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Understanding Weather – not predicting – 26 July 2013

What happened?
One might say at last, winter’s icy fingers came closer to our latitudes. Not actually icy by degrees of temperature, but more where one’s feel is concerned.
Cold can be either an absence of heat or the influx of a cold airmass.
Winter nights imply clear skies hence maximum radiation. The lower daytime altitude of the sun limits its warming potential which when coupled with an airflow from a colder source increases the cold.
Should this airflow be more measurable, we feel it on the skin and the chilling effect becomes debilitating. These are not necessarily apart. Night and wind-chill are often coupled.
Namibia is a long country, extending across 8 degrees of latitude. The potential of temperature variation from north to south is frequently experienced. But this last week tended to even out this potential range. At its coldest, the temperature range (night) descended well into single figures from our northern border to the Orange valley. This is inland from the escarpment, where another cooling factor emerges: altitude. Much of the interior is some 10oC cooler than sea level. But with some airflow, the outward radiation heat loss is limited, due to air-mass mixing. One might well be warmer on top of a hill than in some secluded valley where the still of the night sees quite remarkable cooling. There is also the effect of temperature inversion, emphasizing this higher (warmer) and lower (colder) feature.
Like so many features of weather, this is interestingly complex.
During this last weekend, colder conditions spread west across even our most northerly regions. Daytime sun brought temperatures to above 20oC for much of the north and central inland. But the far south was limited to the mid-teens: couple that with wind flows of even a few knots and wind-chill is more than evident.
For Namibia, daytimes below 20oC will register as cold. Despite 8 hours of sun well above the horizon, its heat can be restricted.

What’s coming?
The focus is on the intense vortex development recurring between the 55 to 65oS latitudes. Core pressures down to 930hPa with upward extent (intensity) above 40,000feet. While their frontal activity has a limited northward extent, this presence restricts intervening anticyclonic ridges from all but transient presence to the sub-continental south. But a brief southerly flow during weekend penetrates the south and possibly central Namibia giving way to an easterly orientation which prevails for the rest of the week beneath the predominating upper air core. Only limited maritime inflow from the east can be expected.
By historic patterns: typical winter, but the absence of frontal activity across our south ensures a continued absence of necessary winter rain across our south, even the coastal Namib lies too far north.

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