David le Page
The “wind event” which hit the Cape Flats this week probably wasn’t a tornado – but tornadoes may be hitting South Africa more frequently. Certain areas of South Africa, particularly the eastern escarpment, are already more prone to tornadoes. Tornadoes are a spin-off from the thunderstorms enjoyed by the Free State and Gauteng.
Whatever the direction of the winds in the Cape Town storm – straight or vortexed – unusually warm sea temperatures probably contributed to their intensity. The cause of that warming cannot be established, though there is some speculation that it is related to La Nia – cooling of equatorial waters and warming of the southern Indian and Pacific oceans.
The Cape storm killed five, injured 180 and left 5 000 homeless. The areas hardest hit were Surrey Estate, Manenberg and Guguletu, where comparatively poor construction probably contributed to the destruction of at least 40 flats, mainly top-storey units, and numerous houses.
In the United States, Texas and Oklahoma are most frequently hit, though the greatest number of deaths is suffered by the more densely populated Massachusetts. Other affected regions are Canada, Australia, Bangladesh, Italy, Spain, Japan and New Zealand.
What do you need for a tornado-producing storm? Low, warm, moist air at low altitudes; cold, dry air at higher altitudes and high surface temperatures.
South Africa has something of the right combination in the Indian Ocean’s warm Agulhas current, the Drakensberg and the highveld’s heat and relative summer dryness.
Lightning and hail frequently accompany tornadoes – 20cm giant hailstones were reported in Welkom in 1993.
Experts believe it less likely that the Cape Flats storm bred a tornado. The storm hit at 6am, hardly a time when surface conditions are high. Unfortunately, the only weather radar that might have picked up more information, situated on Constantiaberg, is currently not operational. In the US, doppler radar is used to measure the speed of winds within storms, allowing prediction times of between 20 and 30 minutes. Speeds up to 450kph have been reliably measured using such equipment.
Misunderstandings about tornadoes are not unusual in South Africa. Frequently, a quite genuine tornado will strike in a small area. Those who suffer incidental damage from the same storm, but not its tornado component, then think they too have been hit by a twister.
Charts of tornado events tend to be concentrated on Gauteng and central KwaZulu-Natal, at least partly because there are more people in those areas to notice and report them.
Last December, then president Nelson Mandela had to lie on the floor of an Umtata store, while his bodyguards covered him with their bodies as a tornado hit surrounding buildings. Since twisters have been known to carry delivery vans several kilometres (Swaziland, 1976), they were either rather overweight or rather fortunate that this ploy worked. Cellars are more reliable.
The local history of tornadoes is sparse but colourful; they tend to occur a fifth as frequently as in the US Midwest. About 170 have been reported since 1905.
Livestock and building materials tend to be favourite themes: cows tossed into Vaal Dam (1954); corrugated iron wrapped around trees (Welkom, 1990); chickens plucked (Pretoria and Bronkhorstspruit, 1949); brick truck overturned (Welkom, 1982).
Greater public awareness, especially following the film Twister, has almost certainly increased the reporting of such events. Despite this, the Umtata tornado, and the Harrismith tornado the previous month, seemed to mark an increase in tornado frequency.
Though this is a simple coincidence of tornadoes hitting populated areas, weather experts speculate that there may be a real increase in their incidence.