Structure of a cyclone
Radar image of cyclone
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 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.
Most of the northern coastline of Australia is covered by the Bureau's weather radar network. For real time images and radar information, see: www.bom.gov.au/weather/radar/
Cyclone danger and impacts
Tropical Cyclones are dangerous because they produce destructive winds, heavy rainfall with flooding and damaging storm surges that can cause inundation of low-lying coastal areas.
Cyclones have wind gusts in excess of 90 km/h around their centres and, in the most severe cyclones, gusts can exceed 280 km/h. These very destructive winds can cause extensive property damage and turn airborne debris into potentially lethal missiles. It is important to remember that, during the passage of the cyclone centre or eye, there will be a temporary lull in the wind, but that this will soon be replaced by destructive winds from another direction.
Heavy rainfall associated with the passage of a tropical cyclone can produce extensive flooding. This can cause further damage and death by drowning. The heavy rain can persist as the cyclone moves inland and decays, hence flooding due to a decayed cyclone can occur a long way from the tropical coast as the remains of a cyclone move into central and southern parts of the continent.
The destructive winds accompanying tropical cyclones also produce phenomenal seas, which are dangerous both for vessels out at sea and those moored in harbours. These seas can also cause serious erosion of foreshores.
Storm surge and tides
Potentially, the most destructive phenomenon associated with tropical cyclones that make landfall is the storm surge. Storm surge is a raised dome of water about 60 to 80 km across and typically about 2 to 5 m higher than the normal tide level. If the surge occurs at the same time as a high tide then the area inundated can be quite extensive, particularly along low-lying coastlines.
Tropical cyclone severity categories
The severity of a tropical cyclone is described in terms of categories ranging from 1 to 5 related to the zone of maximum winds. An estimate of cyclone severity is included in all tropical advices. Remember that the Warning Service is not designed to give an exact statement of conditions at individual locations but will give a general idea of the expected worst conditions. Using this severity scale, communities will be able to assess the degree of cyclone threat and take appropriate action. Damage will vary depending upon factors such as:
- How far you are from the zone of maximum winds;
- How exposed the location is;
- Building standards;
- Vegetation type; and
- Resultant flooding.
The category does not refer to the amount of flooding or storm tides. If a storm tide is expected it will be mentioned separately in the cyclone warning.
|Category||Strongest gust (km/h)||Typical effects|
|1 Tropical Cyclone||Less than 125 km/h
|Minimal house damage. Damage to some crops, trees and caravans.Boats may drag moorings.|
|2 Tropical Cyclone||125 - 164 km/h
|Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small boats may break moorings.|
|3 Severe Tropical Cyclone||165 - 224 km/h
Very destructive winds
|Some roof and structural damage. Some caravans destroyed. Power failure likely.|
|4 Severe Tropical Cyclone||225 - 279 km/h
Very destructive winds
|Significant roofing and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.|
|5 Severe Tropical Cyclone||More than 280 km/h
Extremely destructive winds
|Extremely dangerous with widespread destruction.|
Research has shown that cyclones in the Australian region 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 even loops.
Cyclone tracking maps
Bureau tropical cyclone warning services include cyclone forecast track maps. These maps provide a track of the cyclone showing recent movement, and forecast movement.
Many people like to plot cyclone tracks throughout cyclone events.
Information and background maps: Plotting a cyclone track