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How Are Tropical Cyclones Rated?


Question: How Are Tropical Cyclones Rated?
Tropical cyclones, also called typhoons or (when extremely strong) hurricanes in other parts of the world, are wind and rain storms in the southern hemisphere characterised by low atmospheric pressure at the centre (the eye of the storm) and by clockwise wind motion. In the northern hemisphere the winds rotate counterclockwise.

Answer: Australian tropical cyclones are rated according to wind speeds and range from the relatively weak Category 1 to the most destructive Category 5.

Cyclone Tracy, which razed to the ground in 1974 the Northern Territory capital of Darwin, killed 65 people and injured 145 seriously and more than 500 with varying minor injuries, was rated a Category 4 cyclone. It caused damage to the value of $800 million in 1974 Australian dollars.

The most destructive cyclone to hit Australia occurred in 1899 when more than 400 people died as the storm struck Cape Melville. That cyclone, which also destroyed some 100 peral fishing boats anchored in Princess Charlotte Bay, was never categorised and seems to have remained unnamed.


Australian tropical cyclone categories based on Australian Bureau of Meteorology data:

  • Category 1 with strongest wind gust measured at less than 125 kilometres per hour. Usually causes minimal house damage but may cause damage to some crops, trees and caravans and water craft may drag mooings.

  • Category 2 with strongest wind gust measured at 125-170km/h. May cause minor house damage but significant damage to signs, trees and caravans, and heavy damage to some crops. There may be risk of power failure and small craft my break moorings.

  • Category 3 with strongest wind gust measured at 170-225km/h. May cause roof and structural damage, destroy caravans, cause power failure.

  • Category 4 with strongest wind gust measured at 225-280km/h. May cause significant roofing loss and structural damage, dangerous airborne debris, widespread power failure.

  • Category 5 with strongest wind gust at more than 280km/h. Extremely dangerous and causing widespread destruction.
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