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 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.
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 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.
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 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.
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.
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 to 63 knots, 39 to 73 mph, or 62 to 117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present.
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:
This is the intensity at which tropical cyclones tend to develop an eye, which is an area of relative calm surrounded by the strongest winds of the storm, in the eyewall. The strongest of these storms have had maximum sustained windspeeds recorded at 85 meters per second (165 knot, 190 mph, 305 km/h).
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; a strong Category 4 storm) as Super Typhoons.
The definition of sustained winds recommended by the World Meteorological Organization (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.
An extratropical cyclone is a storm that was once tropical in nature. However, once it passed over land or cool waters, its energy source changed from released heat from condensing water to the difference in temperature between air masses. From space, these storms resemble a comma. 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.
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  (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.
There are seven main basins of tropical cyclone formation:
Unusual formation areas
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.
Formation and development
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 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.
A tropical cyclone can cease to have tropical characteristics in several ways:
Even after a tropical cyclone is said to be extratropical or dissipated, it can still have gale-force winds and drop several inches of rainfall.
Attempts to dissipate cyclones
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 these eyewall replacement cycles happen naturally, and so the success of the program was impossible to gauge.
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.
A mature tropical cyclone can release heat at a rate upwards of 6x1014 watts. (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.
Often, the secondary effects of a tropical cyclone are equally damaging. They include:
Beneficial effects of tropical cyclones
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, 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 northward to the mid-latitudes and polar regions.
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.  (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 at least 10,000 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 and 30 thousand people in the Antilles.
The costliest 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.
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  (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 to ever hit land. For comparison, these speeds are encounted at the center of a strong tornado, but Camille was much larger 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.  (http://www.aoml.noaa.gov/hrd/tcfaq/E1.html) Similarly, a gust caused by 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.)  (http://www.guam.navy.mil/weather.htm)
Tip was also the largest cyclone on record, with a circulation of 1,350 miles (2,170 km) wide. The average tropical cyclone is only 300 miles (480 km) wide. (http://hurricanes.noaa.gov/prepare/structure.htm)
On December 25 1974, Tropical Cyclone Tracy hit Darwin, Australia. It was the most devastating natural disaster to have ever hit an Australian city, destroying around 70% of the homes in Darwin. Fifty people died in Darwin, and 16 at sea. Authorities had managed to evacuate most of Darwin. Although Cyclone Tracy was quite small — the smallest cyclone on record, in fact, little larger than 30 miles (48 km) wide — it was very severe, with winds of up to 135 mph (217 km/h). The damage was estimated to be close to $A400 million, which (at 2004 exchange rates) is approximately equal to $US 280 million.
On March 26, 2004, Cyclone Catarina became the first hurricane ever 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.
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 flooding and mudslides killed over 2,000 people.
Naming of tropical cyclones
Tropical cyclones with winds exceeding 17 metres per second 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. 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 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.
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.