About Tropical Cyclones Note: The information on this page has been extracted from the "Surviving Cyclones" Brochure, a joint publication of the Bureau of Meteorology and Emergency Management Australia . A full copy of this brochure may be obtained from the Regional office of the Bureau of Meteorology in Perth, Darwin or Brisbane. What is a Tropical Cyclone? Tropical Cyclones are low pressure systems that form over warm tropical waters and have gale force winds (sustained winds of 63 km/h or greater and gusts in excess of 90 km/h) near the 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 is an area characterised by light winds and often by clear skies. Eye diameters are typically 40 km but can range from under 10 km to over 100 km. 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