tropical storms

tropical storms develop when the upper 50 metres (165 ft) or more of tropical ocean warms to over 26.5°C (80 °F) in regions that are over 500 kilometres (300 mls) from the equator, where Coriolis forces are strong. They are intense low-pressure cyclonic systems that do not contain fronts between warm and cold air masses. Once their wind speeds reach 17 metres per second, or m/s, (32 knots/59 kph/37 mph) they are termed tropical storms and are given names. Since the energy fuelling these damaging storms is the heat that is rapidly being transferred from the ocean to the atmosphere by convection, while they stay over the ocean they continue to grow in intensity. Once wind speed reaches 33 m/s (64 knots/118 kph/74 mph) they are designated as hurricanes (or typhoons in the North Pacific west of the dateline, or tropical cyclones in the Indian Ocean). The speed with which they can intensify is alarming. In 1988 winds in hurricane ‘Gilbert’ rose from 203 kph (127 mph) to 294 kph (184 mph) in 24 hours as the pressure in the eye dropped from 960 to 888 mb. The highest winds recorded in a hurricane were 304 kph (190 mph) associated with Typhoon ‘Tip’ in 1979, but since such winds often destroy the recording instruments, wind speed records are seldom trustworthy.

The destructive force of tropical storms is not necessarily the result of the winds, as rainfall can cause serious flooding. In 1980 cyclone ‘Hyacinth’ deposited 566 centimetres (223 in.) of rain in ten days on the island of Réunion. Storm surges (see tides) generated by hurricanes cause immense death and destruction. The highest storm surge on record of 13 metres (42 ft) occurred in Australia in 1899, but it is along low-lying coastlines like that of Bangladesh that storm surges have drowned hundreds of thousands of people in a single event.

http://tropical.atmos.colostate.edu/

M. V. Angel