Severe weather (continued)

Tropical cyclones are intense low-pressure systems with average winds exceeding gale force (34 knots/62km/hr.) They are called hurricanes and typhoons in other parts of the world. They have set records for sea-level low pressure (870 hPa), sustained wind (250 km/h, with gusts in excess of 300 km/h) and 24-hour rainfall (more than 1800 mm).

Tropical cyclones form over the warm tropical oceans when sea surface temperatures are above 27°C. Warm waters are critical because tropical cyclones are powered by the release of latent heat from the condensation of water vapour in the atmosphere. Latent heat released daily from one tropical cyclone is equivalent to the energy released by 400 20-megaton hydrogen bombs.

Tropical cyclones decay and die if the supply of water vapour is cut off, so they quickly lose intensity when they move over land or colder waters. They don't develop at the equator because the Coriolis force is zero there; it is not strong enough to sustain the rotation of a cyclone until five or more degrees of latitude from the equator.
See Also Monitoring the weather

The top right diagram shows a simplified plan and cross-section of a typical tropical cyclone. They often have a small, almost calm and clear area at their centre - the eye of the storm. At the surface, winds spiral inwards towards the eye (clockwise in the southern hemisphere) and form an intense circular vortex around the eye. Huge bands of convective cloud spiral in towards the eye. Air ascends in the massive thunderstorms around the eye and in the spiral bands. At high levels air spirals outwards from the cyclone. Beyond the periphery of the storm the air settles gently, compensating for the updraughts within the storm.

Tropical cyclones are notorious for their erratic tracks. They often move very slowly, sometimes at less than 10 km/hr, so communities in their path can suffer a sustained battering. While a typical life cycle is about nine days, a few persist for 20 days or more. Some take days to mature: others intensify explosively to the peak of intensity and destructive power in less than 48 hours.

Extra-tropical cyclones
Large-scale low-pressure systems outside the tropics are called extra-tropical cyclones. The Australian region has two important types. East coast cyclones are about the same size as tropical cyclones but are further south and not as intense. Their clockwise air circulation typically brings gales and heavy rain near the coast to the south of the cyclone's centre, while the weather north of its centre often clears quickly.

Southern Ocean cyclones are giant storms, often 2000km across, generally circling the earth from west to east. Australia is on the northern edge of this cyclone belt. When the belt moves further north in winter, southern Australia's weather tends to be windy and stormy. Such storms often bring cold fronts.

Categories of Severity of Tropical Cyclones
Tropical Cyclones are categorised on a five-point severity scale. (See table below)
The category refers to the severity in the zone of maximum winds close to the 'eye' of the cyclone.

Australia's most damaging tropical cyclone ('Tracy') hit Darwin on Christmas Day 1974. It was a category 4 event.