14 August 2014
The weather over our part of the sub-continent remains firmly in the grip of low pressure control. The week started with a firm cold intrusion from the south, reaching as far north as Otjiwarongo with relatively low night temperatures on Sunday and Monday. But like most of this winter’s cold spells, low pressure control in the upper air quickly dispelled the cold, leading to pleasant, warm to hot afternoons.
As the week progressed, the low pressure trough increased in intensity, causing windy, dusty conditions over the interior. It would be a weather tautology to describe August as windy as that is what it is suppose to be but it is very revealing to find out why, and how the constituent weather elements help create these conditions.
Imagine a line running from Ruacana through Windhoek to Koes. This is called the convergence line and it loosely defines the interface where the South Atlantic high pressure cell meets the reciprocal lower pressure airflow from the north-east. The dividing geological feature is the escarpment east of which the Namibian interior plateau lies at an average elevation of about 1400 metres above sea level. This feature has a remarkable impact on our local weather.
As the season morphs from winter to summer, the core pressures in the South Atlantic high recedes, but when that same cell has crossed the continent, finding itself somewhere south of Madagascar, it increases in strength. Therefore, local surface and upper air conditions are very much a function of this see-saw balance between the South Atlantic high and South-of-Madagascar high (South Indian high). When both start weakening compared to their mid-winter intensities, it creates low pressure conditions over the western half of southern Africa, opening the door for airflow from the north i.e. warmer air flowing from north to south. This flow opposes the South Atlantic high every time it approaches land, and it is above the escarpment that we get very windy conditions starting in the north-west (Kunene) migrating along the convergence line to the south-east (Kalahari).
Below the escarpment on the coastal plain, conditions are more conventional with strong southerly winds south of Swakopmund and fresh northerly winds along the northern half of the coast line.
But the South Altantic high remains a mighty animal and it is not so easily slain by the north south airflow, thus we get these intensely cold nights which quickly turns into warm days, as low pressure control again establishes itself aided in no small measure by solar radiation. This week was again a typical example of these elements in balance. It is what makes August the windy month.
A weak cold front ahead of the approaching high pressure cell has already made landfall during Thursday bringing fresh to strong winds to the coast from Oranjemund up to and beyond Lüderitz. However, this intrusion is strongly countered by the prominent north and north-westerly winds reigning over the northern half of Namibia.
A low pressure trough starts developing over the Kunene mouth during Friday, moving south along the convergence line bringing in cloudy conditions all the way from the Kaokoveld, south-eastward across the interior, right up to the Orange River.
Night temperatures will remain mild during the weekend while day temperature will go to the middle twenties over the interior, and above 30oC more or less north of Otavi. The low pressure trough moving in from Kunene can even lead to very light rainfall over the areas immediately west and north of Etosha.
The Kavango and Caprivi are subject to prominent airflow from the north-east, i.e. Zambia, and there is also a likelihood of light, brief rain.
Some outlooks anticipate colder weather towards the end of next week, but if conditions continue to develop as the past week, this will hardly be noticed on the surface.