BURMA:TROPICAL CYCLONES of the type which killed more than 15,000 people in Burma cause large-scale devastation and loss of life, often in countries least able to cope.
The weather systems, also known as hurricanes and typhoons, start as tropical storms over warm ocean water. The storms rotate around a moving centre of low atmospheric pressure working up a surface wind of more than 117km/hr.
In the northern hemisphere, the Earth's rotation causes the wind to swirl into a low-pressure area in a counterclockwise direction. In the southern hemisphere, the winds rotate clockwise around a low.
Under the right atmospheric conditions, a cyclone can be sustained for a couple of weeks but upon reaching cooler water or land, they rapidly lose intensity.
The effect upon land can be devastating with powerful winds and torrential rain causing widespread damage and massive loss of life.
"In very, very simple terms it is a very extreme version of the depressions that come over Britain," said Stephen Davenport, senior meteorologist at MeteoGroup, the weather division of the Press Association.
"The most significant features are the winds and the rain that cause floods and mudslides and you get a huge amount of rainfall in a very short space of time. The winds are very damaging to houses that are not necessarily built to particularly rigorous standards and a lot of people in Burma have lost their homes in that way."
The name hurricane is given to systems that develop over the Atlantic or the eastern Pacific. In the western north Pacific and Philippines they are called typhoons; in the Indian and South Pacific Ocean they are called cyclones.
Many scientists believe global warming has caused a dramatic increase in the frequency of Atlantic hurricanes and tropical storms. Research published last year showed that in the past 100 years, the average number of tropical cyclones in the Atlantic each year has doubled.
- (PA)