PARIS (AFP) — Tropical cyclones of the kind that killed thousands in Myanmar are immensely powerful low-pressure systems capable of generating 10 times as much energy as the atomic bomb dropped on Hiroshima.
Also known in Asia as typhoons, but more often in the West under the general name of hurricanes, cyclones are storms that rotate around a moving centre of low atmospheric pressure.
They develop over tropical oceans, working up a surface wind of more than 74 miles (120 kilometres) an hour.
Sucking up vast quantities of water, they often produce torrential rains and flooding resulting in major loss of life and property damage.
Cyclones (hurricanes) differ from tornadoes -- while both are atmospheric vortices, a tornado is produced by a single convective storm and has a diameter of hundreds of metres (yards).
A hurricane comprises several to dozens of convective storms and can reach hundreds of kilometres (miles) in diameter.
They blow counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Such storms are formed from simple thunder storms at particular times of the year when the sea temperature is at least 27 degrees Celsius (81 Fahrenheit) down to a depth of 80 metres (250 feet), and the air in the upper atmosphere is cold. The heat and moisture from the water is their source of energy.
They can often last up to two or three weeks, but weaken rapidly when they travel over land or colder ocean waters.
When a cyclone loses strength, usually after landfall, it becomes known as a tropical storm.
The "eye" of a hurricane is a roughly circular area of comparatively light winds and fair weather found at the centre of a severe tropical cyclone, between 30 and 60 kilometres (20 and 40 miles) wide.
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