AIMS - Cyclone Survival - How a Cyclone is Formed

Tropical cyclones are the greatest storms on earth releasing in one day as much condensation heat energy as up to four hundred twenty megaton atomic bombs. Every cyclone is dangerous and must be regarded as a real threat until the danger has clearly passed.

The' "life cycle" of the average cyclone is about seven (7) days but can extend for up to three (3) weeks. They form in the atmosphere over warm ocean areas (with at least 26 degrees Celsius water temperature and in latitude about 5 degrees to 25 degrees either side of the equator).

The exact mechanism that "triggers" them is not fully understood. If conditions are right an ordinary tropical depression or "low" can develop into a tropical cyclone. In the southern hemisphere the winds spiral in a clockwise direction towards its centre where they rise and spill over in an outward flow at high altitude. Summer heat beating on the warm ocean evaporates water creating a deep layer of moist air. The uplift of this moist air in the centre of a low cools it causing the intense rain characteristic of tropical cyclones. Higher in the upper levels the rising air spirals outward removing air faster than it flows in resulting in a fall in barometric pressure. Cyclones vary greatly in character but the one feature they all have in common is a virtually calm centre. This centre or "eye" is generally about sixteen to thirty-two kilometres across. Around this eye are bands of heavy cloud associated with the moist air which spirals in towards the centre of the cyclone.

If a tropical cyclone reaches land the friction of the earth and the loss of sustaining heat energy from the ocean causes the cyclone to "fill" and drop most of its rain. As a cyclone moves forward at only approximately fifteen to twenty-five kilometres per hour there is usually sufficient warning time so that people can prepare for its onset.

The Australian cyclone season is from about November to April with an average of ten cyclones per year being tracked by the Bureau of Meteorology in Australia. Of these, six may be expected to cross the Australian coast each year. Damage caused by cyclones varies widely depending on its path. Most deaths from cyclones occur either from drowning, collapsed buildings or debris, which become lethal projectiles carried along by the wind. To date tropical cyclone Tracey has been Australia's most destructive in terms of damage to a community. It struck Darwin on Christmas Eve 1974. Rainfall totalled 195mm in eight and a half hours and winds exceeded 217 kilometres per hour (before measuring instruments failed) literally tearing the city apart.