David Le Page The extremely cold weather that has hit the country this week may feel aberrant to most of us, but meteorologists and climate modellers are adamant that it’s nothing unusual.
A cold front, or large and unpleasantly cold mass of moist air, moved across the Western Cape on Wednesday afternoon. Preceding it, along with masses of disillusioned Vaalies, was unseasonably warm air, which makes the collision between the two more troublesome. Hence the large amounts of rain which accompanied the front’s progress across the Cape. But that collision with warm air is not the reason for the unusual coldness of the front, which has this week drawn cold air from polar latitudes, says Steve Metcalfe, a forecaster with the South African Weather Bureau.
The region from which any cold front draws air depends on the current position of the South Atlantic high pressure system. The pressure ridge of that system currently extends particularly far south. (It’s termed a ridge just because that’s the shape of the system.) The high pressure system rotates anti- clockwise, that direction being determined by Coriolis forces (the rotation of the Earth). That rotation pulls air northwards. A contributing factor, according to Dr Bruce Hewittson, a climatologist in the University of Cape Town’s department of environmental and geographical sciences, is that the low-pressure system behind the cold front has extended unusually far equator-wards.
The low is in this case closely followed by a high pressure system, which tends to create clear weather. The clear skies in turn lead to substantial and unusual heat loss from the land and atmosphere.