Spiralling force

C. RANGANATHAN

Cyclones, hurricanes and typhoons, differing only in name, transfer energies equal to several megaton nuclear bombs from the ocean to the atmosphere every year. Nearly 70 to 90 cyclonic systems develop all over the globe every year.

The "Coriolis force" or the "deflecting force" caused by the rotation of the earth about its own axis is maximum at poles and progressively decreases to zero at the equator. It is this force that compels the surface winds to spiral towards the "low pressure system". As "Coriolis force" is negligible in the equator-belt between Lat 50 N and Lat 50 S, cyclonic systems do not develop in this region. Tropical cyclones weaken into a depression on reaching Lat 400 N (beyond Tokyo Lat) in the northern hemisphere and Lat 300 S (beyond Mauritius Lat) in the southern hemisphere as the sea-surface temperature drops to 250 C.

Cyclonic systems do not develop or intensify all of a sudden. A day passes for the intensification of a system from one category to a higher category.

Radars and geo-stationary satellites have shown that cyclonic systems move zig-zag like a snake and take loop turns when "steering winds" aloft (30,000 and above) dictate such movements. In 1996, a cyclone heading towards Bangladesh took a loop-turn to head towards Andhra coast and became a severe cyclonic storm. Close to the Andhra coast, it took a second loop-turn to move towards Tamilnadu coast and finally dissipated close to Pondicherry. The same year another severe cyclone heading towards Saurashtra coast took a loop-turn and dissipated in the Arabian sea itself. It is at such times cyclone warnings go completely wrong.

Marginal cyclones do more good than harm in the form of heavy rains that are badly needed for the sustenance of life. But there is no way to either weaken a disastrous cyclone or change its course.

Doubts are often raised whether the intensity of a disastrous cyclone could be reduced by artificial means. The advances in science and technology notwithstanding taming of cyclone appears a bizarre possibility even in distant future.

In America attempts were made to weaken hurricanes by seeding the rain-bearing clouds in the eye-wall region. The idea was to melt the clouds in the eye-wall region as quickly as possible by precipitation so that the diameter of the eye (average 40 km in Indian seas) and the "warm core" associated with it could be enlarged resulting in reduction of surface wind speed. A 30 per cent success was achieved in reducing the wind speed but the estimate was not outside the natural variation of surface wind speed associated with a hurricane. In India, it would be a hazardous and a highly expensive experiment not commensurate with the gains to tame a cyclone .

Density of population, topography, bathymetry (depth and slope of sea-bottom along a cost) and the intensity as well as frequency of cyclones that could strike a coast decide the vulnerability of the place.

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