence lay somewhat further south and covered the hemisphere southward to the edges of Antarctica.
This zone covers the “Roaring Forties” so named from the
roaring sound the wind makes when blowing through the rigging of the
old sailing ships. This windy pattern typifies the rapid progress of
anticyclones and vortices interacting between polar and temperate
wind-flow zones. This year we have seen often on the weather maps
features that were unusual for winter season. Many times these pattern
were typical of a summer pattern yet summer it is not.
The anticyclonic cores have shown a remarkable tendency to build from small cores breaking off from the polar anticyclone. As these pockets break away and move into the Roaring Forties latitudes, they build rapidly, increasing in size and in pressure. Eventually they merge with a ridge of the sub-tropical high pressure belt. This merger does not transform the polar origin of the air so intensely cold air feeds the cold frontal activity of the vortex ahead and swirls this air into lower (northward) temperate latitudes. And as we have recently experienced, it can even feed it into the sub-tropics. Not only that, but there is the ability for sub-tropical anticyclonic ridges to break from the core cell and form a new core which then tend to move southeastward, rather than just east. This weather pattern is rare to say the least. Its message is also mystifying. Such a weather pattern would be associated more with a summer. Why this happens is not clear. Were it summer, these north south north flows would have brought rain across southern Africa. Across the rest of the world though, weather performs within the expected range. Indian monsoon conditions are on time, the Pacific is in a normal state (for once), so it appears that only the mid and higher southern latitudes are twisting and turning. What’s coming The outlook for the next week promises two new cold fronts for the Western Cape but with little northward extension. These systems are following too quickly, the one after the other, to allow serious northward cold air influxes. Somewhere along the line there has to be a break, but the break is not identifiable across the next few days. A cause does seem to be that the anticyclones are taking a course too far to the south to advect their polar or modified polar air northward, instead it is being twisted around and recirculated southward as the new vortex appears: a complicated set-up indeed. Pleasant temperatures by day but still typical winter overnight is expected. The veld is generally dry. The risk of veld fires increases by the day. |
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