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Cyclone Yasi bearing down on north Queensland. (Photo: NASA)

Cyclones: facts and figures

  • BY Marina Kamenev and John Pickrell |
  • February 02, 2011

Just what are cyclones, how do they form and what do the categories mean.

TROPICAL CYCLONES ARE intense, spinning storm systems, with low-pressure centres that can be vast in size. They form over warm oceans and can wreak havoc when they approach the shore.

As the names suggests, tropical cyclones and hurricanes occur in the world's tropics. They require the difference in speed of rotation of the Earth at different latitudes to gather momentum as they spin, and they can form either side of the equator.

Cyclones are called hurricanes in the Atlantic and eastern Pacific, typhoons in Southeast Asia, and cyclones in the Indian Ocean and western Pacific around Australia.

And Cyclone Yasi is a big one, with predicted wind speeds of over 295 km/h, a core region 500 km in diameter, and a zone of associated storm activity spread over 2000 km.

"It's a serious event. It's the biggest one that anyone living today has seen in Queensland," says Professor Jonathan Nott at James Cook University. "We've been lulled into a false sense of security in Queensland because we've been through a fairly quiet time of cyclones since the 1970s."

How a cyclone forms

A tropical cyclone needs two main ingredients: a cluster of thunderstorms and a warm body of water –  at least 27ºC – from which the storm gathers its energy. The warm, tropical ocean under a developing storm evaporates then condenses to form clouds, releasing heat throughout the process.

The heat energy combined with the rotation of the Earth, gets the cyclone spinning and propels it forward. While the cyclone looks savage from the outside, its low-pressure centre, commonly known as the eye, is deceptively calm. This belies the danger of the dense wall of cloud that surrounds it, which is the deadliest part of a cyclone.

Here the strongest winds and greatest rainfall are found.The eye is usually 40 km in diameter, but can range in size from less than 10 km to over 100 km – as is the case with Yasi – depending on the size of the cyclone itself.

"The thing about Cyclone Yasi is its large diameter," says Jonathan. "We commonly get 'midgets' in Queensland - small diameter but still intense tropical cyclones. From time to time we get one of these very, very large one, but the midgets have been more common in recent years. It makes it more unusual for us again to see a large diameter system here." 

Cyclone categories

Tropical Cyclones are characterised by strong continuous winds of more than 63 km/h. Once those winds reach 118 km/h the cyclone is classified as severe.  Australia's Bureau of Meteorology classifies cyclones by the following criteria:

CATEGORY 1
Strongest winds: Gales, 90 - 125 km/h
Negligible house damage. Damage to some crops, trees and caravans. Boats may drag moorings.

CATEGORY 2
Strongest winds: Destructive, 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.

CATEGORY 3
Strongest winds: Very destuctive,165 - 224 km/h
Some roof and structural damage. Some caravans destroyed. Power failures likely.

CATEGORY 4
Strongest winds: Very destuctive, 225 - 279 km/h
Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.

CATEGORY 5
Strongest winds: Very destuctive, more than 290 km/h
Extremely dangerous with widespread destruction.

However, wind is not always the most destructive aspect of a storm. A cyclone can cause massives walls of water – storm surges – to move to shore at a high speed. The force of the water can cause damage to properties near the coast and is likely accompanied by torrential rain.

Storm surges

"The generation of storm surges with tropical cyclones relates to a number of factors such as the drop in pressure, which creates an upward bulge in water in the middle of the storm of up to one metre; the size and intensity of the storm; and the speed of forward movement," says Professor Nick Harvey an expert on coastal processes at Adelaide University. "The forward-left quadrant of the storm is the most dangerous in the Southern Hemisphere, because this is where the forward movement and the clockwise spiral of gale force winds pile up water ahead of the storm."

"Notable storm surges in north Queensland have occurred in 1899 at Bathurst Bay - with dubious reports of a 12.2 m surge when many lives were lost from a pearling fleet - and Cyclone Althea in 1971 which produced a 3.6 m surge north of Townsville," he says.

In Australia, cyclones take place seasonally between October and May. This season, a strong La Niña weather pattern has appeared, leading to warmer waters near the northeastern coast of Australia. This has made Queensland particularly susceptible to tropical storms.

“The relationship of strong La Niña events to active tropical cyclone seasons is the basis for the predictions, issued by the Bureau of Meteorology several months ago, of an active season for 2010/11,” says Professor Neville Nicholls president of the Australian Meteorological and Oceanographic Society, who is based at Monash University in Melbourne.

“We have known for over 30 years that the El Niño – Southern Oscillation, of which the La Niña is one extreme phase, affects the number of tropical cyclones in the Australian region, and that this allows us to predict, months in advance, whether a tropical cyclone season is likely to be active or inactive,” he says.

 

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