TROPICAL CYCLONES

Nature's Gigantic Heat Engine

Tropical Cyclones

Tropical cyclones are violent whirling storms, several hundred of kilometers in diameter, that develop over tropical waters. Viewed from satellites high above the earth, a tropical cyclone appears as a powerful, tightly coiled system with spiral cloud bands emanating outwards. In a mature tropical cyclone, there exists a central region of relatively calm air and little cloudiness known as the eye.

Tropical cyclones are classified as follows:

Tropical Depression - An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 63 kmph or less.

Tropical Storm - An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 64 - 117 kmph.

Typhoon - An intense tropical weather system with a well defined circulation and maximum sustained winds of 118 kmph or higher.

How are tropical cyclones formed?

Tropical cyclones are formed over oceanic regions where the sea surface temperature is 26o C or higher. In the Asian region, tropical cyclones, usually referred to as typhoons, are common during the Northern Hemisphere summer when temperatures over the Pacific Ocean are warm and favourable for convective activity. In the warm tropical ocean, low pressure systems are formed, move westwards and develop from tropical depressions into storms and finally evolve into mature typhoons.

The development from tropical depressions to typhoons depends on three conditions:

Warm waters

Moisture

Wind pattern near the ocean surface that spirals air inwards (cyclonic flows).

Developing cyclones gather heat and energy through contact with warm ocean waters. Heat and moisture by evaporation from the sea surface powers them like giant heat engines. Bands of thunderstorms form allowing the air to warm further and rise higher into the atmosphere.

The eye, whose diameter ranges from 30 to 100 kilometers, coincides with the low pressure centre of the storm. The most violent activity takes place in the area immediately around the eye, called the eye wall. Here, air spirals upwards and outwards at increasing speeds. Some of the air moves inwards and sinks into the eye, creating a cloud-free area.

When are typhoons most frequent?

Typhoons begin to appear in the Western North Pacific from early May, and in increasing numbers until attaining a peak frequency in September. The majority of typhoons developed in the area bounded by latitudes 5oN and 20oN and longitude 130oE and 170oE. The normal passage of typhoons is westwards across the Philippines, recurring northeastwards as they approach the Asiatic land mass.

Name of Tropical Cyclones Name in the Western Pacific and South China Sea (with effect from 1 January 2000)

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