What is a tropical cyclone?

Definition of a tropical cyclone

Tropical cyclones are low pressure systems that form over warm tropical waters. They typically form when:

• atmospheric conditions are favourable, and
• the sea surface temperature is above 26.5 °C.

Tropical cyclones can continue for many days, even weeks. They may follow quite erratic paths.

A tropical cyclone often loses energy and breaks up when it moves:

• over land or cooler oceans, or
• into regions where the atmospheric conditions are unfavourable. For example, due to dry air or as winds change rapidly with height.

Technical definition

In the Australian region, a tropical cyclone is defined as:

• a warm-cored, non-frontal low pressure system of synoptic scale developing over warm waters
• having organised convection and
• a (10-minute mean) wind speed of at least 34 knots or 63 km/h, extending more than halfway around near the centre and
• persisting for at least 6 hours.

Organised convection means an area of thunderstorm activity associated with and organised around the low pressure system.

Mean wind speed is the speed of the wind, averaged over a 10-minute period at a given point.

Tropical lows

The key difference between a tropical low and a tropical cyclone is wind speed. Tropical lows usually have lower wind speed than tropical cyclones but can still bring severe thunderstorms and lots of rain.

Tropical lows are moderate-strength low pressure systems that occur in the tropics, often in the monsoon trough. In Australia, they happen in the northern tropical areas from October to April. This is the foundation for our tropical cyclone season.

While all tropical cyclones start as a tropical low, not all tropical lows become tropical cyclones.

How tropical cyclones form

For a tropical cyclone to form, it needs certain conditions and goes through stages as it develops.

1. Warm ocean water

The ocean water must be at least 26.5 °C. This heat fuels the developing tropical cyclone.

2. Low pressure

A tropical cyclone starts life as a tropical low. Over the ocean, these low pressure systems cause warm, moist air to rise.

3. Force to make it spin

As the warm, moist air rises, the developing tropical cyclone begins to spin. See How cyclones spin on this page.

When there is a cluster of thunderstorms over a warm tropical ocean in an area of low pressure, they can form a band and start rotating around the low pressure area. In the right conditions, the cluster can grow and sustain itself.

4. More moisture

As the storm rotates, it begins to draw in more warm, moist air. This comes from evaporation from the sea or is pulled in at low levels by the wind. This air rises and cools, causing clouds to form.

As the air nears the centre, it spins faster. The winds become stronger, drawing in air more quickly.

5. Cyclone eye and eye wall

Some of the heavier, cool air sinks into the low pressure region at the centre of the tropical cyclone. This creates the relatively calm eye.

The eye is usually about 40 km wide but can range from 10 km to more than 100 km. It has light winds and often clear skies.

Rotating thunderstorms form spiral rainbands around the eye. The strongest winds and heaviest rain are found around the eye wall.

6. A tropical cyclone is born

The low becomes a tropical cyclone when the wind speed is 63 km/h or greater, more than halfway around the centre. The new cyclone is given a name. View our Naming tropical cyclones page.

Video: How do tropical cyclones form?

How long tropical cyclones last

A tropical cyclone can maintain its structure while environmental conditions support it.

How tropical cyclones spin

Tropical cyclones need a little help to start spinning. This mostly comes from forces created by the Earth turning on its axis.

A cyclone forms in an area of low pressure. This area of low pressure draws in surrounding winds. As the Earth rotates, it creates forces that cause the winds to swirl around the low pressure. This helps the cyclone start to spin.

Cyclones at the equator

To have a good chance of developing, a tropical low needs to be far enough away from the equator – usually at least 500 km.

This is because the forces created by the Earth's rotation are weaker near the equator and get stronger towards the poles. A tropical low near the equator is unlikely to get enough of a push to spin.

It is possible for a tropical cyclone to form on or near the equator if there is enough push from the wind. Such systems are rare and tend to be short-lived.

Why tropical cyclones turn in different directions

Tropical cyclones turn in different directions north and south of the equator. They spin:

• anticlockwise in the northern hemisphere
• clockwise in the southern hemisphere.

This is due to the Coriolis effect.

Differences between tropical cyclones, hurricanes, typhoons and tornadoes

Tropical cyclones, hurricanes and typhoons

The terms 'hurricane' and 'typhoon' are regionally specific names for a tropical cyclone.

• Hurricane is used in the Northern Atlantic Ocean, central North Pacific Ocean and the Eastern North Pacific.
• Typhoon is used in the Northwest Pacific Ocean.
• Tropical cyclone is used in the South Pacific and Indian Oceans.

Seasons for tropical cyclones, hurricanes and typhoons

The tropical cyclone season in Australia is officially between 1 November and 30 April.

Hurricanes and typhoons typically develop at different times of the year.

• The Atlantic hurricane season runs from 1 June to 30 November.
• The Eastern Pacific hurricane season runs from 15 May to 30 November.
• Most typhoons form from May to October, although they can happen year-round.

Tornadoes and twisters

Tornado and twister are not the same as tropical cyclones.

Unlike a tropical cyclone, a tornado or twister:

• is much smaller – hundreds of metres rather than hundreds of kilometres.
• forms over land, while tropical cyclones form over sea
• generally lasts from a few seconds up to half an hour. Tropical cyclones last much longer.

Learn more about tornadoes.

Current tropical cyclones

See our Tropical cyclone forecast page.