Understanding Weather – not predicting – 26 July 2012
When applying such words as “normal” to weather and climate in our context, the essence of our arid environment must be considered. In contrast to the other global climate zones where consistency is both hallmark and foundation, our signature is variability across whatever weather detail one cares to look at.
Despite a freezing weekend, the week that was quickly recovered giving us one of our most consistent weather aspects: sunshine. We are now slightly more than a month past the winter solstice and already the effect of more sunshine hours per day can be felt, as was readily observed during the week.
It is still only the end of July yet the sun already shines a whisker over 11 hours each day, increasing by about 1 minute and 17 seconds per day. This leads to the anomaly of temperatures swinging through a 20 degree range in 24 hours, from lows of around 5oC to daily highs of around 25oC. This big difference between day and night temperature is rare elsewhere in the world.
A high pressure system continued to dominate local weather for much of the week. The high pressure core feature is descending air, hence our clear skies. This airmass flows outward to create and maintain both the Torrid Zone convergence and the Tepid (Temperate) Zone vorticity. These are consistent factors of an arid zone climate.
These last few days supplied our normal sunshine, providing warmer days countrywide. The anticyclonic core though was located somewhat away from the expected area, Botswana, and instead sat above central South Africa. Apart from ensuring an easterly airflow, this more southerly position kept the cold fronts of the 3 successive temperate vortex cores of the weekend (and a couple more by mid-week) well to the south of the Cape. No northward cold front presence means no coastal low development (plus-minus Walvis area) thus an absence of normal coastal weather variation. It also means an absence of rain across our south.
This is yet another aspect of the reality of a changing climate: a shift of ranges of activity but the climate expectation goes little altered other than the shift southward.
Again, by mid week welcome warming gave an appreciated return to more ideal temperatures.
As the weekend arrives the upper air sees a development throughout the middle and higher levels of a well-marked trough. This gets closer to the surface by Sunday developing a cut-off vortex southeast of the subcontinent which is a most unusual position. This keeps the high pressure area static until early next week while another intense upper trough forms to the west by Wednesday. Winds will remain easterly with cold mornings, turning north easterly to north, later in the week.
The mild oosweer at the coast departs during the weekend moving to the south and then vanishes as more variable wind patterns develop. For the greater part of the country, little change from the conditions of the past few days is expected.