What is a Cyclone

Cyclone seen from Space

A tropical cyclone is a low-pressure system which develops in the tropics and is sufficiently intense to produce sustained gale force winds of at least 63km/h. If the sustained wind reaches hurricane force of at least 118km/h the system is defined as a severe tropical cyclone. In other parts of the world they are called hurricanes or typhoons.

Tropical cyclones can cause significant phenomena which can adversely, and sometimes favourably impact on communities and the environment. The most common features are destructive winds and heavy rainfall that can lead to flooding. Storm surge, or coastal inundation by seawater, is a lesser known phenomenon but can be the most dangerous element of a cyclone. Though rare in Australia, tornadoes have been reported during cyclone events.

Severe wind

In tropical cyclones, wind gusts in excess of 90km/h can be expected around their centre, or eye, while in the most severe events, gusts can exceed 360km/h. Although the strongest winds are near the eye, damaging winds can extend hundreds of kilometres from the centre. The eye can have quite calm winds and cloud-free skies, but this lull is temporary and is followed by destructive winds from another direction. This is because, from above, the winds spiral around the eye in a clockwise direction (in the Southern Hemisphere). The effect of this on the ground is that winds on opposite sides of the eye blow in different directions.

Wind damage is mostly caused by the maximum gusts in the cyclone. For this reason, the well-known tropical cyclone severity categories used by the Bureau of Meteorology to communicate warnings are based on maximum gust strengths.

Heavy rainfall

Because tropical cyclones form over warm tropical oceans, they generally hold enormous amounts of moisture and can produce heavy rainfall over extensive areas. Rain can create severe impacts by causing floods and landslides and through the direct damage of materials by contact, such as being driven by wind into buildings. Direct damage is generally the result of wind damage to walls, windows or roofs, which allows water to penetrate buildings.

Rainfall can be associated with the tropical cyclone when it impacts on the coast or further inland as it weakens to become a tropical depression. Heavy rain from tropical cyclones or tropical depressions can often reach Australia's more southerly latitudes where the rainfall is a major source of water for the country's inland river and ground water systems. Flooding can wreak havoc over vast areas, inundating land, isolating communities and destroying infrastructure.

Storm surge

Potentially the most dangerous hazard associated with tropical cyclones which make landfall is storm surge. Storm surge has been responsible for more deaths than any other feature of tropical cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere the onshore winds occur to the left of the tropical cyclone's path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast.

The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye. Its influence also depends on the local topography of the seafloor and the angle at which the cyclone crosses the coast. If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.

Interesting fact: Tropical cyclone Tracy is the smallest recorded cyclone, with a radius of gales extending less than 50 kilometres. The largest tropical cyclone on record is Typhoon Tip which had gales over a radius 1100 kilometres in the north-west Pacific Ocean.