03 February 2011

The danger spot of a cyclone

The right top quarter of a tropical cyclone is the most dangerous section of the storm. The wind is at its top speed there and it contains the most debris.


The world is holding its breath as one of the most powerful cyclones on record has started to pound the northeast coast of Australia. What is the most dangerous spot of a cyclone?

The right top quarter of a tropical cyclone is the most dangerous section of the storm. The wind is at top speed and it contains a build-up of debris that has been collected throughout the remaining sections of the hurricane.

Why is this the most dangerous?
The reason for this increase in speed is because as the winds approach this section of the cyclone, their force adds to the steering wind speed. The steering wind speed is the speed at which the cyclone is travelling in a single direction.

If the cyclone is travelling north at 60 km/h, and the internal wind speed is 100 kilometres an hour, the wind speed in the top right quarter of the cyclone will total 160 km/h.

What are tropical cyclones?
Hurricanes and typhoons are different names for tropical cyclones (as they are known in South Africa), featuring winds that can blow at a speed of more than 200 km/h. It can take hours, or even days for a thunderstorm to develop into a tropical cyclone.

A tropical cyclone begins as a tropical storm over the humid Atlantic and Pacific oceans near the equator. As moisture evaporates, it rises and forms enormous amounts of heated air that are forced into a twist by combining with cold atmospheric air.

Hurricanes that affect the US, tend to form near Africa, and drift with the Trade Winds west, veering north toward North America as they come into contact with prevailing easterly winds.

In the northern hemisphere, tropical cyclones rotate counterclockwise (from west to east); in the southern hemisphere, they rotate clockwise (from east to west).

A typhoon is a tropical cyclone located in the western north Pacific basin.

Hurricanes/cyclones and their names
So do they always have women's names? The interesting thing is that until the 1940s, hurricanes only had men's names. Then in the 1950s they were only given women's names. In the late 1970s weather services began alternating between men's and women's names. The first hurricane of the season has a name starting with the letter 'A' , the second 'B' and so on.

But whether they have sweet-sounding names or not, these weather phenomena have been known to flatten towns and forests, and the danger they pose should never be underestimated.

Tropical cyclones in South Africa can occur between November and April - mostly on the northeastern regions of the country. Only a few move in over land and the last time a tropical cyclone caused enormous damage and upheaval was "Domoina" in 1984, and more recently "Eline" in February 2000.

Taming cyclones
Has there ever been an attempt to reduce the strength of tropical cyclones?

A few decades ago, the US government conducted experiments to weaken hurricanes by dropping silver iodide, a substance that serves as effective ice nuclei, into the rain bands of hurricanes.

The reasoning was as follows: hurricanes are fed by the upsurge of warm air in the eye of the storm. The idea was to “freeze” the air around the hurricane with the silver iodide, thereby helping a rain band to grow at the expense of the eye wall. With a weakened convergence to the eye wall (the eyewall is a doughnut-like ring of thurnderstorms that surround the calm eye), the strong inner core winds would also weaken.

This method did not prove successful. The aim today is to gain better understanding of hurricanes in order to improve forecasts.

- Health24, updated February 2011

Read about the link between hurricanes and global warming


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