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Here's everything you need to know about cyclones

Kate Doyle, Tuesday March 19, 2019 - 08:26 EDT
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Tropical cyclones are a part of summer in Australia but what does it take for them to form? - ABC

Cyclones are part and parcel of an Australian summer. In a normal year, about 11 tropical cyclones develop in Australian waters and four make landfall.

But what makes a cyclone and what are their impacts?

Tropical cyclones, hurricanes and typhoons are all names for the same phenomena. The , but for general purposes they are all intense, tropical, low-pressure systems with destructive winds.

What makes a cyclone?

The spiralling superstorms generally form in the warm waters of the tropics and affect the northern Australian coast. But they can travel down the coast or inland and still have big impacts as ex-tropical cyclones.

Some examples of cyclones doing damage include Debbie (2017), whose flooding effects extended into New South Wales, and Oswald (2013) and Wanda (1974) which caused major flooding in Brisbane.

For a cyclone to form there are a few criteria which need to be met:
Warm waters — more than 26.5 degrees Celsius
Humid, rising air to give moisture to the storm
Be about 500km or more away from the equator for the Coriolis effect to kick in
Low vertical shear

The Coriolis effect is when the air gets curved as it moves across the globe because the earth rotates faster around the equator than around the poles. In the same way as you get pushed to one side when you try to walk in a straight line on a playground merry-go-round.

In the Southern Hemisphere, it means winds are pushed to the left, so as the air is drawn in towards the cyclone, it is pushed to the left and results in a clockwise spiralling motion. In the northern hemisphere, hurricanes and typhoons rotate anticlockwise.

Vertical shear is when a layer of air is going at a different speed or direction to the layer of air above it. Too much shear will rip the storm apart, so it loses momentum.

Even if all of these conditions are met, there is no guarantee a cyclone will form. Cyclones are temperamental, and it can be difficult to tell which little disturbance will go rogue and develop into the next big one.


If a cyclone does form, it will feed off all that heat and moisture and spiral into one of the most powerful natural events on the planet.

The strength of a cyclone is expressed in categories. In Australia, category five cyclones are the strongest, bringing the most destructive winds.

Cyclones can be hundreds of kilometres wide, but the strongest winds are around the eye wall, surrounding the eerily calm 'eye' at the centre of the storm.

If you are in a cyclone and the wind stops suddenly, don't come out. You are likely in the eye of the storm, which can take hours to pass over — wait for the all clear before leaving for safety.

The problem with this category system is that it only considers the speed of the winds — not the other impacts of the storm so it can misrepresent the whole impact.

There are other dangers like storm surge, where the cyclone pushes the ocean onto land and can result in water coming up over six metres higher than normal. Storm surge often has the deadliest impact.

Rain can also bring heavy flooding, before, during and after the winds have settled.

Rainfall can be especially bad if the storm is travelling slowly or if it stops over one area.

So and make sure you have a plan in place for how you are going to respond to an emergency, wherever you are.


© ABC 2019

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