Tropical cyclones are low-pressure systems. In the Southern Hemisphere, tropical cyclones have well-defined clockwise wind circulation with a region surrounding the centre of gale force winds (sustained winds of 63km/hr or greater with gusts in excess of 90km/hr). The gale-force winds can extend hundreds of kilometres from the cyclone centre. If the sustained winds around the centre reach 119km/hr (gusts in excess of 170km/hr), then the system is called a severe tropical cyclone. These are referred to as hurricanes or typhoons in other countries.
The circular eye or centre of a tropical cyclone is an area characterised by light winds and often by clear skies. Eye diameters are typically 40km but can range from under 10km to over 100km. They eye is surrounded by a dense ring of cloud, about 16km high, known as the eyewall, which marks the belt of strongest winds and heaviest rainfall.
Tropical cyclones derive their energy from the warm tropical oceans and do not form unless the sea-surface temperature is above 26.5°C, although, once formed, they can persist over lower sea-surface temperatures. Tropical cyclones can persist for many weeks and may follow quite erratic paths. They usually dissipate over land or colder oceans.
Most of the northern coastline of Australia is covered by the Bureau of Meteorology’s weather watch radar network.
Monitoring Cyclones
Research has shown that cyclones in Australian exhibit more erratic paths than cyclones in other parts of the world. A tropical cyclone can last for a few days or up to two or three weeks. Movement in any direction is possible, including sharp turns and loops.