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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

Introduction

Tropical cyclones are low pressure systems which form over the tropical oceans, with the exception of the South Atlantic and the southeast Pacific east of about 140°W. In their most intense state these systems are called hurricanes in the Atlantic, typhoons in the western North Pacific and cyclones in the Bay of Bengal. In areas affected by the intense tropical cyclones, the resultant damages are often extensive, especially in developed coastal areas. The principal damaging forces are storm surge, floods caused by torrential rains and high winds.

Hurricanes begin as weak tropical systems and intensify through a number of characteristic stages to the intense systems when conditions are favourable. To differentiate between the various levels of intensity of these tropical cyclones, the following terminology is used.

• Tropical Disturbance: an area of organized convection which has maintained its identity for at least 24 hours and does not have a detectable circulation in the wind field at the surface.
•  Tropical Depression: a tropical cyclone in which the maximum surface wind 38 mph or 33 knots or less.
• Tropical Storm: a tropical cyclone in which the maximum surface wind is in the range 39-73 mph or 24-64 knots.
• Hurricane: a tropical cyclone with maximum surface winds greater than 74 mph or 64 knots.

In the Atlantic, including the Caribbean Sea and the Gulf of Mexico, the hurricane season run from June 1 to November 30. Although storms sometimes form in May, the hurricane season peaks around the middle of September and diminishes by the end of October. About 80% of the tropical storms and hurricanes occur during this 3-month period, with September being the most active. On average, the number of cyclones reaching tropical storm strength per year in this region is nine, of which six become hurricanes.

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