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Tornadoes: Nature’s hardest weather event to predict

But statistically the July tornadoes of the last week are very unusual indeed.

Global warming and a rise in tornadoes

You may have heard that thunderstorms, lightning, sleet, hail, damaging winds and hurricanes will occur more frequently if the Earth’s climate continues to warm.

But tornadoes? Not so fast

Tornadoes are really beyond the edge of our understanding of things,” says Tony Del Genio, a climatologist at NASA’s Goddard Institute for Space Studies in New York City. “This science is in its infancy.”

Del Genio says tornadoes are nature’s hardest weather event to predict. They are essentially flukes of nature. Unlike hurricanes, they form spontaneously, are short-lived, and traverse a much smaller land mass by comparison.

Many atmospheric conditions need to converge at the right time for tornadoes to form. They need hot, humid air near the ground with a cool air mass above them. They also need strong wind velocity at higher altitudes, known as wind shear, to get them spinning.

So what is the difference between tornadoes and hurricanes?

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A tornado is a violently spiraling funnel cloud that extends from the bottom of a thunderstorm to the ground. It is important not to confuse a tornado with a hurricane or tropical cyclone because tornadoes and hurricanes are very different phenomena. The only similarity between them is that they both contain strong rotating winds that can cause damage.

Size

The largest tornado every observed was 4 km wide, but most tornadoes are about 0.8 km wide. The parent storm clouds that produce tornadoes are generally about 16 km wide. Hurricanes are much larger, ranging from about 160 km to 1600 km wide.

Life cycles

A tornado’s lifetime is short, ranging from a few seconds to a few hours. A hurricane’s life cycle can last from days to weeks.

Wind shear

Tornadoes, and the parent storm clouds that produce them, require strong vertical wind shear and strong horizontal temperature changes to form and survive. Hurricanes thrive in regions of weak vertical wind shear where the horizontal change in atmospheric temperature is small.

Location

Strong tornadoes usually occur over land, while hurricanes almost always form over the ocean.

Wind speeds

The strongest tornadoes can have wind speeds over 483 kph, but even the strongest hurricanes rarely produce wind speeds over 322 kph.

Tornadoes in South Africa

From an analysis of the occurrence of tornadoes in South Africa, it is clear that most of them have been observed in Gauteng, the Free State, KwaZulu-Natal (along a line from Pietermaritzburg to Ladysmith) and the northern region of the former Transkei.

Seasonal Distribution

TORNADO GRAPH (1)

Tornadoes in July are unusual

Most of the events occur in mid-summer from November to January, although a large number of tornadoes have occurred in spring and early summer (September and October) and in the late summer and autumn (February to May). It is also worth mentioning that most tornado events (for which the time of the day were available) occurred in the late afternoon or early evening, typically between 4pm and 7pm.

5 worst tornadoes in SA history prior to July 2016 twisters

Source: Newcastle Advertiser

#1. Mount Ayliff and Tabankulu, 18 January 1999

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The worst recorded tornado in SA history was a violent F4 that struck towns around Mount Ayliff and Tabankulu in the Eastern Cape on 18 January 1999. An estimated 95% of inhabitants of this area were left homeless, 25 people were killed and about 500 were injured. Vehicles were tossed through the air, with one travelling almost 500 meters.

#2. Roodepoort, 26 November 1948

When this tornado struck in Roodepoort on 26 November 1948, 700 homes were destoyed. The tornado in which only four people were killed, left a 64 km path of destruction, touching down 15 times.

When a tornado struck in Roodepoort on 26 November 1948, 700 homes were destroyed. The tornado, in which only four people were killed, left a 64 km path of destruction, touching down 15 times.

#3. Harrismith, 15 November 1998

The third worst tornado touched down in Harrismith on 15 November 1998 destroying 750 homes over a 9 km path.

#4. Utrecht and Glencoe, 3 November 1993

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On 3 November 1993 rural Utrecht and Glencoe in KwaZulu-Natal was hit by a twister in which 40 people lost their homes and seven people were killed.

#5. Welkom, 20 March 1990

A tornado caused structural damage over a 20 km wide path in Welkom and surrounds on 20 March 1990 amounting to more than R230 million. The multi-vortex tornado was part of a 240 km long storm front with a width of 1.7 km.

 
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