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Tropical cyclones explained

Tuesday February 1, 2011 - 15:46 EDT

As Tropical Cyclone Yasi, an extremely large and powerful system, moves towards the north Queensland coast, find out more about these storms and how they cause so much damage.How do cyclones form?

They form with a combination of very warm sea surface temperatures and the right type of outflow. Imagine a tall chimney where all the energy comes into the base and gets sucked up to the top.

There is a lot of energy formed which often results in thunderstorms. In the right part of the ocean that can start circulating and release all that energy into the upper part of the atmosphere. The energy created from the thunderstorms can then add to the whole system and a tropical cyclone will form.

Cyclones rely on the circulation of the Earth, so they form away from the equator which helps the whole system rotate.

Sea temperatures of 27 degrees or above, along with a group of thunderstorms or pre-existing conditions, create ideal weather conditions for a cyclone to form.

To give an example, Hurricane Katrina, which hit New Orleans in 2005, formed in sea temperatures around 32 degrees or warmer. It is currently 31 degrees around the Gulf of Carpentaria and parts of the Coral Sea.

There is a pattern that comes across the Indian and Pacific Oceans called the Madden-Julian Oscillation that can help spin them along. A new high pressure cell with a new surge of south-easterly winds can also contribute.How destructive can cyclones be?

It depends how quickly they are moving and how deep they are. If you have ventral pressure that continues to be created within the tropical cyclone down to about 850 hectopascals, they can be incredibly destructive.

The storm surge, or massive wall of water, that can come out can be akin to a tsunami. It is not just flooding, it is the actual force of the water that is moving at very high speeds as we have seen in southern Queensland, and that can do enormous amount of damage. It is not just the winds, it is what it can do to the top layer of the ocean if conditions are right.

For example, Cyclone Larry made landfall in far north Queensland in 2006 with wind gusts reaching 240 kilometres-an-hour, damaging more than half the homes in Innisfail and leaving a repair bill of $1.5 billion. The weather bureau is now predicting Tropical Cyclone Yasi will be at least twice the diameter of Larry.

Category four Cyclone Tracy hit Darwin on Christmas Eve in 1974 with winds of up to 217kph and killed 71 people. Tracy caused more than $800 million in damage and destroyed about 80 per cent of homes.What does La Nina have to do with cyclones?

La Nina is a system where there are very warm sea temperatures around the coast of Australia, especially around the Coral Sea, and that contrasts with cooler water in the central and eastern Pacific. That gives strong easterly winds which raises the sea level around the Australian coast.

Warm sea temperatures, coupled with upper-level winds which steer the cyclone, are the ideal breeding ground to help tropical cyclones along. We tend to get many more cyclones during La Nina years and they are more likely to hit the northern and eastern Australian coast, as they are doing right now.

Queensland is more vulnerable to cylones because we are close to the Coral Sea which is about 29 degrees and the Gulf of Carpentaria which is about 31 degrees. Other contributors are those upper-level winds which are different during La Nina and that exposes the coast a bit more than during El Nino years, which has the opposite effect. What is the eye of cyclone like?

The pressure is very deep inside the eye of a cyclone and there are very light winds, which can give a false sense of security. There is also an eerie sensation because you may have been subject to 200 kilometre-an-hour winds and suddenly it goes calm. The problem is, the winds can return to being just as strong as the cyclone continues to move over. Does Australia have more cyclones than any other country?

No. There are other parts of the world such as the Philippines, the Gulf of Mexico, Florida and New Orleans that would be subject to more. North-west Western Australia also seems to have a few more cyclones. Generally the western Pacific in the Northern Hemisphere towards Taiwan, China, Korea, Vietnam and the Philippines would be subject to more cyclones than here. But we seem to be getting more intense tropical cyclones since the 1970s. There has been a 100 per cent increase in categories four and five cyclones globally, but there has not been an increase in the number of cyclones.

Australia has cyclone season every summer but because we have had so many years of El Nino - or drought conditions - they have mostly been confined to north-western WA in what is called the Monsoon trough. Because we have had so many years of drought, we have been lulled into a false sense of security and have forgotten what they can do and where they can attack. The Federal Government lists cyclone severity and potential damage they may cause as follows:CategoryWind GustsOcean SwellsDamage1Up to 125km/hr(Gales)1.2 - 1.6mSlight damage. Trees and farmland damaged.2126 - 169km/hr(Destructive)1.7 - 2.5mSignificant Damage. Minor house damage. Severe damage to signs and trees. Heavy damage to crops3170 - 224km/hr(Very Destructive)2.6 - 3.7mStructural damage. House roofs and most likely power failures4225 - 279km/hr(Very Destructive)3.8 - 5.4mSignificant roofing and structural damage. Airborne debris, widespread power failure5Winds above 280km/hr(Very Destructive)More than 5.5mAlmost total destruction and extremely dangerous. Houses flattened, cars over turnedIs climate change a contributor to that?

Climate change would suggest warmer sea temperatures in some parts of the world and temperatures in the Coral Sea this current summer are the highest since records began in the 1900s, so global warming may be a factor in that.

The warmer the atmosphere the more moisture it can contain, and the more thunderstorms you have the more heat is released into the atmosphere through latent heat release. So it implies that would increase either the intensity or number of tropical cyclones. How do cyclone differ from tornados and hurricanes?

Hurricanes are the same as cyclones, it is just a different name. However, cyclones turn anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. That is why tropical cyclones do not form around the equator - it does not have the spin of the Earth to give that direction.

Tornados are born out of thunderstorms and they are tiny by comparison; generally only about 100 metres across.How are cyclone names chosen?

There is a committee at the World Meteorological Organisation in Geneva that decides the names many years in advance. They have a list of names for each region in the world that is responsible for identifying and classifying a tropical cyclone.

Importantly they never use the names of previous cyclones again in case some members of the public think Cyclone Tracy or Hurricane Katrina, for example, have come back. Of course that is impossible but there is a fear in the community that it might have the same impact.


© ABC 2010

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