Cyclones
VIDEO: BTN Wild Weather
PDF: Cyclones – recovering together after a natural disaster
VIDEO: Cyclone Tracy
Cyclone Tracy, which hit Darwin in the small hours of Christmas Day 1974, killed 71 people and devastated 80 per cent of the city.
In the days and weeks following the disaster, most of the traumatised population left the city.
On 28 February 1975 the Whitlam government established the Darwin Reconstruction Commission, which effectively rebuilt the city within three years. Darwin’s near complete destruction led to the introduction of improved building codes across Australia.
Darwin, indeed the whole of Northern Australia, is no stranger to cyclones. However, Cyclone Tracy, which hit Darwin in the small hours of Christmas Day 1974, was among the most destructive ever recorded in Australia. Its vital statistics make for frightening reading:
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wind gusts reached 217 km/h before the anemometer was destroyed
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gales extended to about 40 kilometres from the cyclone’s centre
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there was a storm surge of 1.6 metres in Darwin’s harbour; an estimated 4 metres at Casuarina Beach
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255 mm of rain fell in 12 hours overnight, 145 mm in the two half-hour periods on either side of the eye of the cyclone
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71 people were killed
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145 people were seriously injured; more than 500 received minor injuries
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about 70 per cent of houses suffered serious structural failure
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total damage bill topped $800 million (a colossal sum in 1974).
WHAT IS A TROPICAL CYCLONE?
Tropical cyclones are low pressure systems that form over warm tropical waters. They typically form when the sea-surface temperature is above 26.5°C. Tropical cyclones can continue for many days, even weeks, and may follow quite erratic paths. A cyclone will begin to slow down once it moves over land or over cooler oceans.
HOW DO TROPICAL CYCLONES FORM?
A cluster of thunderstorms can develop over warm tropical oceans. If that cluster continues in an area of low pressure, it can start rotating. If the conditions are just right, the cluster of thunderstorms can grow in size and sustain itself and then develop into a tropical cyclone.
Once developed, a tropical cyclone is like a giant, atmospheric heat engine. The moisture from the warm ocean acts as it's fuel, generating huge amounts of energy as clouds form.
The rotating thunderstorms form spiral rainbands around the centre (eye) of the cyclone where the strongest winds and heaviest rain are found (eye wall), transporting heat 15 km or higher into the atmosphere. The drier cooler air at the top of the atmosphere becomes the exhaust gas of the heat engine.
Some of the cool air sinks into the low-pressure region at the centre of the cyclone, hence causing the relatively calm eye. The eye is usually about 40 km wide but can range from 10 to over 100 km, with light winds and often clear skies. The rest of the cool air spirals outward, away from the cyclone centre, sinking in the regions between the rainbands.
As long as the environmental conditions support this atmospheric heat engine, the tropical cyclone can maintain its structure and even intensify over several days.
Hurricanes, cyclones and typhoons are all types of tropical storms. But what's the difference between them? Well, they are all basically the same thing, but are given different names depending on where they appear.
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Hurricanes are tropical storms that form over the North Atlantic Ocean and Northeast Pacific.
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Cyclones are formed over the South Pacific and Indian Ocean.
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Typhoons are formed over the Northwest Pacific Ocean.