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Tropical cyclones are classified into three main groups, tropical depressions, tropical storms, and a third group whose name depends on the region. A tropical depression is an an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 metres a second (33 knot or 38 mph). A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 metres a second (34-63 knot or 39-73 mph). The term used to describe tropical cyclones with maximum sustained exceeding 33 metres a second, varies depending on region, as follows:
(This terminology is defined in WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland; available online from http://www.bom.gov.au/bmrc/pubs/tcguide/globa_guide_intro.htm)
The definition of sustained winds recommended by the WMO is that of a ten-minute average, and that definition is adopted by most countries. However, a few countries use different definitions: the United States, for example, defines sustained winds based on a 1-minute average wind measured at about 10 metres (33 ft) above the surface.
The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods we associate with this phenomenon.
There is also a polar counterpart to the tropical cyclone, called an arctic cyclone.
Each year, an average of ten tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean. On average, six of these storms become hurricanes each year. In an average 3-year period, roughly five hurricanes strike the United States coastline, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically "major" or "intense" hurricanes (winds greater than 175 km/h or 110 mph).
Hurricanes also strike Mexico, Central America, and Caribbean island nations, often doing intense damage: they are deadlier when over warmer water, and the United States is better able to evacuate people from threatened areas than many other nations.
In October 1998, Hurricane Mitch[?] caused severe flooding and mudslides in Honduras, killing at least 10,000 people and changing the landscape enough that entirely new maps of the nation were needed.
Hurricanes are categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to flooding.
On Christmas Day 1974, Tropical cyclone Tracy hit Darwin, Australia. It was the most devastating natural disaster to have ever hit an Australian city. Around 90% of the homes in Darwin were destroyed. 50 people died in Darwin, and 16 people died at sea. Authorities managed to evacuate most of Darwin. Although cyclone Tracy was quite small, it was very severe, with winds of up to 217 kilometres per hour. The damage was estimated to be close to $400 million (Australian), which (at current exchange rates) is approximately equal to $800 million US.
Tropical cyclones with winds exceeding 33 metres a second are given names. These names are taken from lists which vary from region to region. The lists are decided upon either by national meteorological organizations, or by committees of the World Meteorological Organization. The names on the list are reused; however, tropical cyclones which cause major death or destruction have their names retired.
Atlantic names were originally assigned by the U.S. National Hurricane Centre, and are now maintained by the WMO. Other sets of names are used in the Eastern North Pacific, Central North Pacific, and the Western North Pacific (maintained by who?). The Australian Bureau of Meteorology maintains three lists of names, one for the Western Australian region, one for the Northern Australian region, and one for the Eastern Australian region. There are also Fiji region and Papua New Guinea region names (maintained by who?). The Seychelles Meteorological Service maintains a list for the Southwest Indian Ocean.