More About Cyclones

South Africa's cyclone season spans November to April, peaking in January and February. However, few cyclones get close enough to South Africa to be destructive. Generally, only cyclones that develop north of 25 degrees south put South Africa at risk. When they do make landfall, they typically affect the provinces of KwaZulu Natal, Mpumalanga and Limpopo. Because Kruger National Park straddles both Mpumalanga and Limpopo, and is closer to the coast than much of these provinces, it tends to bear the brunt of the cyclone's anger. Recent examples of destructive cyclones are Domoina (January 1984), Imboa (February 1984) and Eline (February 2000).

What are Cyclones?

Cyclones are born out of intensely low-pressure cells usually at latitudes of between 5 to 20 degrees north or south and have steep pressure gradients. They have an 'eye' at their center consisting of gently subsiding air that results in clear, calm and warm conditions. Their outer edges consist of an enormous wall of typically cumulonimbus clouds where most of the ferocious activity occurs. These cloud walls can reach from sea level to an altitude of about 15 km. Extending outwards from the top of these cloud walls for several kilometers are thick cirrus and alto-stratus clouds.

Near the ground, cyclones produce strong cyclonic winds, that in the southern hemisphere rotate clockwise, and that can reach speeds well in excess of 100 km per hour.

Cyclones vary in diameter from 200 to 2000 km.

Moving at speeds of around 200 km per day and always in a westerly direction, cyclones track very erratic paths that are notoriously difficult to predict.

In the Pacific Ocean, cyclones are called Typhoons, whereas in the Atlantic they are called Hurricanes.

How do Cyclones Form?

Cyclones typically form at latitudes of between 5 to 20 degrees north or south over warm oceans when the wind pattern near the surface spirals air inwards. They gather moisture, heat and energy from the ocean until thunderstorms form that further heat the air. The warm air together with the spiraling winds drive rising corkscrews of air with a low pressure center.

Cyclones usually survive for about a week depending upon atmospheric and oceanic conditions, but disintegrate when encountering cold water or land, and especially rough terrain.

Why are Cyclones so Destructive?

Cyclones reek havoc in three ways.

• Torrential rain - Cyclones can produce as much as 3000 mm of rain, which can result in destructive flooding.
• Strong winds - Cyclones produce wind speeds ranging from zero in the eye to well over 100 km per hour at its circumference, with the strongest winds being experienced at its leading edge.
• Storm surge - Cyclones generate a rapid rise in sea level and wind-driven waves that can reach heights exceeding 6 m. Storm surge is the biggest cause of damage and death resulting from cyclones.

Images courtesy of the Australian Government Bureau of Meteorology and futura-sciences.com