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Hurricane and Typhoon redirect here. For other uses, see Hurricane (disambiguation) and Typhoon (disambiguation).
Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004.
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Hurricane Isabel in 2003 as seen from the International Space Station
Hurricane Isabel

A tropical cyclone is a storm system characterized by a large low-pressure center, a closed low-level circulation and a spiral arrangement of numerous thunderstorms that produce strong winds and heavy rainfall. Tropical cyclones feed on the heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as Nor'easters, European windstorms and polar lows, leading to their classification as "warm core" storm systems. Most tropical cyclones originate in the doldrums, approximately ten degrees from the Equator.

The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, as well as to their formation in maritime tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with anticlockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on its location and intensity, a tropical cyclone may be referred to by names such as "hurricane", "typhoon", "tropical storm", "cyclonic storm", "tropical depression" or simply "cyclone".

Types of cyclone: 1. A "Typhoon" is a tropical cyclone located in the North-west Pacific Ocean which has the most cyclonic activity and storms occur year-round. 2. A "Hurricane" is also a tropical cyclone located at the North Atlantic Ocean or North-east Pacific Ocean which have an average storm activity and storms typically form between May 15 and November 30. 3. A "Cyclone" is a tropical cyclone that occurs in the South Pacific and Indian Oceans.

Terms for tropical cyclones

Eye of Typhoon Odessa, Pacific Ocean, August 1985

Depending on the region, different terms are used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h):

  • hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline
  • 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 and South Pacific Ocean east of 160°E.
  • cyclone unofficially in the South Atlantic Ocean

In other areas, hurricanes have been called Bagyo in the Philippines and Taino in Haiti.

Etymology

The word typhoon has two possible origins:

  • From the Chinese 大風 (daaih fūng (Cantonese); dà fēng (Mandarin)) which means "great wind". (The Chinese term as 颱風 táifēng, and 台風 taifu in Japanese, has an independent origin traceable variously to 風颱, 風篩 or 風癡 hongthai, going back to Song 宋 (960-1278) and Yuan 元(1260-1341) dynasties. The first record of the character 颱 appeared in 1685's edition of Summary of Taiwan 臺灣記略).
  • From Urdu, Persian or Arabic ţūfān (طوفان) < Greek tuphōn (Τυφών).

Portuguese tufão is also related to typhoon. See tuphōn for more information.

The word hurricane is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracán.

The word cyclone came from the Greek word "κύκλος", meaning "circle".

Overall mechanics of a tropical cyclone

Hurricanes form when the energy released by the condensation of moisture in rising air causes a positive feedback loop. The air heats up, rising further, which leads to more condensation. The air flowing out of the top of this “chimney” drops towards the ground, forming powerful winds.

Structurally, a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. Its primary energy source is the release of the heat of condensation from water vapor condensing at high altitudes, the heat ultimately derived from the sun. Therefore, a tropical cyclone can be thought of as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth. Continued condensation leads to higher winds, continued evaporation, and continued condensation, feeding back into itself. This gives rise to factors that give the system enough energy to be self-sufficient and cause a positive feedback loop where it can draw more energy as long as the source of heat, warm water, remains. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The orbital revolution of the Earth causes the system to spin, giving it a cyclone characteristic and affecting the trajectory of the storm.

The factors to form a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist and allow it to create a feedback loop by maximizing the energy intake possible, for example, such as high winds to increase the rate of evaporation, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.

Condensation as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena, and because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones, for example, draw their energy mostly from pre-existing horizontal 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 condensation of this moisture is driven by the high winds and reduced atmospheric pressure in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly.

Scientists at the National Center for Atmospheric Research estimate that a hurricane releases heat energy at the rate of 50 to 200 trillion watts -- about the amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes [1].

While the most obvious motion of clouds is toward the center, tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine. This outflow produces high, thin cirrus clouds that spiral away from the center. The high cirrus clouds may be the first signs of an approaching hurricane.

Formation

Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.

The formation of tropical cyclones is the topic of extensive ongoing 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 (79.7 degrees Fahrenheit) to at least a depth of 50 meters (164 feet). The moisture in the air above the warm water is the energy source for tropical cyclones.
  2. Upper-atmosphere conditions conducive to thunderstorm formation. Temperature 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 tropical oceans.
  4. A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan was the strongest storm to form closer than 10 degrees from the equator; it started forming at 9.7 degrees north.)
  5. Low vertical wind shear (change in wind speed or direction over height). High wind shear can break apart the vertical structure of a tropical cyclone.

Tropical cyclones occasionally form despite not meeting these conditions.

Only specific weather disturbances can result in tropical cyclones. These include:

  1. Tropical waves, or easterly waves, which, as mentioned above, are westward moving areas of convergent winds. This often assists in the development of thunderstorms, which can develop into tropical cyclones. Most tropical cyclones form from these. A similar phenomenon to tropical waves are West African disturbance lines, which are squally lines of convection that form over Africa and move into the Atlantic.
  2. Tropical upper tropospheric troughs, which are cold-core upper level lows. A warm-core tropical cyclone may result when one of these (on occasion) works down to the lower levels and produces deep convection.
  3. Decaying frontal boundaries may occasionally stall over warm waters and produce lines of active convection. If a low level circulation forms under this convection, it may develop into a tropical cyclone.

Times of formation

Tropical storms and hurricanes by month, for the period 1944-2005
(North Atlantic region)
Month Total Average
January–April 4 0.1
May 8 0.1
June 35 0.6
July 58 0.9
August 173 2.8
September 224 3.6
October 114 1.8
November 32 0.5
December 6 0.1
Source: NOAA + additions for 2001-05

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 from late August through September. The statistical peak of the North Atlantic hurricane season is September 10. 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.

Worldwide, an average of 80 tropical cyclones form each year.

Locations of formation

Most tropical cyclones form in a worldwide band of thunderstorm activity called the Intertropical convergence zone (ITCZ).

Nearly all of them form between 10 and 30 degrees of the equator and 87% form within 20 degrees of it. Because the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator [2], where the Coriolis effect is weakest. However, it is possible for tropical cyclones to form within this boundary if there is another source of initial rotation. These conditions are extremely rare, and such storms are believed to form at most once per century. Hurricane Ivan of 2004 developed within 10 degrees of the equator. A combination of a pre-existing disturbance, upper level divergence and a monsoon-related cold spell led to Typhoon Vamei at only 1.5 degrees north of the equator in 2001. It is estimated that such conditions occur only once every 400 years.

Major basins

There are seven main basins of tropical cyclone formation:

Unusual formation areas

Hurricane Vince on October 9, 2005 at 2300 UTC near the Madeira Islands.

The following areas spawn tropical cyclones only very rarely.

  • Southern Atlantic Ocean: A combination of cooler waters, the lack of an ITCZ, 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, Hurricane Catarina (sometimes also referred to as Aldonça), which made landfall in Brazil in 2004 as a Category 1 hurricane, and a smaller storm in January 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.
  • Eastern South Pacific: Tropical cyclone formation is rare in this region; when they do form, it is frequently linked to El Niño episodes. Most of the storms that enter this region formed farther west in the Southwest Pacific. They affect the islands of Polynesia in exceptional instances.
  • 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. However, there is debate on whether these storms were tropical in nature.
  • Northeastern Atlantic Ocean: In October 2005, Hurricane Vince formed near Madeira, then moved northeastward, passing south of the Portuguese south coast, and made landfall in southwestern Spain as a tropical storm. Vince's origin was the northernmost in the eastern Atlantic ever recorded, and Vince was the first storm in recorded history to reach the Iberian Peninsula as a tropical cyclone, i.e. before being transformed into an extratropical low or absorbed into other systems of low pressure.
  • Australia: SW Pacific Basin includes the eastern part of Australia and the Fiji area.
  • Australia: SE Indian Basin includes the eastern part of the Indian ocean and the northern and western part of the Australian basin.
  • Southern South China Sea Tropical cyclones normally do not develop in the Southern South China Sea due to its close proximity to the equator. Areas within ten degrees laditude of the equator do not experience a significant coriolis force, a vital ingredient in tropical cyclone formation. However, in December 2001, Typhoon Vamei formed in the Southern South China Sea and made landfall in Malaysia. It caused flooding in southern Malaysia and some damage in Singapore. It formed from a thunderstorm formation in Borneo that moved into the South China Sea.

Average Season

Basin Season Start Season End Tropical Storms (34-63 knots) Tropical Cyclones (>63 knots) Category 3+ Tropical Cyclones (>95 knots)
Northwest Pacific Year Round Year Round 26.7 16.9 8.5
Northeast Pacific May November 16.3 9.0 4.1
Southwest Indian October May 13.3 6.7 2.7
North Atlantic June November 10.6 5.9 2.0
Australia Southwest Pacific October May 10.6 4.8 1.9
Australia Southeast Indian October May 7.3 3.6 1.6
North Indian April December 5.4 2.2 0.4

Structure and classification

Structure of a hurricane

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 very intense thunderstorm activity that make up the inner portion of the hurricane. This contains the eye wall, and the eye itself. The classic hurricane contains a symmetrical CDO, which means that it is perfectly circular and round on all sides.
  • 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: It is the area directly around the eye of the cyclone where the winds are the highest, the clouds reach furthest into the atmosphere and the precipitation is the heaviest. The heaviest damage caused by tropical cyclones occurs where the eyewall crosses over land.
  • 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.

Intensities of tropical cyclones

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. Government weather services assign first names to systems that reach this intensity (thus the term named storm).

At hurricane intensity, a tropical cyclone tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of the circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 10 to 50 miles (16 to 80 kilometers) wide in which the strongest thunderstorms and winds circulate around the storm's center.

The circulation of clouds around a cyclone's center imparts a distinct spiral shape to the system. Bands or arms may extend over great distances as clouds are drawn toward the cyclone. The direction of the cyclonic circulation depends on the hemisphere; it is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 85 m/s (165 knots, 190 mph, 305 km/h). Intense, mature hurricanes can sometimes exhibit an inward curving of the eyewall top that resembles a football stadium: this phenomenon is thus sometimes referred to as stadium effect.

Eyewall replacement cycles naturally occur in intense tropical cyclones. When cyclones reach peak intensity they usually - but not always - have an eyewall and radius of maximum winds that contract to a very small size, around 5 to 15 miles. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger.

Categories and ranking

Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds, a Category 5 hurricane has the highest. The rankings are not absolute in terms of effects. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. In fact, tropical systems of less than hurricane strength can produce significant damage and human casualties, especially from flooding and landslides.

The National Hurricane Center classifies hurricanes of Category 3 and 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) and used by most weather agencies is that of a 10-minute average. The U.S. weather service defines sustained winds based on 1-minute average speed measured about 10 meters (33 ft) above the surface.

Other storm systems

An extratropical cyclone is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses. From space, extratropical storms have a characteristic "comma-shaped" cloud pattern. Extratropical cyclones can also be dangerous because their low-pressure centers cause 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 winds but are not given individual names. However, two powerful extratropical cyclones that ravaged France, Germany, and the United Kingdom in December 1999, "Lothar" and "Martin", were named due to their unexpected power (equivalent to a category 1 or 2 hurricane). 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 a polar low.

Movement and track

Large-scale winds

Although tropical cyclones are large systems generating enormous energy, their movements over the earth's surface are often compared to that of leaves carried along by a stream. That is, large-scale winds—the streams in the earth's atmosphere—are responsible for moving and steering tropical cyclones. The path of motion is referred to as a tropical cyclone's track.

The major force affecting the track of tropical systems in all areas are winds circulating around high-pressure areas. Over the North Atlantic Ocean, tropical systems are steered generally westward by the east-to-west winds on the south side of the Bermuda High, a persistent high-pressure area over the North Atlantic. Also, in the area of the North Atlantic where hurricanes form, trade winds, which are prevailing westward-moving wind currents, steer tropical waves (precursors to tropical depressions and cyclones) westward from off the African coast toward the Caribbean and North America.

Coriolis effect

The earth's rotation also imparts an acceleration (termed the Coriolis Acceleration or Coriolis Effect). This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents (i.e. in the north, the northern part of the cyclone has winds to the west, and the Coriolis force pulls them slightly north. The southern part is pulled south, but since it is closer to the equator, the Coriolis force is a bit weaker there). Thus, tropical cyclones in the Northern Hemisphere, which commonly move west in the beginning, normally turn north (and are then usually blown east), and cyclones in the Southern Hemisphere are deflected south, if no strong pressure systems are counteracting the Coriolis Acceleration. The Coriolis acceleration also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds. (Much of that is due to the conservation of angular momentum - air is drawn in from an area much larger than the cyclone such that the tiny angular velocity of that air will be magnified greatly when the distance to the storm center shrinks.)

Interaction with high and low pressure systems

Finally, when a tropical cyclone moves into higher latitude, its general track around a high-pressure area can be deflected significantly by winds moving toward a low-pressure area. Such a track direction change is termed recurve. A hurricane moving from the Atlantic toward the Gulf of Mexico, for example, will recurve to the north and then northeast if it encounters winds blowing northeastward toward a low-pressure system passing over North America. Many tropical cyclones along the U.S. East Coast and in the Gulf of Mexico are eventually forced toward the northeast by low-pressure areas which move from west to east over North America.

Forecasting

Hurricane Epsilon strengthened and organized in the Central North Atlantic Ocean despite highly unfavorable conditions. This unusual system defied most NHC forecasts and demonstrated the difficulties of predicting tropical cyclones.

Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system.

With their understanding of the forces that act on tropical cyclones, and a wealth of data from earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. High-speed computers and sophisticated simulation software allow forecasters to produce computer models that forecast tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. But while track forecasts have become more accurate than 20 years ago, scientists say they are less skillful at predicting the intensity of tropical cyclones. They attribute the lack of improvement in intensity forecasting to the complexity of tropical systems and an incomplete understanding of factors that affect their development.

Landfall

Officially, "landfall" is when a storm's center (the center of the eye, not its edge) reaches land. Naturally, storm conditions may be experienced on the coast and inland well before landfall. In fact, for a storm moving inland, the landfall area experiences half the storm before the actual landfall. For emergency preparedness, actions should be timed from when a certain wind speed will reach land, not from when landfall will occur.

Unusual landfall areas

The following areas rarely have a recorded landfall of a tropical cyclone:

Europe: Because of its high latitude, Europe has had only one recorded landfall made by a tropical system (Hurricane Vince of 2005)[citation needed] and relatively few landfalls by extratropical remnants of tropical cyclones such as Hurricane Debbie of 1961 and Hurricane Charley of 1986.

Azores: Like Europe, the Azores have a some recorded landfalls of hurricanes and tropical storms.

Canary Islands: Until Tropical Storm Delta of 2005, the Canary Islands were rarely affected by any tropical storm or hurricanes.

West African Coast: No recorded landfall of a tropical storm or hurricane although some come close but bypass the area.

Cape Verde Islands: Some records of landfall made by a tropical storm or hurricane, most notably 1982's Tropical Storm Beryl that killed 115 people.

Venezuela: Rarely a tropical storm or hurricane makes landfall in this country. Notable examples are 1993's Tropical Storm Bret and Hurricane Joan of 1988.

California: Rarely a tropical storm or hurricane have ever affected California. Notable storms were a tropical storm in 1939 and a hurricane in 1858.

New Zealand: On rare circumstances, a cyclone or two have made landfall in that country.

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. The storm loses strength slower over flatter or marshy areas than over mountainous terrain which disrupts the surface circulation of the storm more.
  • It remains in the same area of ocean for too long, sucking up all the warm water. Without warm surface water, the storm cannot survive.
  • It experiences wind shear, causing the convection to lose direction and the heat engine to break 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. (Such, however, can re-strengthen the non-tropical system as a whole.)
  • 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.
  • An outer eye wall forms (typically around 50 miles from the center of the storm), choking off the convection toward the inner eye wall. Such weakening is generally temporary unless it meets other conditions above.

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

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. The winds of Hurricane Debbie dropped as much as 30 percent, but then regained their strength after each of two seeding forays. In an earlier episode, disaster struck when a hurricane east of Jacksonville, Florida, was seeded, promptly changed its course, and smashed into Savannah, Georgia. Because there was so much uncertainty about the behavior of these storms, the federal government would not approve seeding operations unless the hurricane had a less than 10 percent chance of making landfall within 48 hours. This placed severe restrictions on the project, and when the Navy pulled out in 1972, it all but killed any further attempts at hurricane seeding in the Atlantic.

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 [3]. However, it has been suggested by some that we can change the course of a storm during its early stages of formation, (detailed by an article, Controlling Hurricanes, Scientific American, 2005), such as using satellite to alter the environmental conditions or, more realistically, spreading degradable film of oil over the ocean, which prevent water vapour from fueling the storm.

Monitoring, observation and tracking

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 available only 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 possible only 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 [4]. The aircraft used are WC-130 Hercules and WP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft fly directly into the cyclone and take direct and remote-sensing measurements. The aircraft also launch GPS dropsondes inside the cyclone. These sondes measure temperature, humidity, pressure, and especially winds between flight level and the ocean's surface.

A new era in hurricane observation began when a remotely piloted Aerosonde, a small drone aircraft, was flown through Tropical Storm Ophelia as it passed Virginia's Eastern Shore during the 2005 hurricane season. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare[5].

Tropical cyclones far from land are tracked by weather satellites using visible light and infrared bands. These satellite images are received regularly on half hour intervals. As the hurricane approaches land, the cyclone can also be imaged remotely by a nationwide system of Doppler radar. Land-based Doppler radars play a crucial role during landfall because they give forecasters the ability to see the storms location and intensity minute by minute.

Recently, university researchers have begun to deploy mobile weather stations fortified to withstand hurricane-force winds. The two largest programs are the Florida Coastal Monitoring Program [6] and the Wind Engineering Mobile Instrumented Tower Experiment [7]. During landfall, the NOAA Hurricane Research Division compares and quality controls reconnaissance aircraft data—which include flight-level, GPS sonde and stepped frequency microwave radiometer wind speed estimates—to wind speed data transmitted in real-time from weather stations erected near or at the coast. The National Hurricane Center uses these data to evaluate conditions at landfall and to verify its forecasts.

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 (if there were any) are "retired" and new names are chosen to take their place.

See also: Lists of tropical cyclone names

Naming schemes

The WMO's Regional Association IV Hurricane Committee selects the names for Atlantic Basin and central and eastern Pacific storms.

In the Atlantic and Eastern North Pacific regions, feminine and masculine names are assigned alternately in alphabetic order during a given season. The "gender" of the season's first storm also alternates year to year: the first storm of an odd-numbered year gets feminine name, while the first storm of an even-numbered year gets a masculine name. Six lists of names are prepared in advance, and each list is used once every six years. Five letters — "Q," "U," "X," "Y" and "Z" — are omitted in the Atlantic; only "Q" and "U" are omitted in the Eastern Pacific, so the format accommodates 21 or 24 named storms in a hurricane season. Names of storms may be retired by request of affected countries if they have caused extensive damage. The affected countries then decide on a replacement name of the same gender (and if possible, the same ethnicity) as the name being retired.

If there are more than 21 named storms in an Atlantic season or 24 named storms in an Eastern Pacific season, the rest are named as letters from the Greek alphabet: the 22nd storm is called "Alpha," the 23rd "Beta," and so on. This was first necessary during the 2005 season when the names Alpha, Beta, Gamma, Delta, and Epsilon were all used. There is no precedent for a storm named with a Greek Letter causing enough damage to justify retirement; how this situation would be handled is unknown.

Further information: List of notable tropical cyclones

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.

History of tropical cyclone naming

For several hundred years after Europeans arrived in the West Indies, hurricanes there were named after the saint's day on which the storm struck.

The practice of giving storms people's names was introduced by Clement Wragge, an Anglo-Australian meteorologist at the end of the 19th century. He used feminine names and the names of politicians who had offended him.

During World War II, tropical cyclones were given feminine 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 feminine. 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 masculine names to the nomenclature. It was also in 1979 that the practice of preparing a list of names before the season began. The names are usually of English, French or Spanish origin in the Atlantic basin, since these are the three predominant languages of the region where the storms typically form.

Renaming of tropical cyclones

In most cases, a tropical cyclone retains its name throughout its life. However, a tropical cyclone may be renamed in several occasions.

1. A tropical storm enters the southwestern Indian Ocean from the east

In the south Indian Ocean, RSMC la Reunion names a tropical storm once it crosses 90°E from the east, even though it has been named. In this case, the Joint Typhoon Warning Center (JTWC) will put two names together with a hyphen.

Examples: Bertie-Alvin (2005)

2. A tropical storm crosses from the Atlantic into the Pacific, or vice versa, before 2001

It was the policy of National Hurricane Center (NHC) to rename a tropical storm which crossed from Atlantic into Pacific, or vice versa.

Examples: Cesar-Douglas (1996), Joan-Miriam (1988)

In 2001, when Iris moved across Central America, NHC mentioned that Iris would retain its name if it regenerated in the Pacific. However, the Pacific tropical depression developed from the remnants of Iris was called Fifteen-E instead. The depression later became tropical storm Manuel.

NHC explained that the Iris had dissipated as a tropical cyclone prior to entering the eastern North Pacific basin, the new depression was properly named Fifteen-E, rather than Iris.

In 2003, when Larry was about to move across Mexico, NHC attempted to provide greater clarity:

Should Larry remain a tropical cyclone during its passage over Mexico into the Pacific, it would retain its name. However, a new name would be given if the surface circulation dissipates and then regenerates in the Pacific.

Up to now, there has been no tropical cyclone retaining its name during the passage from Atlantic to Pacific, or vice versa.

3. Uncertainties of the continuation

When the remnants of a tropical cyclone redevelop, the redeveloping system will be treated as a new tropical cyclone if there are uncertainties of the continuation, even though the original system may contribute to the forming of the new system.

Example: TD 10-TD 12 (2005)

4. Human faults

Sometimes, there may be human faults leading to the renaming of a tropical cyclone.

Example: Ken-Lola (1989)

Effects

The aftermath of Hurricane Katrina in Gulfport, Mississippi. Katrina was the most costly tropical cyclone in United States history.

A mature tropical cyclone can release heat at a rate upwards of 6x1014 watts [8]. 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.
Graphic illustrating storm surge

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. One of the most common post-hurricane injuries is stepping on a nail in storm debris, leading to a risk of tetanus or other infection. Infections of cuts and bruises can be greatly amplified by wading in sewage-polluted water.
  • Power outages - Tropical cyclones often knock out power to tens or hundreds of thousands of people (or occasionally millions if a large urban area is affected), 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

Although cyclones take an enormous toll in lives and personal property, they may bring much-needed precipitation to otherwise dry regions. Hurricane Camille averted drought conditions and ended water deficits along much of its path. Hurricane Floyd did the same thing in New Jersey in 1999. The destruction caused by Camille on the Gulf coast spurred redevelopment as well, greatly increasing local property values. On the other hand, disaster response officials point out that redevelopment encourages more people to live in clearly dangerous areas subject to future deadly storms (as shown by the effects of Hurricane Katrina). Of course, many former residents and businesses do relocate to inland areas away from the threat of future hurricanes as well.

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

While the number of storms in the Atlantic has increased since 1995, there seems to be no signs of a global trend; the global number of tropical cyclones remains about 90 ± 10. [9].

Atlantic storms are certainly becoming more destructive financially, since five of the ten most expensive storms in United States history have occurred since 1990. This can to a large extent be attributed to the number of people living in susceptible coastal area, and massive development in the region since the last surge in Atlantic hurricane activity in the 1960s.

Often in part because of the threat of hurricanes, many coastal regions had sparse population between major ports until the advent of automobile tourism; therefore, the most severe portions of hurricanes striking the coast often went unmeasured. The combined effects of ship destruction and remote landfall severely limit the number of intense hurricanes in the official record before the era of hurricane reconnaissance aircraft and satellite meteorology. Although the record shows a distinct increase in the number and strength of intense hurricanes, therefore, experts regard the early data as suspect.

The number and strength of Atlantic hurricanes may undergo a 50-70-year cycle. Although more common since 1995, few above-normal hurricane seasons occurred during 1970-1994. Destructive hurricanes struck frequently from 1926-60, including many major New England hurricanes. A record 21 Atlantic tropical storms formed in 1933, only recently exceeded in 2005. Tropical hurricanes occurred infrequently during the seasons of 1900-1925; however, many intense storms formed 1870-1899. During the 1887 season, 19 tropical storms formed, of which a record 4 occurred after 1 November and 11 strengthened into hurricanes. Few hurricanes occurred in the 1840s to 1860s; however, many struck in the early 1800s, including an 1821 storm that made a direct hit on New York City which some historical weather experts say may have been as high as Category 4 in strength.

These unusually active hurricane seasons predated satellite coverage of the Atlantic basin that now enables forecasters to see all tropical cyclones. Before the satellite era began in 1961, tropical storms or hurricanes went undetected unless a ship reported a voyage through the storm. The official record, therefore, probably misses many storms in which no ship experienced gale-force winds, recognized it as a tropical storm (as opposed to a high-latitude extra-tropical cyclone, a tropical wave, or a brief squall), returned to port, and reported the experience.

Global warming?

A common question is whether global warming can or will cause more frequent or more fierce tropical cyclones. So far, virtually all climatologists seem to agree that a single storm, or even a single season, cannot clearly be attributed to a single cause such as global warming or natural variation [10]. The question is thus whether a statistical trend in frequency or strength of cyclones exists. The U.S. National Oceanic and Atmospheric Administration says in their Hurricane FAQ that "it is highly unlikely that global warming has (or will) contribute to a drastic change in the number or intensity of hurricanes." [11].

Regarding strength, a similar conclusion was consensus until recently. This consensus is now questioned by K. Emanuel (2005) (Nature 436, 686–688, preprint). In this article, K. Emanuel states that the potential hurricane destructiveness, a measure which combines strength, duration, and frequency of hurricanes, "is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multidecadal oscillations in the North Atlantic and North Pacific, and global warming." K. Emanuel further predicts "a substantial increase in hurricane-related losses in the twenty-first century".

Along similar lines, P.J. Webster et al. published an article in Science 309, 1844-1846 examining "changes in tropical cyclone number, duration, and intensity" over the last 35 years, a period where satellite data is available. The main finding is that while the number of cyclones "decreased in all basins except the North Atlantic during the past decade" there is a "large increase in the number and proportion of hurricanes reaching categories 4 and 5". I.e., while the number of cyclones decreased overall, the number of very strong cyclones increased.

Both, Emanuel and Webster et al., consider the sea surface temperature as of key importance in the development of cyclones. The question then becomes: what caused the observed increase in sea surface temperatures? In the Atlantic, it could be due to the Atlantic Multidecadal Oscillation (AMO), a 50–70 year pattern of temperature variability. K. Emanuel, however, found the recent temperature increase was outside the range of previous oscillations. So, both a natural variation (such as the AMO) and global warming could have made contributions to the warming of the tropical Atlantic over the past decades, but an exact attribution is so far impossible to make. [12]

While Emanuel analyzes total annual energy dissipation, Webster et al. analyze the slightly less relevant percentage of hurricanes in the combined categories 4 and 5, and find that this percentage has increased in each of six distinct hurricane basins: North Atlantic, North East and North West Pacific, South Pacific, and North and South Indian. Because each individual basin may be subject to intra-basin oscillations similar to the AMO, any single-basin statistic remains open to question. But if the local oscillations are not synchronized by some as-yet-unidentified global oscillation, the independence of the basins allows joint statistical tests that are more powerful than any set of individual basin tests. Unfortunately Webster et al. do not undertake any such test.

Under the assumption that the six basins are statistically independent except for the effect of global warming, Stoft has carried out the obvious paired t-test and found that the null-hypothesis of no impact of global warming on the percentage of category 4 & 5 hurricanes can be rejected at the 0.1% level—there is only a 1 in 1000 chance of simultaneously finding the observed six increases in the cat-4&5 percentages. This statistic needs refining because the variables being tested are not normally distributed with equal variances, but it may provide the best evidence yet that the impact of global warming on hurricane intensity has been detected.

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. [13]

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 country 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 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 mbar and maximum sustained wind speeds of 190 mph (305 km/h). It weakened before striking Japan. Tip does not hold the record for fastest sustained winds in a cyclone alone; Typhoon Keith in the Pacific, and Hurricane Camille and Hurricane Allen in the North Atlantic currently share this record as well [14], although recorded wind speeds 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 encountered at the center of a strong tornado, but Camille was much larger and long-lived than any tornado.

Typhoon Nancy in 1961 had recorded wind speeds of 213 mph (343 km/h), but recent research indicates that wind speeds from the 1940s to the 1960s were gauged too high, and this is no longer considered the fastest storm on record. [15] 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 non-tornadic 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.) [16]

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. [17]

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

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 June 2001 had its name retired for killing 41 people and causing over $5 billion damage in East Texas, even though it never became a hurricane; the damage from Allison was mostly due to flooding, not winds or storm surge. 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.

On August 28 2005, Hurricane Katrina made landfall to Louisiana and nearby states. The U.S. National Hurricane Center, in its August review of the tropical storm season stated that Katrina was probably the worst natural disaster in U.S. history. Its death toll is above 1000, mainly from flooding and the aftermath. It is also estimated to have caused $200 billion in damages. Before that, the most costly (in money, not human terms) storm had been 1992's Hurricane Andrew, which caused an estimated $25 billion in damage in Florida.

See also

Look up tropical cyclone in Wiktionary, the free dictionary.

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