In the tropics, there is a broad zone of low pressure which stretches either side of the equator. Within this area of low pressure, the air is heated over the warm tropical ocean. This air rises, causing thundery showers to form. These showers usually come and go, but from time to time, they group together into large clusters of thunderstorms. This creates a flow of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure at the surface.
There are various trigger mechanisms required to transform these cloud clusters into a tropical cyclone. These trigger mechanisms depend on several conditions being in place at the same time. The most influential factors are:
a source of warm, moist air derived from tropical oceans with sea surface temperatures normally in excess of 26 °C
winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form
winds which do not vary greatly with height - known as low wind shear. This allows the storm clouds to rise vertically to high levels
sufficient distance from the equator to provide spin or twist
Tropical storms can become very large, up to 800 km across, and this is large enough for coriolis to become a factor. This force, caused by the rotation of the Earth, helps the spin of the column of rising air.
Diagram showing how cyclones develop
Although developed in the USA, the Saffir-Simpson Hurricane Wind Scale is used to rank tropical cyclone wind strength in many parts of the world.
If you would like [further information on Tropical storms] (http://www.metoffice.gov.uk/weather/tropicalcyclone) you can find it on the Met Office site.
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