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Tropical cyclone

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(Redirected from Typhoon)

In meteorology, a tropical cyclone (or tropical storm, typhoon, or hurricane, depending on strength and location) is a type of low-pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.

Contents

A heat engine

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 the heat of condensation from water vapor condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.

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.

This use of condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain 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. As a result, when a tropical cyclone passes over land, its strength will diminish.

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 per second (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the spiral shape of more powerful storms. It is already becoming a low-pressure system, however, hence the name "depression".

A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 meters per second (34-63 knots, 39-73 mph, or 62-117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present.

Eye of , Pacific Ocean, August
Enlarge
Eye of Typhoon Odessa, Pacific Ocean, August 1985

The term used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h) varies depending on region of origin, 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 is the intensity at which tropical cyclones tend to develop an eye, which is an area of relative calm surrounded by the eyewall, which contains the strongest winds of the storm. They also carry a distinct spiral shape to them, and spin in a cyclonic fashion depending on the hemisphere; counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. The strongest of these storms have had maximum sustained windspeeds recorded at over 85 meters per second (165 knot, 190 mph, 305 km/h).

In other places in the world, hurricanes have been called 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.

The U.S. National Hurricane Center classifies hurricanes of Category 3 or above as Major Hurricanes. The Joint Typhoon Warning Center classifies typhoons with wind speeds of at least 150 mi/h (67 m/s or 241 km/h, equivalent to a strong Category 4 storm) as Super Typhoons.

The definition of sustained winds recommended by the World Meteorological Organization (WMO) is that of a 10-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.

An extratropical cyclone refers to a storm that derives its energy from horizontal temperature gradients typically found over higher latitudes. A cyclone originating over the tropics can become extratropical as it moves toward higher latitudes if its energy source changes from released heat through condensing water to the difference in temperature between air masses. From space, extratropical storms have a characteristic "comma-shaped" cloud pattern. Extratropical cyclones still can be dangerous because their continuing low pressure causes powerful winds.

In the United Kingdom and Europe, some severe northeast Atlantic cyclonic depressions are referred to as "hurricanes," even though 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."

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


Location

Missing image
Hurricane_from_satellite.jpg
Hurricane Anita approaching landfall on the coast of Mexico in September 1977.

Nearly 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 [1] (http://www.bom.gov.au/bmrc/pubs/tcguide/ch1/figures_ch1/figure1.9.htm) (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.

Major basins

There are seven main basins of tropical cyclone formation:

  • Western North Pacific Ocean: Tropical storm activity in this region frequently affects China, Japan, the Philippines, and Taiwan. This is by far the most active basin, accounting for one third of all tropical cyclone activity in the world. National meteorology organizations, as well as the Joint Typhoon Warning Center (JTWC) are responsible for issuing forecasts and warnings in this basin.
  • Eastern North Pacific Ocean: This is the second most active basin in the world, and is also the most dense (a large number of storms for a small area of ocean). Storms that form in this basin can affect western Mexico, Hawaii and on extremely rare occasions, California. The Central Pacific Hurricane Center is responsible for forecasting the western part of this area, and the National Hurricane Center for the eastern part.
  • South Western Pacific Ocean: Tropical activity in this region largely affects Australia and Oceania, and is forecast by Australia and Papua New Guinea.
  • Northern Indian Ocean: This basin is actually divided into two areas, the Bay of Bengal and the Arabian Sea, with the Bay of Bengal dominating (5 to 6 times more activity). Hurricanes which form in this basin have historically cost the most lives — most notably, the 1970 Bhola cyclone killed 200,000. Nations affected by this basin include India, Bangladesh, Sri Lanka, Thailand, Burma, and Pakistan, and all of these countries issue region forecasts and warnings. Rarely, a tropical cyclone formed in this basin will affect the Arabian Peninsula.
  • Southeastern Indian Ocean: Tropical activity in this region affects Australia and Indonesia, and is forecast by those nations.
  • Southwestern Indian Ocean: This basin is the least understood, due to a lack of historical data. Cyclones forming here impact Madagascar, Mozambique, Mauritius, and Kenya, and these nations issue forecasts and warnings for the basin.
  • North Atlantic Basin: The most well studied of all tropical basins, the North Atlantic includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Tropical cyclone formation here varies widely year to year, ranging from over twenty to just one. The average is ten. The United States, Mexico, Central America, the Caribbean Islands and Canada are affected by storms in this basin. Forecasts for all storms are issued by the National Hurricane Center (NHC) based in Miami, Florida, and the NHC issues warnings on storms for all nations in the region; the Canadian Hurricane Centre, based in Halifax, Nova Scotia, also issues forecasts and warnings for storms expected to affect Canadian territory and waters. Hurricanes that strike Mexico, Central America, and Caribbean island nations, often do 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. Many of the more intense Atlantic storms are Cape Verde-type hurricanes, forming just west of Africa near the Cape Verde islands.

Unusual formation areas

The following areas spawn tropical cyclones only very rarely.

  • Southern Atlantic Ocean: A combination of cooler waters, the lack of an Inter-tropical Convergence Zone, and wind shear makes it very difficult for the Southern Atlantic to support tropical activity. However, three tropical cyclones have been observed here — a weak tropical storm in 1991 off the coast of Africa, Cyclone Catarina (sometimes also referred to as Aldonē”©, which made landfall in Brazil in 2004 as a Category 1 hurricane, and a smaller storm in January of 2004, east of Salvador, Brazil. The January storm is thought to have reached tropical storm intensity based on scatterometre winds.
  • Central North Pacific: Shear in this area of the Pacific Ocean severely limits tropical development. However, this region is commonly frequented by tropical cyclones that form in the much more favorable Eastern North Pacific Basin.
  • Mediterranean Sea: Storms which appear similar to tropical cyclones in structure sometimes occur in the Mediterranean basin. Such cyclones formed in September 1947, September 1969, January 1982, September 1983, and January 1995. There is debate on whether these storms were tropical in nature.

Timing

Tropical storms and hurricanes by month, for the period 1944-2000
(North Atlantic region)
Month Total Average
January–April 3 0.1
May 8 0.1
June 31 0.5
July 50 0.9
August 151 2.6
September 198 3.5
October 100 1.8
November 26 0.5
December 4 0.1
Source: NOAA (http://www.aoml.noaa.gov/hrd/Landsea/deadly/Table8.htm)

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.

Structure

A strong tropical cyclone consists of the following components.

  • Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.
  • Warm core: tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
  • Central Dense Overcast (CDO): The Central Dense Overcast is a dense shield of rain bands and thunderstorm activity surrounding the central low. Tropical cyclones with symmetrical CDO tend to be strong and well developed.
  • Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
  • Eyewall: The eyewall is a circular band of intense convection and winds immediately surrounding the eye. It has the most severe conditions in a tropical cyclone. Intense cyclones show eye-wall replacement cycles, in which outer eye walls form to replace inner ones. The mechanisms that make this occur are still not fully understood.
  • Outflow: the upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.

Formation and development

The formation of tropical cyclones is still the topic of extensive research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:

  1. Sea surface temperatures above 26.5 degrees Celsius to at least a depth of 50 meters. Warm waters are the energy source for tropical cyclones. When these storms move over land or cooler areas of water they weaken rapidly.
  2. Upper level conditions must be conducive to thunderstorm formation. Temperatures in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.
  3. A pre-existing weather disturbance. This is most frequently provided by tropical waves - non rotating areas of thunderstorms that move through the world's tropical oceans.
  4. A distance of approximately 10 degrees or more from the equator (2004's Hurricane Ivan, the strongest storm to be so close to the equator, started its formation at 9.7 degrees north). The Coriolis Effect initiates and helps maintain the rotation of a tropical cyclone. The absence of this effect at and near the equator prohibits development.
  5. Lack of vertical wind shear (change in wind velocity over height). High levels of wind shear can break apart the vertical structure of a tropical cyclone, prohibiting development.

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 extratropical 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.

Dissipation

A tropical cyclone can cease to have tropical characteristics in several ways:

  • It moves over land, thus depriving it of the warm water it needs to power itself, and quickly loses strength. Most strong storms become disorganized areas of low pressure within a day or two of landfall. There is, however, a chance they could regenerate if they manage to get back over open warm water. If a storm is over mountains for even a short time, it can rapidly lose strength. This is, however, the cause of many storm fatalities, as the dying storm unleashes torrential rainfall, and in mountainous areas, this can lead to deadly mudslides.
  • It remains in the same area of ocean for too long, consuming all the heat available and dissipating.
  • It experiences wind shear, causing the convection to lose direction and the heat engine breaks down.
  • It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms.
  • It enters colder waters. This does not necessarily mean the death of the storm, but the storm will lose its tropical characteristics. These storms are extratropical cyclones.

Even after a tropical cyclone is said to be extratropical or dissipated, it can still have gale-force (or even hurricane-force) winds and drop several inches of rainfall.

Artificial dissipation

In the 1960s and 1970s, the United States government attempted to weaken hurricanes in its Project Stormfury by seeding selected storms with silver iodide. It was thought that the seeding would disrupt the storm's eyewall, causing it to collapse and thus reduce the winds. However, it was later discovered that eyewall disruption happens naturally as part of eyewall replacement cycles, and so the success of the program was impossible to gauge.

Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans; covering the ocean in a substance that inhibits evaporation; or blasting the cyclone apart with nuclear weapons. These approaches all suffer from the same flaw: tropical cyclones are simply too large for any of them to be practical [2] (http://www.aoml.noaa.gov/hrd/tcfaq/C5f.html).

Observations

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. Surface level observations are generally only available if the storm is passing over an island or a coastal area, or it has overtaken an unfortunate ship. 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. In the Atlantic basin, these flights are regularly flown by US government Hurricane Hunters. The aircraft used are WC-130 Hercules and WP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft fly directly into the storm and take direct and remote-sensing measurements. The aircraft also launch dropsondes inside the cyclone, giving a continuous set of measurements from flight level to the ocean surface.

Tropical cyclones far from land are tracked by weather satellites using visible light and infrared bands. Nearer to land, the cyclone can also be imaged remotely by radar.

Effects

Missing image
Destruction_following_hurricane_andrew.jpg
The aftermath of Hurricane Andrew, the costliest tropical cyclone in United States history.

A mature tropical cyclone can release heat at a rate upwards of 6x1014 watts.[3] (http://www.noaa.gov/questions/question_082900.html) 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 four ways.

  • High winds - Hurricane strength winds can damage or destroy vehicles, buildings, bridges, etc. High winds also turn loose debris into flying projectiles, making the outdoor environment even more dangerous.
  • Storm surge - Tropical cyclones cause an increase in sea level, which can flood coastal communities. This is the worst effect, as cyclones claim 80% of their victims when they first strike shore.
  • Heavy rain - The thunderstorm activity in a tropical cyclone causes intense rainfall. Rivers and streams flood, roads become impassable, and landslides can occur.
  • Tornado activity - The broad rotation of a hurricane often spawns tornadoes. While these tornadoes are normally not as strong as their non-tropical counterparts, they can still cause tremendous damage.

Often, the secondary effects of a tropical cyclone are equally damaging. They include:

  • Disease - The wet environment in the aftermath of a tropical cyclone, combined with the destruction of sanitation facilities and a warm tropical climate, can induce epidemics of disease which claim lives long after the storm passes.
  • Power outages - Tropical cyclones often knock out power to tens of thousands of people, prohibiting vital communication and hampering rescue efforts.
  • Transportation difficulties - Tropical cyclones often destroy key bridges, overpasses, and roads, complicating efforts to transport food, clean water, and medicine to the areas that need it.

Beneficial effects of tropical cyclones

The human toll of cyclones cannot be overlooked. 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, the destruction caused by Camille on the Gulf coast spurred redevelopment, multiplying many times the land values that existed before the storm. However, disaster officials point out that this is not necessarily a good thing; it just encourages more people to live in what is clearly a danger area for deadly storms.

Additionally, hurricanes actually help to maintain global heat balance by moving warm, moist tropical air to the mid-latitudes and polar regions.

Notable cyclones

Main article: List of notable tropical cyclones

Tropical cyclones that cause massive destruction are fortunately rare, but when they happen, they can cause damage in the thousands of lives and the billions of dollars.

The deadliest tropical cyclone on record is a 100 mph (160 km/h, Category 2) storm that hit the densely populated Ganges Delta region of East Pakistan (now Bangladesh) on November 13, 1970. It killed anywhere from 200,000 to 500,000 people. The Indian Ocean basin has historically been the deadliest, with three storms since 1900 killing over 100,000 people, each in Bangladesh. [4] (http://encarta.msn.com/media_701500587_761565992_-1_1/Major_Hurricanes_Typhoons_Cyclones_and_other_Storms_since_1900.html)

In the Atlantic basin, three storms have killed more than 10,000 people. Hurricane Mitch during the 1998 Atlantic hurricane season caused severe flooding and mudslides in Honduras, killing about 18,000 people and changing the landscape enough that entirely new maps of the nation were needed. The Galveston Hurricane of 1900, which made landfall at Galveston, Texas as an estimated Category 4 storm, killed 6,000 to 12,000 people, and remains the deadliest natural disaster in the history of the United States. The deadliest Atlantic storm on record was the Great Hurricane of 1780, which killed between 20,000 and 30,000 people in the Antilles.

The most costly storm was 1992's Hurricane Andrew, which caused an estimated $25 billion in damage in Florida and the U.S. Gulf Coast, and remains the most destructive natural disaster in United States history.

Missing image
Typhoonsizes.jpg
The relative sizes of Typhoon Tip, Tropical Cyclone Tracy, and the United States.

The most intense storm on record was Typhoon Tip in the northwestern Pacific Ocean in 1979, which had a minimum pressure of only 870 mb and maximum sustained windspeeds of 190 mph (305 km/h). Fortunately, it weakened before striking Japan. Tip does not, however, hold alone the record for fastest sustained winds in a cyclone; Typhoon Keith in the Pacific, and Hurricane Camille and Hurricane Allen in the North Atlantic currently share this record as well [5] (http://www.weatherwatchers.org/tropical/1998/13/mitch.html), although recorded windspeeds that fast are suspect, since most monitoring equipment is likely to be destroyed by such conditions.

Camille was the only storm to actually strike land while at that intensity, making it, with 190 mph (305 km/h) sustained winds and 210 mph (335 km/h) gusts, the strongest tropical cyclone of record to ever hit land. For comparison, these speeds are encounted at the center of a strong tornado, but Camille was much larger and long-lived than any tornado.

Typhoon Nancy in 1961 had recorded windspeeds of 213 mph (343 km/h), but recent research indicates that windspeeds from the 1940s to the 1960s were gauged too high, and this is no longer considered the fastest storm on record. [6] (http://www.aoml.noaa.gov/hrd/tcfaq/E1.html) Similarly, a gust caused by Super Typhoon Paka over Guam was recorded at 236 mph (380 km/h); however, this reading had to be discarded, since the anemometer was damaged by the storm. Had it been confirmed, this would be the strongest wind ever recorded at the Earth's surface. (The current record is held by a non-hurricane wind registering 231 mph (372 km/h) at Mount Washington in New Hampshire.) [7] (http://www.guam.navy.mil/weather.htm)

Tip was also the largest cyclone on record, with a circulation 1,350 miles (2,170 km) wide. The average tropical cyclone is only 300 miles (480 km) wide. The smallest storm on record, 1974's Cyclone Tracy, which devastated Darwin, Australia, was roughly 30 miles (50 km) wide. [8] (http://hurricanes.noaa.gov/prepare/structure.htm)

Hurricane Iniki in 1992 was the most powerful storm to strike Hawaii in recorded history, hitting Kauai as a Category 4 hurricane, killing six and causing $3 billion in damage.

The first recorded South Atlantic hurricane
Enlarge
The first recorded South Atlantic hurricane

On March 26, 2004, Cyclone Catarina became the first recorded South Atlantic hurricane. 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.

A tropical cyclone need not be particularly strong to cause memorable damage; Tropical Storm Allison in 2001 had its name retired for killing 41 people and causing over $5 billion damage in Texas, even though it never became a hurricane. Hurricane Jeanne in 2004 was only a tropical storm when it made a glancing blow on Haiti, but the flooding and mudslides it caused killed over 3,000 people.

Naming of tropical cyclones

Storms reaching tropical storm strength (winds exceeding 17 metres per second, 38 mph, or 62 km/h) are given names, to assist in recording insurance claims, to assist in warning people of the coming storm, and to further indicate that these are important storms that should not be ignored. 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.

See also: Lists of tropical cyclone names

History of tropical cyclone naming

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.

The modern method of ascribing peoples names to storms was introduced by Clement Wragge, an Anglo-Australian meterologist at the end of the 19th century. As well as girls names, he also used the names of politicians who had offended him.

During World War II, tropical cyclones were given female names, mainly for the convenience of the forecasters and in a somewhat ad hoc manner. For a few years afterwards, names from the Joint Army/Navy Phonetic Alphabet were used.

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, beginning in 1953 the practice of systematically naming tropical storms and hurricanes was initiated by the United States National Hurricane Center, and is 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. This was also the first year that a list of names was prepared before the season began.

The naming process

The WMO's 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). Names of storms may be retired at the request of affected countries if they have caused extensive damage to life and property.

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. Japan Meteorological Agency uses a secondary naming system in Western North Pacific that numbers a typhoon on the order it formed resetting on December 31 of every year. The Typhoon Songda in September 2004 is internally called the typhoon number 18 and is recorded as the typhoon 0418 with 04 taken from the 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.

See also

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