Typhoons: Asia's megastorms
By CNN's Clarence Fong
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 East Pacific, they are called hurricanes; over the West Pacific, they are called typhoons; and over the Indian, 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 4 groups according to maximum sustained winds near the center:
- Tropical depressions (less than 62 kmh)
- Tropical storms (63-87 kmh)
- Severe tropical storms (88-117 kmh)
- and typhoons (more than 117 kmh).
Different weather agencies have slightly different classifications.
For example, the US Joint Typhoon Warning Center does not have severe tropical storms, but 'super typhoons' with maximum sustained winds over 240 kmh.
Tropical cyclones over 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.