Definition of a tropical cyclone
Tropical cyclones are low pressure systems that form over warm tropical waters.
A low pressure system is an area where the air pressure is lower than surrounding areas. All tropical cyclones start life as a tropical low.
Tropical cyclones typically form when:
- atmospheric conditions are favourable, and
- the sea surface temperature is above 26.5 °C.
They can continue for many days, even weeks, and 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 maximum mean wind speed of 34 knots or 63 km/h (10-minute mean) or greater 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 tropical cyclones spin.
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. See Naming tropical cyclones.
Video: Understanding tropical cyclones
Australia's tropical cyclone season runs from November to April. It is possible to see cyclones outside this time, but it is rare.
Within the Australian region, tropical cyclones start as low pressure systems over warm tropical waters to the north of our country. Australia's most cyclone-prone area is the northwest coast of Western Australia between Broome and Exmouth. But we do also see cyclones forming outside this area, including over northern Queensland and off the Northern Territory's Top End. And it's within these tropical areas where we are most likely to see direct impacts from a cyclone. But we can also feel the effects for places as far south as Perth and also northern New South Wales.
Cyclones need 4 main ingredients to form.
- First, warm water of at least 26.5 °C. This acts as fuel for a tropical cyclone.
- Second, an area of unsettled weather such as a low pressure system which becomes the focus of cyclone formation.
- Third, an atmosphere that supports thunderstorms. Lots of warm rising tropical air fuelling thunderstorms which gives strength to a cyclone.
- But how do these ingredients actually come together to form a cyclone? Well, as the Earth rotates, it generates forces that causes winds to flow clockwise around a low pressure system or area of unsettled weather in the southern hemisphere. And this causes a cyclone to spin. These forces are weakest near the equator itself. And so, we don't typically see cyclones forming within about 500 km of the equator. As developing cyclone starts to spin, it starts to pull in warm moist air that spins into the centre of the cyclone. As warm moist air moves in, it starts to spin even faster and wind speeds increase. When the wind blows at 63 km an hour, more than halfway around the centre for at least 6 hours, the system is named a tropical cyclone. Warm air drawn into the centre, then rises, and then cools. This causes clouds to form that draw in more warm, moist air to further strengthen the cyclone.
Cyclones are categorised by their wind speed. Category 1 cyclones have wind gusts up to 125 km an hour, while category 5 cyclones, the strongest, have wind gusts of more than 280 km an hour.
Satellite footage shows what a tropical cyclone looks like from above. And we can see the classic rainbands spiralling clockwise into the centre of the cyclone. Within these spiral rainbands, we see air rising and condensing to form rain and thunderstorms.
And if cyclone reaches category 3 strength or higher, these spiral bands may wrap tightly enough around the centre to form an eye. And it's near the eye wall where we do see the strongest winds as well as heaviest rain. But in the eye itself, this is an area of relative calm. We see light winds and also sometimes clear skies.
Within the cyclone, strong winds can cause significant damage, but wind isn't the only hazard. And so tropical cyclone categories on their own don't tell you how severe the impacts could be.
Heavy rain is another hazard. And although cyclone wind speeds do decrease as it moves over land, heavy rain can continue, causing devastating flooding.
Cyclones with the strongest wind speed don't necessarily bring the heaviest rain. And so it is possible for a category 1 cyclone to cause worse flooding than a category 5 cyclone.
In addition, storm surges are also dangerous. These are caused by winds and low pressure when the sea rises above the expected tide. When this happens close to the highest tide level of a year, this can swamp low-lying areas and flow for kilometres inland.
The Bureau forecasts cyclones and warns you about them so you can prepare and stay safe. If you live or travel to areas which could be impacted by cyclones, it is important that you have the latest information. You can check this information on the Bureau's website or on the BOM weather app.
And on the BOM Weather app, you can set up notifications to get warnings direct to your phone. And to learn more about tropical cyclones, be sure to visit the tropical cyclone knowledge centre on our website.
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.
They are different names for the same type of storm – a violently rotating column of air over land. It is associated with a severe thunderstorm.
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 in our Severe weather knowledge centre.