Wind speed reached 57?km/h in Sunday’s storm

2013-01-10 00:00

THE highest recorded wind speed at the height of Sunday’s storm was 57 km/h, said Michael Savage, a senior professor at the University of KwaZulu-Natal.

Savage has given a breakdown of the storm, which lasted from about 4.30 pm to 4.54 pm. He said the air temperature high in the shade was 32,4°C, resulting in a thunderstorm developing in the late afternoon.

“The wind gust at two metres, the highest recorded wind speed for the day, was 57 km/h at 4.37 pm at the agrometeorology main tower of the University of KwaZulu-Natal,” Savage told The Witness.

“This corresponds to a Beaufort [scale] wind number of seven [high wind, moderate gale, near gale]. In high winds, it is the wind gust that does the damage, even though the gust may be for a very short period.”

The wind speed measurements for the thunderstorm are shown by the solid line curve in the graph.

The wind speed high was 47 km/h from almost north. At heights of five metres (dashed line of the chart) and ten metres (dashed-dotted curve) wind speed increases.

So in open areas, a 10-metre-high tree could experience winds of in excess of 80 km/h as shown.

This would explain the extensive damage caused to trees in open areas, with damage to power lines in some areas. Savage said knowing the wind speed for the tenth storey of a building is almost an impossible calculation.

“Not only does wind speed change with height above ground in a complex way, but it varies a lot from place to place and is reduced significantly in urban areas due to protection provided by walls, buildings, etc. In more open areas, more damage may occur,” he said.

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