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Typhoons: Asia's megastorms

Typhoons unleash massive amounts of energy

HONG KONG, China (CNN) -- Tropical cyclones are heat engines formed over the vast expanses of water that form the world's oceans.

The energy released by a single cyclone system could, if there were a means to harness, provide enough power to supply a city for several years.

Around the world tropical cyclones are given different names.

Over the north Atlantic and eastern Pacific, they are called hurricanes; over the western Pacific, they are called typhoons; and over the Indian Ocean, they are called cyclones.

But they are all essentially the same phenomenon.

They form initially as thunderstorms over warm and humid tropical oceans with an unstable atmosphere.

A rotating mechanism induced by the rotation of the Earth -- the same effect that makes water spiral down a plughole -- causes the clouds to spin and become a tropical cyclone.

Weak vertical wind shear in the atmosphere is required for the clouds to develop high up into an area known as the tropopause, some 10 kilometers aloft.

Good ventilation by an upper-level anticyclone then keeps the whole storm engine running.

In Asia, tropical cyclones are classified into four groups according to maximum sustained winds near the center:

Tropical depressions (less than 62 kilometers per hour);

Tropical storms (63-87 kilometers per hour);

Severe tropical storms (88-117 kilometers per hour);

Typhoons (more than 117 kilometers per hour).

Different weather agencies have slightly different classifications.

For example, the U.S. Joint Typhoon Warning Center does not have severe tropical storms, but 'super typhoons' with maximum sustained winds more 240 kilometers per hour.

Tropical cyclones across the northern hemisphere generally move in a west-northwestward direction.

The movement will be affected by other weather systems such as the sub-tropical ridge, westerly trough, or interaction with another tropical cyclone.

As these systems are constantly changing in location and intensity, the movement of tropical cyclones can vary greatly and it is always a challenge for weather agencies to produce an accurate prediction.

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