Australia: The Land Where Time Began

A biography of the Australian continent 


The term cyclone was coined by Piddington (1855), from the Greek word for the coil of snake, referring to the spiralling air flow. Cyclones form when layers of near surface air have a central area of low pressure, the air flowing to the base spiralling cyclonically towards the centre. Cyclones have different direction in the different hemispheres, in the Northern Hemisphere the spin is anticlockwise and clockwise in the Southern Hemisphere. The spin acquired by air flowing to the central low pressure system is caused by the Coriolis Effect, spin about a local vertical axis. The air flowing to the low pressure centre concentrates this rotation, gaining speed of rotation the closer as it approaches the centre. At the equator there is no local vertical axis spin, the Coriolis effect not operating here. The result is that cyclones don't form at the equator, at least in the normal sense.

There are 2 main families of cyclones, tropical cyclones and mid-latitude or subtropical cyclone, the 'lows', low pressure systems that are normally seen in weather reports. Because of its clockwise spin in the Southern Hemisphere, the air flowing on the western margin of a low pressure system draws colder southern air towards the equator and the leading edge, the eastern margin, brings warmer tropical air to the more southerly regions. A cold front is the narrow zone that often occurs where the 2 air masses meet. The southern parts of Australia receive most of their rain from cold fronts associated with the cyclone type.

Tropical cyclones

The tropical cyclones affecting Australia form over tropical water around the more northerly regions of the continent where there is normally warm enough water, from the east coast along the north coast to the more northerly parts of the west coast. The inner core is where the strongest winds and heaviest rain of the system are located, around a central eye of calm winds and generally clear skies. Fronts do not form in tropical cyclones. They are known by several names in different parts of the world, hurricanes in the North Atlantic, typhoon ('great wind') in Asia and in Japan the word kamikaze ('divine wind') derived from the typhoon that destroyed the Mongol fleet that was on its way to invade Japan in the 14th century. In the earlier times of European settlement in Australia there were a number of other terms for them, one of which was cockeyed Bobs. In Australia cyclones are allocated a category based on the strength of their winds, from 1 for the weakest, to 5 for the most severe.

The eye of a cyclone is an area of very low pressure where the weather is calm, no rain or strong winds. In most cyclones the eye is about 40 km wide, but can reach to more than 100 km wide, as was the case with the category 5 cyclone, Yasi, that crossed the Queensland coast on the 4th of February 2011, it has been estimated by Dr. David Moore from the University of Leicester that the eye was bigger than the island of Anglesey.

The circular eye wall in the centre of the cyclone has clouds extending from near the ocean surface up to 12-18 km. As the horizontal pressure gradients weaken with height, the eye wall leans outward by an amount that increases with height, allowing the strong winds that are carried upwards to spread out. Away from the eye the clouds are in the form of spiral bands of deep tropical convection that can extend up to 1000 km from the eye.

A feature of cyclones that can be catastrophic for people or property in its path is that the strongest winds, near the eye, occur close to the surface. In the case of Yasi, wind speeds were reported that gusted to around 300 km/h, estimated by the Bureau of Meteorology to have reached 285 km/h.

Tropical cyclones differ from mid latitude lows in that they gain most of their energy from the heat of the ocean they pass over, and in the case of tropical oceans they can be warm, having been heated by the sun. The mid-latitude latitude lows obtain most of their energy from the large-scale temperature gradient between the tropics and the poles. In the tropics the warmed water evaporates at a high rate than cooler water and the air passing over it is also warm, so holds more water, leading to warm air holding large amounts of water vapour. The air near the surface is also heated by the sun, leading to rising columns of this warm air that holds a lot of water. As this air is carried high into the atmosphere in the updrafts it cools leading to the development of convective clouds as the water it held at higher temperatures for water droplets.

Sources & Further reading

  1. G. Holland & P. McBride in Webb, Eric K, (1997), Windows on Meteorology, Australian Perspective, CSIRO Publishing.


Author: M. H. Monroe
Last Updated 01/10/2011
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