Was it a tornado that hit Boksburg? The answer is blowing in the wind

2019-01-18 17:13
(Supplied)

(Supplied)

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Gauteng residents witnessed a spectacular thunderstorm on Thursday that was characterised by strong winds, heavy rains and hail.

It was most certainly not the worst storm the region had seen to date, but its intensity led some to believe that they had witnessed a tornado.

In Boksburg, east of Johannesburg, reports of a tornado started to pour in after corrugated iron roofs were blown off a church and a block of flats. Carports were also ripped from the ground and blown into the street while trees were damaged.

Mental images of tornadoes often simulate those from disaster movies such as Into the Storm or Twister, but, in reality, tornadoes that destructive are mostly the stuff of fiction, especially on the Highveld.

Tornadoes more frequent than expected

According to Puseletso Mofokeng, senior forecaster at the South African Weather Service, it is unlikely that it was a tornado residents observed, but that doesn't mean that tornadoes never occur in South Africa. In fact, it happens more frequently than one might expect, especially in summer rainfall areas, but at a much lesser scale than Hollywood would like you to believe.

"There was a bit of a hook [on Thursday], a very gentle hook, which is a signal that appears before a tornado occurs. What happened in Boksburg was that there was a line of storm approaching from outside Gauteng into the southern areas. This storm had what we call 'gust front'. Simply explained, a gust front is air that moves out of the storm ahead of it.

"So, the damage caused in the Boksburg area was because of gust front feeding the storm following closely behind it with air."

Mofokeng said he ruled out the possibility of a tornado because the storm was moving too fast.

"If it were a tornado, it would have been a very short one," Mofokeng said.

"Straight winds can cause a lot of damage, it doesn't have to rotate [like a tornado]."

Mofokeng said he could base much of his assessment on pictures of the aftermath, and that a tornado would typically not leave behind pieces of straight corrugated iron in the same spot where winds had ripped them off.

"They would be scattered."

Tornadoes are, however, not a scarce phenomenon in South Africa, Mofokeng said.

"Tornadoes occur a lot in Gauteng, Free State and in KwaZulu-Natal, especially in November, December and January."

SA has highest rate in Africa

November and December are typically when tornadoes tend to peak in South Africa.

"Having said that, anything can happen, so we often see tornadoes in January as well.

"When I say 'a lot', I don't mean as many as in the US, for example. But compared to the rest of Africa, South Africa has the highest probability of tornadoes occurring."

Tornadoes do not even occur every year.

"It depends on the temperature of the Indian Ocean. As it stands, we've had around five tornadoes this summer alone, the worst being on November 18 last year in Bethlehem, and a second on December 18 in Pietermaritzburg.

"On January 5 this year, we saw three tornadoes – one around Lydenburg, one on the border of Swaziland, and another small one that didn't even reach the ground."

These frequent occurrences can be attributed to the higher temperatures observed in the Mozambique Channel and Agulhas Current, Mofokeng said.

"But most tornadoes that occur in South Africa are quite small and do not cause major damage."

1999 tornado left 5 dead, 5 000 homeless

Tornadoes are categorised according to the Enhanced Fujita (FJ) scale, which measures the speed of winds categorised as tornadoes. FJ0 is the lowest, at approximately 100km/h to 140km/h, causing low damage, while FJ5 is the highest, at more than approximately 300km/h, causing severe damage.

"Those kind of tornadoes that we get here (FJ0) don't cause any damage at all. Usually people don't even see them. These tornadoes are in the majority [in South Africa].

"The strongest tornado recorded in South Africa happened in Cape Town in August 1999 and was rated as FJ4 (approximately 267km/h to 321km/h)."

That tornado left five dead, 177 injured and 5 000 homeless.

"Usually, our tornadoes are not as strong as that particular one," Mofokeng emphasised.

Mofokeng says South Africans don't need to panic when they hear the word 'tornado'.

"It is not a curse. People need to take precautions only when we issue a warning."

So, was it a tornado that hit Boksburg? It's hard to tell for sure, but all things considered, probably not.

Read more on:    johannesburg  |  weather
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