Cyclone

Australia's cyclone season is usually from December to April and affects most of the Queensland coast. The greatest threat lies north of the Tropic of Capricorn.

Tropical Cyclones are low pressure systems. They form over warm tropical waters and have gale force winds near their centre. Technically they are defined as a non-frontal low pressure system of synoptic scale developing over warm waters having organised convection and a maximum mean wind speed of 34 knots or greater extending more than half-way around near the centre and persisting for at least six hours.

The gale force winds can extend hundreds of kilometres from the cyclone centre. If the sustained winds around the centre reach 118 km/h (gusts in excess 165 km/h). 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 has light winds and often clear skies. The eye is surrounded by a dense ring of cloud about 16 km high known as the eye wall 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 days and may follow quite erratic paths. They usually dissipate over land or colder oceans.

Australian Council of State Emergency Services 2013