Understanding Weather – not predicting – 09 August 2012
“Blow, blow thou winter wind, thou art so unkind” goes the mediaeval acknowledgement of our current description: Wind chill factor, and its presence was obvious for much of Namibia during the past few days. The further south one went the more unkind the experience! Wind is the factor by which we can feel air. It is the atmospheres’ response to unequal distribution of air masses and their variation of barometrically measured pressures. While only more recently understood, navigators especially have, for the last 1000 years, put to use winds’ strength and direction to manipulate sailing vessels across a wide range of the seas far beyond land-bound horizons.
So where does this match the experience of the past few days?
Of course cold outbreaks have a regular history where our winter climate is concerned. The variation is that the strength might be similar, but the direction (and its source) varies from the accepted norm.
Last weekend saw the development of well-marked trough move in from the southeast Atlantic. Its advance into the continent was coupled with the push of the succeeding anticyclonic ridge around (south of) the slowing vortex core, cutting it off, so leaving it stranded above the southern parts of the subcontinent. Situations like this bring precipitation, this was no exception. Namibia’s experience was limited. In South Africa however, widespread snow was recorded. Extreme cold coupled with precipitation in July are rare here, on our calendar July is the least rain-prone month for all but the south, far south west in particular, so we missed this range of excess.
To the rear of the cut-off low, a deep south to southeasterly flow invaded. This flow began from as far away as the 60oS latitude, circulating beneath mainly overcast skies. The ability to warm up was restricted: so the cold just blew in. As the low pressure synoptics dawdled, deep inflows persisted for another 48 hours. Afternoon warmth, insolation, was limited by this wind chill factor. Sheltered places with slack wind but maximum overnight radiation recorded frost and ice up to Wednesday night.
The pronounced low pressure core over the ocean southeast of the Eastern Cape Province in SA, was leaning back over the subcontinent leading to jet streams in excess of 200kts at the 30,000 feet level.
This Friday yet another trough is approaching the Cape. This is diverted southeastward, but on its northern side, a cooler westerly flow is felt by Sunday, but quickly displaced as the system departs and dissipates by nightfall. The respite is brief. A new intense trough develops over the mid-Atlantic made obvious by Monday’s increasing winds. Moving inland it brings another cold southerly inflow (extending far to the north) until Thursday. Forecasts are not unanimous but it seems Wednesday and Thursday next week will be bitterly cold. There is the possibility of rain in the Lüderitz, Oranjemund, Rosh Pinah triangle with a good chance that this may turn into snow.