Tropical cyclone

This article is about the weather phenomenon. For other uses see Hurricane (disambiguation) or Typhoon (disambiguation).

Hurricane Anita approaching landfall on the coast of Mexico, September 1977

In meteorology, a tropical cyclone (informally, a typhoon or hurricane) is a type of low-pressure system which generally forms in the tropics, but not on the equator. Tropical cyclones could not exist without the Coriolis effect and hence cannot form or travel near the equator, as shown by this map . They do not form over land, and having made landfall soon die. The cyclone is accompanied by thunderstorms, and a circulation of winds near the Earth's surface, which is clockwise in the Southern hemisphere and counter-clockwise in the Northern hemisphere.

Table of contents

1 Classification and terminology

2 Hurricanes in the Atlantic

3 Notable cyclones

4 Naming of tropical cyclones

5 See also

6 External links

Classification and terminology

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 organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 metres a second (33 knots or 38 mi/h).

Eye of Typhoon Odessa,Pacific Ocean, August 1985

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 mi/h).

The term used to describe tropical cyclones with maximum sustained winds exceeding 33 metres a second, varies depending on region, as follows:

hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline, and the South Pacific Ocean east of 160°E
typhoon in the Northwest Pacific Ocean west of the dateline
severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E
severe cyclonic storm in the North Indian Ocean
tropical cyclone in the Southwest Indian Ocean

(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/ch1/ch1_3.htm).

In the UK and Europe some severe north-east Atlantic cyclonic depressions are referred to as "hurricanes," even although they rarely originate in the tropics. These European windstorms can generate hurricane-force windspeeds but are not given individual names.

In other places in the world, hurricanes have been called Willy-Willies (singular Willy-Willy) in Australia, Baguio in the Philippines, Chubasco in Mexico, and Taino in Haiti.

Hurricanes are rated on a 1-5 scale based on wind intensity called the Saffir-Simpson Hurricane Scale.

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 meters (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 associated with this phenomenon.

There is also a polar counterpart to the tropical cyclone, called an arctic cyclone.

Hurricanes in the Atlantic

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 mi/h).

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.

The first recorded South Atlantic hurricane

On March 26, 2004, Cyclone Catarina became the first-ever hurricane observed in the south Atlantic Ocean. Because of limited observation ability, hurricanes may have formed there prior to 1960, when weather satellites began monitoring the Earth's oceans. Previous South Atlantic cyclones in 1991 and 2004 reached only tropical storm strength.

Notable cyclones

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. Fifty people died in Darwin, and sixteen 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.

A 100-mph tropical cyclone hit the densely populated Ganges Delta region of East Pakistan (now Bangladesh)s on November 13, 1970, which killed an estimated 500,000 people (this is regarded as the 20th century's worst cyclone disaster).

The Great Galveston Hurricane of 1900, which made landfall at Galveston, Texas as an estimated category 4 storm, killed 6,000-12,000 people. It remains the United States' deadliest natural disaster.

Naming of tropical cyclones

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.

To help in their identification, in the early 1950's the practice of naming tropical storms and hurricanes was initiated by the United States National Hurricane Center and are now maintained by the WMO. In keeping with the common English language practice of referring to inanimate objects such as boats, trains, etc., using the female pronoun "she", names used were exclusively female. The first storm of the year was assigned a name beginning with the letter "A", the second with the letter "B", etc. However, since tropical storms and hurricanes are primarily destructive, it was gradually realized by the National Weather Service that the naming practice could be considered inadvertantly sexist, and in 1979 the use of male names, in addition to female names, was initiated. Currently, female and male names during a given season are assigned alternately, still in alphabetic order. The "gender" of the first storm of the season also alternates year to year. The lists of names is prepared in advance, and reused periodically, except that the names of particularly destructive storms are "retired".

Other sets of names are used in the Eastern North Pacific, Central North Pacific, and the Western North Pacific, maintained by the WMO Typhoon Committee. The Australian Bureau of Meteorology maintains three lists of names, one for each of the Western, Northern and Eastern Australian regions. There are also Fiji region and Papua New Guinea region names. The Seychelles Meteorological Service maintains a list for the Southwest Indian Ocean.