There are always clusters of clouds over the tropical oceans of the globe. When one of these occurs simultaneously and at the same location as certain other meteorological conditions, a tropical cyclone will form. The conditions for its formation and development include sea-surface temperature above 26°C, high relative humidity from the surface upwards to about six km, spiralling inflow of winds at low levels, and divergent (outflow) winds aloft. At the mature stage, the tropical cyclone can cover an area of some hundreds of kilometres in diameter. It will have a very low surface atmospheric pressure at its centre, where an eye will form. The eye will be a relatively cloud and rain-free area with light winds and with a diameter of a few dozens of kilometres. It will be surrounded by a wall of clouds producing the heaviest rainfall and the strongest winds. In the most severe cyclones, the surface wind speed exceeds 200 km/h. Change in intensity is usually a gradual process. For example, a tropical cyclone will weaken slowly when it moves over land or over cold water. However, there are relatively rare cases when pre-hurricane intensity suddenly develops into severe hurricanes within a period of hours. Every year about 80 tropical cyclones form over warm tropical oceans.

● TERMINOLOGY AND MOVEMENT

As tropical cyclones gain in intensity ? with surface wind speeds exceeding 118 km/h, and bringing with them turbulent clouds and torrential rainfall ? they are referred to as hurricanes in the western hemisphere, typhoons in the western North Pacific and cyclonic storms or tropical cyclones in other areas. They become real hazards for marine and aeronautical navigation. In simplified terms, it may be said that they move in the direction of the steering current, that is the wind flow in which they are embedded. With light or rapidly changing steering current, tropical cyclones will move erratically, with tracks showing sudden changes in direction or even loops.

● IMPACT

Many tropical cyclones move inland causing both loss of life and extensive destruction of property. Although the terms used to describe their intensity relate to surface wind speed, the death and devastation they bring are due not only to strong winds, but also to the floods and storm surges they cause as well as other associated phenomena, such as tornadoes and landslides. Their effects range in severity up to among the worst of all sudden-onset natural disasters, with a death toll of nearly 300,000 people in one extreme case in Bangladesh (1970). Records show that Hurricane Andrew is the tropical cyclone which caused the most damage, estimated at US$ 26.5 billion, when it struck Florida and Louisiana (USA) and the Bahamas in 1992. During the recent hurricane season in the Atlantic, Ivan, a Category 5 hurricane caused a death toll in Haiti approaching 3,000 and property damage in the region estimated at US$20 billion.

In these examples of tropical cyclones which caused disasters, it was helpful to be able to identify them by name or, at least, by identification codes. This is only one of the many ways in which identification is useful. To millions of people, a tropical cyclone is an unforgettable event whose name will long be remembered.

● WARNING SYSTEMS

A particularly important aspect of tropical cyclones, as distinct from some other natural hazards, is the worldwide availability of early warning systems as a basis for preparedness action and, hence, disaster mitigation. As a result of international cooperation and coordination, and with the aid of meteorology and modern technology, such as satellites, weather radars and computers, all tropical cyclones around the globe are now being monitored from their early stages of formation and throughout their life time by designated Regional Specialized Meteorological Centres (RSMCs) located in Honolulu, Miami, Nadi (Fiji), New Delhi, Tokyo, and La Réunion, as well as by other centres of national Meteorological and Hydrological Services. These centres also provide advisories on the behaviour of the tropical cyclones, their movement and changes in intensity and on associated phenomena ? principally storm surges and floods. They issue timely warnings to all those who are threatened. The activities are coordinated at the global and regional levels by the World Meteorological Organization through its World Weather Watch and Tropical Cyclone Programmes.

© LE MONDE