Meteorological Information What are they When and where Life cycle Structure The Cyclone Threat Northern Territory Cyclone Tracy Warning System Cyclone Tracy Oral Histories Front Page	Life cycle of a tropical cyclone The complete life cycle of a tropical cyclone usually spans about 9 days but may be only 2 or 3 days or more than 20 days. Formation The formation of a tropical cyclone is dependent upon a number of favourable environmental conditions which are frequently present in the Inter Tropical Convergence Zone. These include a warm ocean surface (above 260C) and several physical parameters contributing to a deeply humid and unstable atmosphere. The formation process begins in an area of low pressure coinciding with vigorous convective cloud in the tropics between about 50 and 220 latitude. Usually the cloud cluster drifts slowly towards the west as the convection increases and winds begin spiralling in towards the system centre. Prematurity As the tropical low becomes further organised and the surface winds reach gale force it is then declared a tropical cyclone according to international convention. Satellite and radar observations of the system show the distinctive spiral banding pattern. Full Maturity If the ocean and atmosphere environment continues to be favourable the cyclone may continue to intensify as it moves poleward. The cloud system becomes more circular in shape and develops a distinct eye. This is the severe cyclone stage where the cyclone is at its most dangerous. Approximately half of the cyclones that form progress to full maturity.   Decay Tropical cyclones normally decay when they move into a less favourable environment, either over land or the cooler waters in higher latitudes. The rate of decay varies with the circumstances. A tropical cyclone moving into mid-latitude westerlies may be quickly sheared apart by strong upper winds, or it may react with a frontal system and persists for several more days. Similarly, a cyclone moving over land normally dissipates rapidly due to loss of its energy source, namely the warm ocean surface. However in northern Australia cyclones moving inland are frequently observed to persist as rain depressions for a number of days bringing widespread flood rains, and may even redevelop if they move over the ocean once more.
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