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Caribbean weather - Weather systems in the tropics

by Mr. Horace Burton, Chief Meteorologist, Caribbean Institute for Meteorology and Hydrology (CIMH).

Hurricanes

The Structure of the Hurricane

A hurricane is a large revolving storm, varying in size from 300 to 1500 km in diameter and accompanied by violent destructive winds, heavy rains, and high waves and tides. The mature system is characterised by a circular mass of cloud which contains the "eye" and a number of bands which spiral into the centre of the system. Wind speeds increase inward from the periphery of the storm and reach their maximum values in the circular band which surrounds the eye.  The most intense convection, heaviest rainfall and most severe thunderstorms are also found in this region which is known as the eyewall which extends outward 30 to 50 km or more from the centre.

The eye of the hurricane is characterized by clear or lightly clouded skies, due to the subsiding air in this region, and calm winds. This calm is deceptive, bordered as it is by the hurricane-force winds and torrential rains. The diameter of the eye varies between 5 and 50 km with a typical value of about 20 km. The surface pressure attains its minimum value in the eye. For a typical hurricane this value can be about 950 mb, although pressures may fall below 900 mb in very intense hurricanes.

The spiral bands are also regions of active thunderstorms and strong winds which are associated with the air converging toward the centre of the storm. Sometimes these bands, which can extend to the outer regions of the hurricane, may cause just as much damage as the eyewall itself.

The hurricane circulation extends throughout the troposphere with the tropopause usually defining its upper limit.  The intensity of the circulation is strongest in the lower troposphere and being warm core it graduals weakens with height. The air which streams in toward the centre in the lower levels rises in the eyewall. At upper levels the rising air flows outward and at the outskirts of the storm there is sinking motion.

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