Tropical Cyclone Debbie is about to join a long list of cyclones that have menaced northern Australia, here’s what you need to know about cyclones.
What is a cyclone?
Tropical cyclones are intense low-pressure systems that form over warm ocean waters at low latitudes.
They form when the sea-surface temperature is above 26.5C, drawing their energy from warm tropical waters.
Cyclones produce gales, torrential rain and storm surges of varying intensity, depending on how low the storm’s central pressure becomes.
They can last for days but usually dissipate when they move over land or cooler water.
The Australian cyclone season is from November 1 to April 30 each year.
The northwestern coast of Western Australia, between Broome and Exmouth, is Australia’s most cyclone-prone area.
How much damage can a cyclone cause?
The destructive winds, heavy rainfall and storm surges can cause extensive damage and lead to flooding, landslides and dangerous sea conditions.
Tropical cyclones are categorised from one to five based on the wind speed. A category one has gusts less than 125km/h and a category five has winds surpassing 280km/h.
The most destructive winds occur in an area known as the eye wall, which surrounds an area of relative calm in the eye, at the centre of the system.
The sharpness of the eye’s appearance in satellite images is a pointer to the intensity of the cyclone.
Australia holds the record for the world’s strongest non-tornado wind gust. That was 408km/h, when Severe Tropical Cyclone Olivia roared over Barrow Island, off the Pilbara coast in Western Australia, on April 10, 1996.
Severe tropical cyclone Tracy is probably the most infamous of Australia’s cyclones.
At least 66 people died when Tracy, a category four storm, struck Darwin early on Christmas Day, 1974, virtually flattening the city with winds near the eye estimated at 260km/h.
Despite its intensity and incredible destructive power, Tracy was tiny. The World Meteorological Organisation recognises Tracy as the world’s smallest tropical cyclone, with gale force winds extending only 50km/h from its eye.
Cyclone Mahina, which struck Bathurst Bay in far north Queensland on March 5, 1899, holds two records. It remains Australia’s deadliest natural disaster, killing an estimated 410 people when it scuttled a sheltering pearling fleet.
It also holds the world record for the highest storm surge ever recorded. Debris from the surge was found 13 metres above sea level.
Cyclone Yasi, which struck the far north Queensland coast in February 2011, is one of the largest cyclones to hit Australia in recent years.
It was a category five storm, the most intense on the Australian cyclone scale, with a peak wind gust of 285km/h.
What’s in a name?
Cyclones have many names depending on their location around the world.
In the south Pacific and Indian Oceans, they are known as cyclones.
In the northwest Pacific, they are known as typhoons.
In the Atlantic and northeast Pacific Oceans, they are known as hurricanes.
How cyclones are named varies among meteorological organisations and countries. For some, it’s as simple as making their way through an alphabetical list of names, while others start afresh each year.
The Australian Bureau of Meteorology created a new list, with 104 names, before the 2008-09 cyclone season. There were three lists before then.
In the 19th century an Australian forecaster, Clement Wragge, used the names of South Sea Island girls, and then later used the names of the wives of politicians he disliked. When an especially destructive cyclone affects Australia, its name is retired.
The Australian cyclone category system
Australia uses a different scale to the rest of the world when rating the intensity of cyclones.
Outside Australia, tropical cyclones that we know as category one are regarded elsewhere as mere tropical storms, although they have the eye formation that characterises all tropical cyclones.
A category three cyclone, the first category in what are known in Australia as severe tropical cyclones, corresponds roughly to a category two on the international Saffir-Simpson hurricane scale.
- Category one: Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings. Strongest winds gust 90-125km/h.
- Category two: Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings. Strongest winds are regarded as destructive, gusting to 125-164km/h.
- Category three: Some roof and structural damage. Some caravans destroyed. Power failures likely. Strongest winds are regarded as very destructive, gusting to 165-224 km/h.
- Category four: Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures. Strongest winds gust to 225-279km/h.
- Category five: Extremely dangerous with widespread destruction. Strongest winds gusts of more than 280km/h.