Tropical cyclones, known as hurricanes in the Atlantic and eastern and central Pacific, typhoons in the western North Pacific, and simply tropical cyclones elsewhere, are some of the most potentially destructive storms of our atmosphere. They are the strongest of sequence of rotating tropical stroms of progressively increasing wind strength. A tropical depression is the weakest, and defined as a low pressure system with a closed surface wind circulation with speeds less than 27 konts (50kph). With mean wwind speeds above 27 konts (50kph) the system becomes known as a tropical cyclone. A tropical storm is a cyclone with mean wind speed between 34 konts (63kph) and 63 knots (117kph)whilst the hurricane or typhoon is a tropical cyclone with speeds of 64 konts and above (118kph).
In recent years hurricanes such as Gloria (1985), Hugo (1989) and Andrew (1992) have caused tremendous damage along the eastern seaboard of the United States but with relatively little loss of life. However, sstorms of similar intensivity such as david (1979) and Joan (1988), caused major loss of life in the Dominican respublic and Nicaragua. This differential impact on developed and developing countries in a clear reflection of variations in vulnerability.
Although storms aare most obviously associated with high wind speed and even tornadoes the main cause of damage and loss of life is actually due to the heavy rainfall, flooding and, in coastal areas, the tidal surgues which often accompany the storms. The storm surge is the rapid rise of the sea surface which occurs as a storm approaches the coastline. It is caused by the drop in atmospheric pressure- a 100 mb fall in surface pressure results on sea-surface rise of about one metre- the piling up of water along the coast generated by strong anshore winds, and the decreasing water depth encountered by storm as it nears the coast. Surges can be considerable.
Major cyclones are not new. Theyare a characteristic ffeature of certain areas of the tropics where suitable conditions of high seasurface temperature, high atmospheric humidity and atmospheric wind structure allow their development (Fig. 1). Most cyclones form during the period late sping through to late autumn for each hemisphere. Not all formation areas are equally productive in spawning storms. The western North Pacific produces the greatest number of cyclones,with about 30 per cent of the global total. The Indian ocean is also important, with the Atlantic and East PPacific storms being slighty less frequent. Interestingly, no storms develop in the tropical South Atlantic nor the eastern South Pacific where surface water temperatures are usually too cool. Unfortunately we still do not have a clear idea of how cyclone frequencies vary for each are, as they mostly accur over tropical oceans. Historically, surface observations were dependment upon island locations or shipping routes. In the area of Atlantic Ocean an Caribbeanafected by hurricanes there are many possible observing sites. However over the East Pacific region especially, the number of sites is small. Many storms could develop, mature and decay without being observed by a surface station. It is only with the development of geostationary satellites that it has become possible to produce a more accurate climatology of the frequency of tropical storms in each of these areas. A consequence of the improved observations is that the number of eastern Pacific storms is greater than had previously been believed but within the Atlantic region the number of storms has not changed.
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