About Cyclones

Cyclones IconTropical Cyclones are formed over the ocean in the area around the equator, between the Tropic of Cancer and the Tropic of Capricorn.

 

 

 

 

 

About Cyclones

Tropical Cyclones are formed over the ocean in the area around the equator, between the Tropic of Cancer and the Tropic of Capricorn.

Cyclones

Cyclones are giant, spinning, whirlwind storms. They are called hurricanes in North America and typhoons in Asia. In order for a cyclone to be identified as a cyclone, it must be travelling over 119 km per hour and it must have been formed over the ocean in a tropical region. These great storms are usually dangerous and can cause damage to everything in their path. A cyclone will often bring with it heavy rains that can cause flooding.

In order for a cyclone to form, the ocean waters need to be warm, at least 26°C. Above the warm ocean, water evaporates and forms clouds. If there is low air pressure where the clouds are formed, it pulls them in and they begin to rotate. It is the earth rotating and spinning on its axis that causes the cyclone’s clouds to rotate. Clouds will continue to form and begin spinning more.

This is the stage when it can lose its momentum or develop into a mature cyclone. Even if it has developed into a mature cyclone, it can still grow in size and increase its wind speed. In order for it to be categorised as a cyclone, its average sustained wind speed needs to exceed 63 km per hour. To be classified as severe, the average sustained wind speed needs to exceed 118 km per hour.

Once a cyclone arrives over land, its strength weakens and it begins to fade out. This is due to the lack of moisture and heat compared to the ocean over which it was formed.

How often do they occur?

Cyclone season in Australia is between November and April, however cyclones can still occur in May. The season for hurricanes and typhoons in the Northern Hemisphere is between June and November.

According to the Bureau of Meteorology, Australia has, on average, 13 cyclones a year. Half of these occur in the western regions.

Categories and severity

Cyclones are categorised according to their strength. There are five categories: Category 1 is the weakest and Category 5 is the strongest.

Cyclone Strength

Category Wind Gusts Ocean Swells Damage
1 Up to 125km/hr -Gales 1.2 - 1.6m - Slight Damage Trees  and farmland damaged
2 126 - 169km/hr - Destructive 1.7 - 2.5m - Significant Damage  Minor house damage. Severe damage to signs and trees. Heavy damage to crops
3 170 - 224km/hr - Very Destructive 2.6 - 3.7m - Structural Damage House roofs and most likely power failures
4 225 - 279km/hr - Very Destructive 3.8 - 5.4m - Significant roofing and structural damage Airborne debris, widespread power failure
5 Winds above 280km/hr - Very Destructive More than 5.5m - Almost total destruction and extremely dangerous Houses flattened, cars overturned

 



 

 

 

 

 

 

Every cyclone has an ‘eye’

The eye is in the centre of the cyclone and can vary in size from ten km to 100 km, depending on the severity of the storm. Because there is the least amount of air pressure in the eye, it produces clear weather with light wind, no clouds, no rain and some sunshine. Do not be fooled by the eye: the storm is not over yet. This is only the middle of the storm. Depending on the wind gusts, the eye may pass in a few minutes or it may take several hours.

You are advised to stay indoors during the passing of the eye of the storm, because the cyclone will continue. Always listen for the official word that the cyclone has passed and when it is safe to leave your shelter. When going outside, be aware of fallen power lines, debris and damage left behind after the cyclone.

Did you know?

  • The average life of a cyclone is one week.
  • After the eye passes and the other side of the cyclone hits, the wind blows with equal strength but in the opposite direction.
  • Tropical cyclones from the Southern Hemisphere spin clockwise, while hurricanes and typhoons in the Northern Hemisphere spin anti-clockwise.
  • Cyclone Tracy was Australia’s most destructive cyclone.
  • Cyclones are assigned names, which are chosen from a list.

Names of cyclones

The name of each cyclone is taken from a list of approximately 104 names that are used only for cyclones in the Australian Region. If a cyclone begins and is named in another region, it will keep that name when it enters Australia’s zone of responsibility. The names are usually chosen in sequence, returning to the beginning when the end of the list is reached. Names alternate between male and female and if a cyclone has a significant effect in the Australian Region (such as Cyclone Tracy) the name will be removed from the list as a mark of respect.

For a full list of the names, visit the Bureau of Meteorology website www.bom.gov.au

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