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 dissipate once it moves over land or over cooler oceans.

Austrlia Cyclones Image

Tropical cyclones defined

A more technical definition of a tropical cyclone is:

A non-frontal low pressure system of synoptic scale developing over warm waters having organised convection and a maximum mean wind speed of 34 knots or greater extending more than half-way around near the centre and persisting for at least six hours.

How do tropical cyclones form?

A cluster of thunderstorms can develop over warm tropical oceans. If that cluster persists 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.

Figure 1: Diagram of a tropical cyclone system
Figure 1: Diagram of a tropical cyclone system

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.

Impacts of tropical cyclones

Tropical cyclones are dangerous because they can produce extreme winds, heavy rainfall with flooding and damaging storm surge that can cause inundation of low-lying coastal areas.

Wind

Cyclones have gale force winds with wind gusts in excess of 90 km/h around their centre. In the most severe cyclones, gusts can exceed 280 km/h. These winds can cause extensive property damage and turn airborne debris into potentially lethal missiles. It is important to remember when the eye of a cyclone passes over a location, there will be a temporary lull in the wind, but that this will soon be replaced by destructive winds from another direction.

Rain

Heavy rainfall associated with the passage of a tropical cyclone can produce extensive flooding. This can cause further damage. The heavy rain can persist as the cyclone moves inland and weakens into a low pressure system, hence flooding due to an ex-tropical cyclone can occur a long way from where the cyclone made landfall.

Storm surge

As well as extreme winds, a tropical cyclone can cause the sea to rise well above the highest tide levels of the year when it comes ashore. These storm surges are caused mainly by strong, onshore winds and also reduced atmospheric pressure. Potentially, the storm surge is the most dangerous hazard associated with a tropical cyclone. Read more...

Tropical cyclone severity categories

The severity of a tropical cyclone is described in terms of categories ranging from 1 (weakest) to 5 (strongest) related to the maximum mean wind speed as shown in this table.
Note: corresponding approximate wind gusts are also provided as a guide. Stronger gusts may be observed over hilltops, in gullies and around structures.

Category Maximum Mean Wind (km/h) Typical Strongest Gust (km/h) Typical Effects
1 63 - 88 < 125 Damaging winds. Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.
2 89 - 117 125 - 164 Destructive winds. Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings.
3 118 - 159 165 - 224 Very destructive winds. Some roof and structural damage. Some caravans destroyed. Power failures likely. (e.g. Clare, Olwyn)
4 160 - 199 225 - 279 Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures. (e.g. Tracy, Debbie, Lam)
5 > 200 > 279 Extremely dangerous with widespread destruction. (e.g. Vance, Marcia, Yasi)