Why storms are worsening

2009-01-07 00:00

THE SA Weather Services has warned that severe storms — which have killed scores and left thousands homeless this summer — are far from over.

Weather forecaster Ricus Lombard said it is uncommon to have the number of storms we have been experiencing over the past few months.

Lombard, who is based at Durban’s Weather Bureau, said not only are these storms more frequent, but they have grown in severity.

On average, KZN typically has three to four severe thunderstorms a month over summer, but Lombard said we have had at least four in a week lately. He believes this is as a result of changes in the global weather system. “We were actually talking about this at the office the other day.

Durban weather has changed in the last four months. It is not usual to have 24 degrees as the average temperature in December. We don’t usually have the amount of cloud we have been having and the overcast weather.”

Previously, the main weather system for the eastern coast was the Indian Ocean High (IOH) pressure system.

He said we used to get north-easterlies, which usually meant clear weather, but now with the IOH situated further south than normal, the winds are more easterly and onshore.

According to Lombard, this has resulted in more moisture, producing more cloud. He said the higher availability of moisture intensifies thunderstorms by giving them more surface moisture to feed on.

Lombard said storms are difficult to forecast because they are very small. However, the damage they cause can be severe.

While many storm victims have described the recent storm as like a tornado, Lombard explained that it is more likely that these were microbursts. Although the two are different, their effects can be similar.

He said in a storm, gusts of strong wind — microbursts — hit in different directions, in the form of an intense down-draft.

The strongest gusts are likely to occur in front of the approaching storm because the downdraft gains momentum and strength since it is travelling in the same direction as the approaching storm.

“Let’s say the storm is coming from the west and the wind is blowing in a west to east direction.

If the storm is travelling at 60 km/h and the gust is 90 km/h, the downdraft travelling in the same direction as the storm will then be going at 120 km/h … It is not uncommon for down drafts to damage one tree and not touch the one next to it.”

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