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In meteorology, a tropical cyclone (or typhoon or hurricane, depending on where it forms) is a type of low-pressure system which generally forms in the tropics. While some, particularly those that make landfall in populated areas, are regarded as highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.
Structurally, a tropical cyclone is a large, rotating area of clouds, wind, and thunderstorm activity. The primary energy source of a tropical cyclone is the release of heat of condensation from water condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.
This use of condensation as a driving force is the primary difference setting tropical cyclones apart from other meteorological phenomena, such as mid-latitude cyclones, which draw energy mostly from pre-existing temperature gradients in the atmosphere. To drive its heat engine, a tropical cyclone must stay over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle.
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 per second (33 knots or 38 mi/h or 62 km/h).
A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 metres per second (34-63 knots or 39-73 mi/h or 62-117 km/h).
The term used to describe tropical cyclones with maximum sustained winds exceeding 33 metres per second (63 knots or 73 mi/h or 117 km/h), varies depending on region of origin, 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/ch1/ch1_3.htm .)
An extratropical cyclone is a storm that was once tropical in nature. However, since it passed over land or cool waters, its energy source changed from releasing heat from condensing water to difference in temperature in air masses. From space, they resemble a comma. Extratropical cyclones can still be dangerous because they are still low in pressure, which causes powerful winds.
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 British shipping forecasts, winds of force 12 on the Beaufort scale are described as 'hurricane force'.
In other places in the world, hurricanes have been called Willy-Willies (singular Willy-Willy) in Australia, Bagyo in the Philippines, Chubasco in Mexico, and Taino in Haiti.
Hurricanes are categorized on a 1-to-5 scale 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.
National Hurricane Center classifies hurricanes of Category 3 or above as Major Hurricanes. Joint Typhoon Warning Center classifies typhoons with wind speeds of at least 150 mi/h (67 m/s or 241 km/h) as Super Typhoons.
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 associated with this phenomenon.
There is also a polar counterpart to the tropical cyclone, called an arctic cyclone.
Almost all tropical cyclones form within 30 degrees of the equator and 87% form within 20 degrees of it. Since the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator (where the Coriolis effect is weakest). However it is possible for tropical cyclones to form within this boundary if another source of initial rotation is provided. These conditions are extremely rare and such storms are believed to form at a rate of less than one a century.
Most tropical cyclones form in a worldwide band of thunderstorm activity known as the Intertropical convergence zone (ITCZ).
Worldwide, an average of 80 tropical cyclones form each year.
There are seven main basins of tropical cyclone formation:
The following areas spawn tropical cyclones only very rarely.
Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns.
In the north Atlantic, a distinct hurricane season occurs from June 1 to November 30, sharply peaking in early September. The northeast Pacific has a broader period of activity, but in a similar timeframe to the Atlantic. The northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the north Indian basin, storms are most common from April to December, with peaks in May and November.
In the southern hemisphere, tropical cyclone activity begins in late October, and ends in May. Southern hemisphere activity peaks in mid-February to early March.
A strong tropical cyclone consists of the following components.
The formation of tropical cyclone is still the topic of extensive research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:
Tropical cyclones can occasionally form despite not meeting these conditions. A combination of a pre-existing disturbance, upper level divergence, and a monsoon related cold spell led to the creation of Typhoon Vamei at only 1.5 degrees north of the equator in 2001. It is estimated that the factors leading to the formation of this typhoon occur only once every 400 years.
When a tropical cyclone reaches higher latitudes or passes over land, it may merge with weather fronts or develop into a frontal cyclone, also called extra-tropical cyclone. In the Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach Europe as a European windstorm.
Intense tropical cyclones pose a particular observation challenge. As they are a dangerous oceanic phenomenon, weather stations are rarely available on the site of the storm itself, unless it is passing over an island or a coastal area, or an unfortunate ship is caught in the storm. Even in these cases, real-time measurement taking is generally only possible in the periphery of the cyclone, where conditions are less catastrophic.
It is however possible to take in-situ measurements, in real-time, by sending specially equipped reconnaissance flights into the cyclone. These are flown by four-engine turboprop aircraft, which take direct and remote-sensing measurements and launch dropsondes inside the cyclone.
The cyclone can also be imaged remotely by radar, and by weather satellites using visible light and infrared.
A mature tropical cyclone can release heat at a rate upwards of 2x1019 watts. This is two hundred times the total rate of human electrical production, and is equivalent to detonating a 10 megaton nuclear bomb every 20 minutes. Tropical cyclones on the open sea cause large waves, heavy rain, and high winds, disrupting international shipping and sometimes sinking ships. However, the most devastating effects of a tropical cyclone occur when they cross coastlines, making landfall. A tropical cyclone moving over land can do direct damage in 4 ways.
Often, the secondary effects of a tropical cyclone are equally damaging. They include:
The 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.
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 16 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 $A 400 million, which (at current exchange rates) is approximately equal to $US 280 million.
A 100 mph (160 km/h) tropical cyclone hit the densely populated Ganges Delta region of East Pakistan (now Bangladesh) on November 13, 1970, which killed an estimated 500,000 people (this is regarded as the 20th century's worst cyclone disaster).
In August, 1992, Hurricane Andrew became the most destructive natural disaster in the history of the United States of America.
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.
On March 26, 2004, Cyclone Catarina became the first-ever hurricane observed in the south Atlantic Ocean. Previous South Atlantic cyclones in 1991 and 2004 reached only tropical storm strength. Hurricanes may have formed there prior to 1960 but were not observed until weather satellites began monitoring the Earth's oceans in that year.
September 2004: Hurricane Ivan: Ivan reached "unprecedented" intensity at low latitudes, reaching 135 mph (215 km/h) (Category 4) at only 10.6° N. It later went on to maintain Category 5 strength for over a full day and strike several islands and the United States.
Tropical cyclones with winds exceeding 33 metres per second are given names. These names are taken from lists which vary from region to region and are drafted a few years ahead of time. The lists are decided upon, depending on the regions, either by committees of the World Meteorological Organization (called primarily to discuss many other issues), or by national weather services involved in the forecasting of the storms.
Each year, the names of particularly destructive storms are "retired" and new names are chosen to take their place.
For several hundred years after the arrival of Europeans in the West Indies, hurricanes there were named after the saint's day on which the storm struck. During World War II, tropical cyclones were given female names, mainly for the convenience of the forecasters and in a somewhat ad hoc manner.
The modern naming convention came about in response to the need for unambiguous radio communications with ships and aircraft. As transportation traffic increased and meteorological observations improved in number and quality, several typhoons, hurricanes or cyclones might have to be tracked at any given time. 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, some considered this practice sexist. The National Weather Service responded to these concerns in 1979 with the introduction of male names to the nomenclature.
The World Meteorological Organization's (WMO) Regional Association IV Hurricane Committee selects the names for Atlantic Basin and central and eastern Pacific storms. Currently, in the Atlantic and Eastern North Pacific regions, 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. Six lists of names are prepared in advance, and reused on a six-year cycle (a different list is used for each year).
In the Central North Pacific region, the name lists are maintained by the Central Pacific Hurricane Center in Honolulu, Hawaii. Four lists of Hawaiian names are selected and used in sequential order without regard to year.
In the Western North Pacific, name lists are maintained by the WMO Typhoon Committee. Five lists of names are used, with each of the 14 nations on the Typhoon Committee submitting two names to each list. Names are used in the order of the countries' English names, sequentially without regard to year.
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.
The human toll of cyclones cannot have a price put on it. However, cyclones may bring much-needed precipitation to otherwise dry regions. An appreciable percentage of Japan's rainfall is due to typhoons. Hurricane Camille averted drought conditions and ended water deficits along much of its path.
Additionally, hurricanes actually help to maintain global heat balance by moving warm, moist tropical air northward to the mid-latitudes and polar regions.
In the 1960's and 70's, the United States government attempted to weaken hurricanes in its Project Stormfury by seeding with silver iodide. It was thought that the seeding would cause changes in the structure of the hurricane, essentially disrupting the eyewall to collapse and thus reduce the winds. However, it was later determined that this already happens naturally.
Regarding the frequency of hurricanes, numbers have varied widely .
All this applies only to Atlantic.
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