Deaths included the crews of 100 vessels and an estimated 100 local aborigines. Cyclone Tracy - To date, TC Tracy has been Australia's most destructive in terms of damage to a community. It struck Darwin on Christmas Eve, 1974. If conditions are right, an ordinary tropical depression, or 'low' can develop into a tropical cyclone. In the southern hemisphere, in a 'low ', 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. The Central 'Eye' Cyclones vary greatly in character but the one feature they all have in common is a virtually calm centre with clear sky. Around this eye are bands of heavy cloud, associated with the moist air which spirals in towards the centre of the cyclone. These clouds, known as the 'eye wall', may be 15 km high and mark the ring of strongest winds and heaviest rainfall.