- What are they
- When and where
- Life cycle
- Structure
- The Cyclone
Threat - Northern
Territory - Cyclone Tracy
- Warning System
High Winds
The extreme wind speeds in tropical cyclones are directly related to the steep pressure gradients near the cyclone centre. Wind damage increases exponentially with increased wind speed. In severe cyclones the maximum sustained winds can approach 200 km/h with short period gusts closer to 300 km/h. Such violent winds can devastate natural vegetation and all but the strongest man-made structures.
Heavy Seas
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Storm Surge
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Before/after photos showing the destruction of a stately southern mansion by storm surge in the wake of Hurricane Camille 1969. Photos courtesy Bureau of Meteorology.
As a tropical cyclone crosses the coast the combination of low pressure near the centre and strong onshore winds can produce a large increase in sea level, called a storm surge. This can bring about sea waves breaking into areas not normally affected, producing absolute destruction of buildings or other facilities. In highly vulnerable areas such as the Ganges River Delta of Bangladesh, where a large population inhabits the fertile flats close to sea level, the results can be disastrous. Storm surges in this area have resulted in death tolls exceeding 100,000.
Flood Rains and
Landslides
Some of Australia's heaviest rainfalls have occurred in association with tropical cyclones and their decay over land. The highest recorded 24 hour fall was at Mount Bellenden Ker in Queensland during Cyclone Peter. In February 1974 Cyclone Wanda produced huge floods in Brisbane and southeast Queensland causing 14 deaths and a total damage bill around $1.6 billion. Flooding due to tropical cyclones has frequently caused hardship and occasional loss of life in coastal areas of the Northern Territory. However the benefits of widespread rainfall brought to the dry pastoral areas of northern Australia usually far outweigh the costs.
In some areas of the world (especially the Philippines, China and Japan), torrential rain brought by tropical cyclones can produce landslides in mountainous terrain, sometimes with disastrous results. In early November 1991 on the Philippines island of Leyte an estimated 6,000 people died and a further 20,000 were left homeless when flood rains in the wake of Typhoon Thelma caused flash flooding, landslides and a burst dam. Hardest hit was the port city of Ormoc where a huge mudslide occurred on nearby hillsides denuded by extensive logging operations.