Tropical cyclone

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In meteorology, a tropical cyclone is a storm system with a closed circulation around a center of low pressure, driven by heat energy released as moist air drawn in over warm ocean waters rises and condenses. The name underscores their origin in the tropics and their cyclonic nature. They are distinguished from other cyclonic storms, such as extratropical storms (such as nor'easters) and polar lows by the heat mechanism that fuels them.

Depending on their strength and location, a tropical cyclone can be called a tropical depression, tropical storm, hurricane, or typhoon, among other names.

They can carry extremely high winds, tornadoes, torrential rain, and storm surge onto coasts, leading to mudslides and flash floods in addition to wind damage. Though the effects on populations and ships can be catastrophic, tropical cyclones carry away heat that builds up in the tropics, and have been known to relieve and end droughts in areas they impact. They are a part of the larger atmospheric circulation that maintains equilibrium in the environment.

Contents

Mechanics of tropical cyclones

Image:Hurricane profile graphic.gif

Structurally, a tropical cyclone is a large, rotating system of clouds, wind, and thunderstorms. 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. Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy;<ref name=NHCC5C>NHC Tropical Cyclone FAQ Subject C5c accessed March 31, 2006</ref> the faster winds and lower pressure associated with them in turn cause increased surface evaporation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation.<ref name="NOAA Question of the Month">NOAA Question of the Month for August 2000 accessed March 31, 2006</ref> This gives rise to factors that provide the system with 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, an effect known as the Coriolis effect, giving it a cyclonic 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.<ref name="BOM Question 6">Bureau of Meteorology FAQ Question 6 accessed March 31, 2006</ref> Because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere.<ref name="BOM Question 6"/> 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 accelerated by the high winds and reduced atmospheric pressure in the storm, resulting in a positive feedback loop. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly.<ref name=NHCC2>NHC Tropical Cyclone FAQ Subject C2 accessed March 31, 2006</ref>

Scientists at the National Center for Atmospheric Research estimate that a hurricane releases heat energy at the rate of 50 to 200 trillion watts.<ref name="NOAA Question of the Month"/> By comparison, this is about the amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes or <ref name=UCAR>University Corporation for Atmospheric ResearchHurricanes: Keeping an eye on weather's biggest bullies accessed March 31, 2006</ref> or 200 times the total energy production capacity of the entire world.<ref name="NOAA Question of the Month"/>

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

Image:Atlantic hurricane graphic.gif

The formation of tropical cyclones is the topic of extensive ongoing research, and is still not fully understood. Six general factors are necessary to make tropical cyclone formation possible, although tropical cyclones may occasionally form despite not meeting these conditions:

  1. Water temperatures of at least 26.5 °C (80°F)<ref name="NHCA15">NHC Tropical Cyclone FAQ Subject A15 accessed March 30, 2006</ref> down to a depth of at least 50 m (150 feet). Waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.<ref name="NHCA16">NHC Tropical Cyclone FAQ Subject A16 accessed March 30, 2006</ref>
  2. Rapid cooling with height. This allows the release of latent heat, which is the source of energy in a tropical cyclone.<ref name="NHCA15"/>
  3. High humidity, especially in the lower-to-mid troposphere. When there is lots of moisture in the atmosphere, conditions are more favourable for disturbances to develop.<ref name="NHCA15"/>
  4. Low wind shear. When wind shear is high, the convection in a cyclone or disturbance will be disrupted, blowing the system apart.<ref name="NHCA15"/>
  5. Distance from the equator. This allows the Coriolis force to deflect winds blowing towards the low pressure center, causing a circulation. The approximate distance is 500 km (310 miles) or 10 degrees.<ref name="NHCA15"/>
  6. A pre-existing system of disturbed weather. The system must have some sort of circulation as well as a low pressure center.<ref name="NHCA15"/>

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.

Locations of formation

Most tropical cyclones form in a worldwide band of thunderstorm activity called the Intertropical Discontinuity (ITD), also called the Intertropical Convergence Zone (ITCZ).

Nearly all of these systems 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 <ref name=BOMmap>Bureau of Meteorology Worldwide Tropical Cyclone Tracks 1979-88</ref>, 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. 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.<ref name="USAToday Vamei">Template:Cite news</ref>

However, the hurricanes that enter the Atlantic Ocean off the coast of Africa have actually originated from the Indian Ocean. The storms develop over the Indian Ocean and head westward. As the disturbances move west, they hit eastern Africa, and the moisture from these disturbances builds up as the storm system moves over the mountains in eastern Africa. As the moisture leaves the mountains, high level winds propell it west as it crosses the continent of Africa. Once the disturbance has cleared Africa, it moves off the west coast of Africa by the Cape Verde Islands. Of course not all these storms turn into tropical storms or hurricanes, but the ones that do move into the warm areas of the Atlantic Ocean and develop into full-fledged hurricanes. Originally, these mammoth storms are formed by disturbances in the Indian Ocean.Template:Fact

Major basins

There are seven main basins of tropical cyclone formation. They are the north Atlantic Ocean, the eastern and western parts of the Pacific Ocean, the southwestern Pacific, and the southwestern and southeastern Indian Oceans, and the northern Indian Ocean. Worldwide, an average of 80 tropical cyclones form each year.<ref name=NHCE10>NHC Tropical Cyclone FAQ Subject E10 accessed March 31, 2006</ref>

Zones and Forecasters<ref name=NHCF1>NHC Tropical Cyclone FAQ Subject F1 accessed March 31, 2006</ref>
Basin WMO Regional Specialized Meteorological Center(s)
North Atlantic National Hurricane Center
Eastern North Pacific National Hurricane Center & Central Pacific Hurricane Center
Western North Pacific Japan Meteorological Agency
North Indian Indian Meteorological Service
South Pacific Fiji Meteorological Services & New Zealand Meteorological Service & Papua New Guinea National Weather Service & Australian Bureau of Meteorology
Eastern South Indian Australian Bureau of Meteorology
Western South Indian Méteo France
  • North Atlantic Basin: The most-studied of all tropical basins, it includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Tropical cyclone formation here varies widely from year to year, ranging from over twenty to one per year. The average is about ten. The United States Atlantic coast, Mexico, Central America, the Caribbean Islands and Bermuda are frequently affected by storms in this basin. Venezuela, the south-east of Canada and Atlantic "Macaronesian" islands are also occasionally affected. Hurricanes that strike Mexico, Central America, and Caribbean island nations, often do intense damage, as hurricanes are deadlier over warmer water. Additionally, they can hit the coast of the U.S., especially Florida, North Carolina, the U.S. Gulf Coast and occasionally New Jersey, New York and New England (usually hurricanes weaken to tropical storms before they reach these northern regions). The coast of Atlantic Canada receives hurricane landfalls on rare occasions, such as Hurricane Juan in 2003. Many of the more intense Atlantic storms are Cape Verde-type hurricanes, which form off the west coast of Africa near the Cape Verde islands.
  • Western North Pacific Ocean: Tropical storm activity in this region frequently affects China, Japan, the Philippines, and Taiwan, but also many other countries in South-East Asia, such as Vietnam, South Korea and Indonesia, plus numerous Oceanian islands. This is by far the most active basin, accounting for one-third of all tropical cyclone activity in the world. The eastern coasts of Taiwan and Philippines also have the highest tropical cyclone landfall frequency in the world.Template:Fact
  • Eastern North Pacific Ocean: This is the second most active basin in the world, and the most dense (a large number of storms for a small area of ocean). Storms that form here can affect western Mexico, Hawaii, northern Central America, and on extremely rare occasions, California.
  • South Western Pacific Ocean: Tropical activity in this region largely affects Australia and Oceania.
  • Northern Indian Ocean: This basin is divided into two areas, the Bay of Bengal and the Arabian Sea, with the Bay of Bengal dominating (5 to 6 times more activity). This basin's season has an interesting double peak; one in April and May before the onset of the monsoon, and another in October and November just after. 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, Myanmar, and Pakistan. 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.
  • 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.

Unusual formation areas

Image:Hurricane Vince October 9 2005 2300 UTC.jpg The following areas spawn tropical cyclones only very rarely.

  • South Atlantic Ocean: A combination of cooler waters and wind shear makes it very difficult for the South 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 Aldonça), which made landfall in Brazil in 2004 at Category 1 strength, and a smaller storm in January 2004, east of Salvador, Brazil. The January storm is thought to have reached tropical storm intensity based on scatterometer winds.
  • Central North Pacific: Shear in this area of the Pacific Ocean severely limits tropical development, with no storms having formed here since 2002. However, this region is commonly frequented by tropical cyclones that form in the much more favorable Eastern North Pacific Basin.<ref name=CPHC>Central Pacific Hurricane Center archives accessed March 31, 2006</ref>
  • 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.Template:Fact
  • Mediterranean Sea: Storms which appear similar to tropical cyclones in structure sometimes occur in the Mediterranean basin. Examples of these "Mediterranean tropical 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.<ref name=NHCF1/>
  • Temperate subtropics: areas further than thirty degrees from the equator are not normally conducive to tropical cyclone formation or strengthening, and areas more than forty degrees from the equator are very hostile to such development. The primary limiting factor is water temperatures, although higher shear at increasing latitudes is also a factor. These areas are sometimes frequented by cyclones moving poleward from tropical latitudes. On rare occasions, such as in 1988<ref name=UnisysAlberto>Unisys Alberto "Best-track" accessed March 31, 2006</ref> and 1975<ref name=Unisys12>Unisys "12" "Best-track" accessed March 31, 2006</ref> may form or strengthen in this region.
  • Low latitudes. Areas within approximately ten degrees latitude 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 formed from a thunderstorm formation in Borneo that moved into the South China Sea.<ref name="UnisysVamei">Unisys Vamei "Best-track" accessed March 30, 2006</ref>

Times of formation

Worldwide, tropical cyclone activity peaks in late summer when water temperatures are the warmest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active.<ref name=NHCG1>NHC Tropical Cyclone FAQ Subject G1 accessed March 31, 2006</ref>

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.<ref name=NHCG1/>

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.<ref name=NHCG1/>

Seasons and Numbers of storms<ref name=NHCE10/><ref name=NHCG1/>
Basin Season Start Season End Tropical Storms (>34 knots) Tropical Cyclones (>63 knots) Category 3+ Tropical Cyclones (>95 knots)
Northwest Pacific Year Round Year Round 26.7 16.9 8.5
South Indian October May 20.6 10.3 4.3
Northeast Pacific May November 16.3 9.0 4.1
North Atlantic June November 10.6 5.9 2.0
Australia Southwest Pacific October May 10.6 4.8 1.9
North Indian April December 5.4 2.2 0.4

Structure and classification

Image:Hurricane structure graphic.jpg 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 3 km to 320 km (2 miles to 200 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
  • Eyewall: A band around the eye of greatest wind speed, where clouds reach highest and precipitation is heaviest. The heaviest wind damage occurs where a hurricane's eyewall passes 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

Image:Td19.jpg Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, 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 m/s (33 kt, 38 mph, or 62 km/h). It has no eye, and does not typically have the organization or the spiral shape of more powerful storms. It is already 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 32 m/s (34–63 kt, 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 and typhoon intensity, a system with sustained winds greater than 33 m/s (64 kt, 74 mph, or 118 km/h), a tropical cyclone tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of 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 10–50 mi (16–80 km) 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 and clockwise in the Southern Hemisphere. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 85 m/s (165 kt, 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 the 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

Template:Saffir-Simpson small 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. For instance, a Category 2 hurricane that strikes a major urban area will likely do more damage than a large Category 5 hurricane that strikes a mostly rural region. 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 mph (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 10 m (33 ft) above the surface.<ref name="NWSM Defs">http://www.weather.gov/directives/sym/pd01006004curr.pdf</ref><ref name="FEMA glossary">Template:Cite web</ref>

The Australian Bureau of Meteorology uses a 1-5 scale called tropical cyclone severity categories. Unlike the Saffir-Simpson Hurricane Scale, severity categories are based on estimated maximum wind gusts, which are a further 30-40% stronger than the 10-minute average sustained winds.<ref name="BoM winds">http://www.bom.gov.au/info/marine/marpamp.shtml</ref> Severity categories are scaled lower than the Saffir-Simpson Scale - for example, a severity category 2 tropical cyclone is roughly equivalent to a strong tropical storm or a weak Saffir-Simpson category 1 hurricane.

Australian Category<ref>http://www.bom.gov.au/info/cyclone/#severity</ref> Maximum wind gusts (km/h) Maximum sustained winds (km/h)<ref>Comparison between strongest gust and suatained winds from Perth Tropical Cyclone Warning Center</ref> Corresponding Beaufort Force<ref>Comparison between Cyclone Category System and Beaufort Scale from Brisbane Tropical Cyclone Warning Center</ref>
1 ≤125 63-88 Gale (8-9)
2 125-169 89-117 Storm (10-11)
3 170-224 118-159 Hurricane (12)
4 225-279 160-199
5 ≥280 ≥200

The sustained winds given in the table are based on a 10-minute average.<ref name="BoM winds"/>

The Australian Bureau of Meteorology also offers this comparison table between classification systems:<ref>http://www.bom.gov.au/catalogue/warnings/WarningsInformation_TC_Ed.shtml#Cat</ref>

Tropical Storm Classifications
Australian Name Australian Category North America US Saffir-Simpson Category Scale NW Pacific Arabian Sea /Bay of Bengal SW Indian Ocean South Pacific
Tropical Low - Tropical Disturbance - Tropical Disturbance Depression Tropical Disturbance Tropical Disturbance
Tropical Low - Tropical Depression - Tropical Depression Deep Depression Tropical Depression Tropical depression
Tropical Cyclone 1 Tropical Storm - Tropical Storm Cyclonic Storm Moderate Tropical Storm Tropical Cyclone (Gale)
Tropical Cyclone 2 Tropical Storm - Severe Tropical Storm Severe Cyclonic Storm Severe Tropical Storm Tropical Cyclone (Storm)
Severe Tropical Cyclone 3 Hurricane 1 Typhoon Very Severe Cyclonic Storm Tropical Cyclone Tropical Cyclone (Hurricane)
Severe Tropical Cyclone 4 Hurricane 2-3 Typhoon Very Severe Cyclonic Storm Intense Tropical Cyclone Tropical Cyclone (Hurricane)
Severe Tropical Cyclone 5 Hurricane 4-5 Typhoon Super Cyclonic Storm Very Intense Tropical Cyclone Tropical Cyclone (Hurricane)

Japan, the Philippines, Hong Kong, Macau and Taiwan use the following scale to classify tropical cyclones. This scale is also for regional exchange among Typhoon Committee members. <ref>http://www.wmo.ch/web/www/TCP/OperationPlans/TCP-232005.pdf</ref>

Classification Maximum sustained winds (km/h) Maximum sustained winds (knots) Corresponding Beaufort Force
Tropical Depression ≤62 ≤33 ≤7
Tropical Storm 63-88 34-47 Gale (8-9)
Severe Tropical Storm 89-117 48-63 Storm (10-11)
Typhoon ≥118 ≥64 Hurricane (12)

Note:

  • The sustained winds given in the table are based on a 10-minute average.
  • Japan and Taiwan use another scale in their own language.
  • The Philippines merges the category 'Severe Tropical Storm' with 'Tropical Storm' when issuing public advisories.
  • China uses a very similar scale except for the following:

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 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

Image:Epsilon ISS012-E-10097.jpg 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.

For a list of notable and unusual landfalling hurricanes, see list of notable tropical cyclones.

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 lose their strength very rapidly after landfall, and become disorganized areas of low pressure within a day or two. 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 its structure. However, many storm fatalities occur in mountainous terrain, as the dying storm unleashes torrential rainfall which can lead to deadly floods and mudslides.
  • It remains in the same area of ocean for too long, drawing heat off of the ocean surface until it becomes too cool to support the storm. 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 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 cause supercooled water in the outer rainbands to freeze, causing the inner eyewall to collapse and thus reducing 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.<ref name="Whipple ch. 5">Whipple, A. (1982, 1984)"Storm" p. 151 Time Life Books ISBN 0-8094-4312-0</ref>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. The project was dropped after it was discovered that eyewall replacement cycles occur naturally in strong hurricanes, casting doubt on the result of the earlier attempts. Today it is known that silver iodide seeding is not likely to have an effect because the amount of supercooled water in the rainbands of a tropical cyclone is too low.<ref name=NHCC5A>NHC Tropical Cyclone FAQ Subject C5a accessed April 2, 2006</ref>

Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans; dropping large quantities of ice into the eye at very early stages so that latent heat is absorbed by ice at the entrance (storm cell perimeter bottom) instead of heat energy being converted to kinetic energy at high altitudes vertically above; 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.<ref name=NHCC5F>NHC Tropical Cyclone FAQ Subject C5f accessed April 2, 2006</ref>

However, it has been suggested by some that we can change the course of a storm during its early stages of formation,Template:Fact such as using satellites to alter the environmental conditions or, more realistically, spreading a degradable film of oil over the ocean, which prevent water vapor 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. <ref name="Hurricane Hunters">Hurricane Hunters homepage accessed March 30, 2006</ref> 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.<ref name="SunHerald">Bowman, L. "Drones defy heart of storm". South Mississippi Sun-Herald accessed March 30, 2006</ref>

Tropical cyclones far from land are tracked by weather satellites capturing visible and infrared images from space, usually at half-hour to quarter-hour intervals. As a storm approaches land, it can be observed by land-based Doppler radar. Radar plays a crucial role around landfall because it shows a storm's location and intensity minute by minute.

Recently, academic researchers have begun to deploy mobile weather stations fortified to withstand hurricane-force winds. The two largest programs are the Florida Coastal Monitoring Program <ref name=FCMP>Florida Coastal Monitoring Program project overview accessed March 30, 2006</ref> and the Wind Engineering Mobile Instrumented Tower Experiment. <ref name=WEMITE>WEMITE homepage accessed March 30, 2006</ref> During landfall, the NOAA Hurricane Research Division compares and verifies data from reconnaissance aircraft, including wind speed data taken at flight level and from GPS dropwindsondes and stepped-frequency microwave radiometers, to wind speed data transmitted in real time from weather stations erected near or at the coast. The National Hurricane Center uses the data to evaluate conditions at landfall and to verify forecasts.

Naming of tropical cyclones

Template:Hurricane main Storms reaching tropical storm strength 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 offices 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.

Naming schemes

Template:Further 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 twenty-second storm is called "Alpha," the twenty-third "Beta," and so on. This was first necessary during the 2005 season when the list was exhausted. 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.

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. Since 1981, the numbering system had been the primary system to identify tropical cyclone among Typhoon Committee members and it is still in use. International numbers are assigned by Japan Meteorological Agency on the order that a tropical storm forms while different internal numbers may be assigned by different NMCs. The Typhoon "Songda" in September 2004 was internally called the typhoon number 18 in Japan but typhoon number 19 in China. Internationally, it is recorded as the TY Sonda (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. There, a new list is used each year.

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 Lindley Wragge, an Anglo-Australian meteorologist at the end of the 19th century. He used girls' names, the names of politicians who had offended him, and names from history and mythology.<ref name=NHCB1>NHC Tropical Cyclone FAQ Subject B1 accessed March 30, 2006</ref><ref name="BOM Question 13">Bureau of Meteorology FAQ Question 13 accessed March 31, 2006</ref>

During World War II, tropical cyclones were given feminine names, mainly for the convenience of the forecasters and in a somewhat ad hoc manner. In addition, George R. Stewart's 1941 novel Storm help to popularize the concept of giving names to tropical cyclones.<ref name=NHCJ4>NHC Tropical Cyclones FAQ Subject J4 accessed March 31, 2006</ref>

From 1950 to 1953, 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. Naming is now maintained by the World Meteorological Organization.

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 World Meteorological Organization 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. In the southern hemisphere, male names were given to cyclones starting in 1975.<ref name="BOM Question 13"/>

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 southwestern Indian Ocean, Metéo France in Réunion 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 include Cyclone Adeline-Juliet in early 2005 and Cyclone Bertie-Alvin in late 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 include Hurricane Cesar-Douglas in 1996 and Hurricane Joan-Miriam in 1988.<ref name=NHCE15>NHC Tropical Cyclone FAQ Subject E15 accessed March 30, 2006</ref>
    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 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.<ref name=NHCManuel>NHC Tropical Storm Manuel Report accessed March 31, 2006</ref>
    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."<ref name=NHCLarry>NHC Tropical Storm Larry Discussion Number 16 accessed March 31, 2006</ref>
    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. One example is TD 10-TD 12 from 2005.
  4. Human errors
    Sometimes, there may be human faults leading to the renaming of a tropical cyclone. This is especially true if the system is poorly organized or if it passes from the area of responsibility of one forecaster to another. Examples include Tropical Storm Ken-Lola in 1989<ref name=JTWCKenLola>JTWC Ken-Lola Report accessed March 30, 2006</ref> and Tropical Storm Upana Chanchu in 2000<ref name=Padgett>Padgett, G. Monthly Global Tropical Cyclone Summary for July 2000 accessed March 30, 2000</ref>

Effects

Image:Cyclone deaths.gif A mature tropical cyclone can release heat at a rate upwards of 6x1014 watts.<ref name="NOAA Question of the Month"/> 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 historically cyclones claimed 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. Inland areas are particularly vulnerable to freshwater flooding, due to residents not preparing adequately.<ref name=NHCFlooding>National Hurricane Preparedness Week: Inland Flooding accessed March 31, 2006</ref>
  • Tornado activity - The broad rotation of a hurricane often spawns tornadoes. Also, tornadoes can be spawned as a result of eyewall mesovortices which perisist until landfall. While these tornadoes are normally not as strong as their non-tropical counterparts,Template:Fact they can still cause tremendous damage.

Image:Hurricane katrina damage gulfport mississippi.jpg

Often, the secondary effects of a tropical cyclone are equally damaging. These 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. Large areas of standing water caused by flooding also contribute to mosquito-borne illnesses.
  • 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 Allen ended the Texas drought of 1980. Hurricane Camille averted drought conditions and ended water deficits along much of its path.<ref name=Christopherson>Christopherson, R. (1992) "Geosystems An Introduction to Physical Geography" pp 222-224. Macmillan Publishing Company New York. ISBN 0-02-322443-6</ref>

In addition, the destruction caused by Camille on the Gulf coast spurred redevelopment as well, greatly increasing local property values.<ref name=Christopherson/> 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. Hurricane Katrina is the most obvious example, as it devastated the region that had been revitalized by Hurricane Camille. 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. James Lovelock has also hypothesised that by raising nutrients from the sea floor to surface layers of the ocean, hurricanes also increase biological activity in areas where life would be difficult through nutrient loss in the deeper reaches of the ocean.Template:Fact

Long term trends in cyclone activity

While the number of storms in the Atlantic has increased since 1995, there seems to be no signs of a global trend; the annual global number of tropical cyclones remains about 90 ± 10. <ref name=EmanuelHomepage>Kerry Emanuel's page on Tropical Cyclones accessed March 30, 2006</ref>.

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 <ref name=realclimate>realclimate.org accessed March 20, 2006</ref>. 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." <ref name="NHCG4">NHC Tropical Cyclone FAQ Subject G4 accessed March 30, 2006</ref>

Regarding strength, a similar conclusion was consensus until recently. This consensus is now questioned by Kerry Emanuel. In an article in Nature, <ref name=EmanuelNature>Nature Vol. 436, pp 686–688 accessed March 20, 2006</ref> 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".<ref name=EmmanuelPreprint>Preprint of a paper by Kerry Emanuel accessed March 20, 2006</ref>

Along similar lines, P.J. Webster and others published an article<ref name=zfacts1>Webster Science 2005 Hurricanes accesed March 20, 2006</ref> in Science <ref name="ScienceMag">Science. Volume 309, pp 1844-1846</ref> 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". That is, 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. 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. <ref name=realclimate/>

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, <ref name=zfacts2>Zfacts accessed March 20, 2006</ref> 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 and 5 hurricanes can be rejected at the 0.1% level. Thus, there is only a 1 in 1000 chance of simultaneously finding the observed six increases in the percentages of Category 4 or 5 hurricanes. 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

Template:Hurricane main Tropical cyclones that cause massive destruction are fortunately rare, but when they happen, they can cause damage in the range billions of dollars and disrupt or end thousands of lives.

The deadliest tropical cyclone on record hit the densely populated Ganges Delta region of East Pakistan (now Bangladesh) on November 13, 1970, likely as a Category 3 tropical cyclone. It killed an estimated 500,000 people. The North Indian basin has historically been the deadliest, with several storms since 1900 killing over 100,000 people, each in Bangladesh.<ref name=Encarta1>Encarta Online accessed March 31, 2006</ref>

In the Atlantic basin, at least 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.<ref name=NHCMitch>NHC Mitch Report accessed March 31, 2006</ref> The Galveston Hurricane of 1900, which made landfall at Galveston, Texas as an estimated Category 4 storm, killed 8,000 to 12,000 people, and remains the deadliest natural disaster in the history of the United States.<ref name=NHCPastDeadly>National Hurricane Center The Deadliest Atlantic Tropical Cyclones, 1492-1996 accessed March 31, 2006</ref> The deadliest Atlantic storm on record was the Great Hurricane of 1780, which killed about 22,000 people in the Antilles.<ref name=NHCPastDeadly/>

Image:Typhoonsizes.jpg 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 <ref name="Weatherwatchers Mitch">WeatherwatchersWeatherwatchers page on Hurricane Mitch accessed March 30, 2006</ref>, 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 on record at landfall. For comparison, these speeds are encountered at the center of a strong tornado, but Camille, like all tropical cyclones, was much larger and long-lived than any tornado.

Typhoon Nancy in 1961 had recorded wind speeds of 215 mph (345 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. <ref name=NHCE1>NHC Tropical Cyclone FAQ Subject E1 accessed March 30, 2006</ref> Similarly, a surface-level gust caused by Typhoon Paka on Guam was recorded at 236 mph (380 km/h); had it been confirmed, this would be the strongest non-tornadic wind ever recorded at the Earth's surface, but the reading had to be discarded since the anemometer was damaged by the storm.<ref name=NWSPaka>National Weather Service Super Typhoon Paka's (1997) Surface Winds Over Guam accessed March 30, 2006</ref>

Tip was also the largest cyclone on record, with a circulation of tropical storm-force winds 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. <ref name=NHCE5>NHC Tropical Cyclone FAQ Subject E5 accessed March 30, 2006</ref>

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.<ref name=CPHCIniki>Central Pacific Hurricane Center Iniki report accessed March 31, 2006</ref> Other destructive Pacific hurricanes include Pauline<ref name=NHCPauline>NHC Pauline Report accessed March 31, 2006</ref> and Kenna.<ref name=NHCKenna>NHC Kenna Report accessed March 31, 2006</ref>

Image:Brazil hurricane.jpg 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. Tropical cyclones 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 Thelma, in November 1991 killed thousands in the Philippines even though it never became a typhoon; the damage from Thelma was mostly due to flooding, not winds or storm surge.<ref name=JTWCThelma>Joint Typhoon Center Thelma report accessed March 31, 2006</ref> In 1982, the unnamed tropical depression that eventually became Hurricane Paul caused the deaths of around 1,000 people in Central America due to the effects of its rainfall.<ref name=MWRPaul>American Meteorological Society "Eastern North Pacific Tropical Cyclones of 1982" [May 1983 Monthly Weather Review accessed March 31, 2006</ref>

On August 29 2005, Hurricane Katrina made landfall in Louisiana and Mississippi. 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.<ref name="NHC Atlantic Monthly Report for August 2005">August 2005 Atlantic Tropical Weather Summary accessed March 31, 2006</ref> Currently, its death toll is at least 1,604, mainly from flooding and the aftermath in New Orleans, Louisiana. It is also estimated to have caused an estimated $75 billion in damages. Before Katrina, the costliest system in monetary terms had been 1992's Hurricane Andrew, which caused an estimated $39 billion (2005 USD) in damage in Florida.<ref name=NHCKatrina>NHC Katrina Report accessed March 31, 2006</ref>

Regional storm terminology

Image:Typhoon.jpg

Terms used in weather reports for tropical cyclones that have surface winds over 64 knots (73.6 mph) or 32 m/s vary by region:

  • Hurricane: Atlantic basin and North Pacific Ocean east of the International date line
  • Typhoon: Northwest Pacific west of the dateline
  • Severe tropical cyclone: Southwest Pacific west of 160°E and the southeast Indian Ocean east of 90°E
  • Severe cyclonic storm: North Indian Ocean
  • Tropical cyclone: Southwest Indian Ocean and the South Pacific east of 160°E.
  • Cyclone (unofficially): South Atlantic Ocean

There are many regional names for tropical cyclones, including Bagyo in the Philippines and Taino in Haiti.

Origin of storm terms

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).Template:Fact
  • From Urdu, Persian or Arabic ţūfān (طوفان) < Greek tuphōn (Τυφών).Template:Fact

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

The word hurricane is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracán.<ref name=NHCB4>NHC Tropical Cyclone FAQ Subject B4 accessed April 15, 2006</ref>

The word cyclone was coined by a Captain Henry Piddington. who used it to refer to the storm that blew a freighter in circles in Mauritius in February of 1845.<ref name=Whipplep53>Whipple, p. 53</ref>

Other storm systems

Template:Seealso Many other forms of cyclone can form in nature. Several of these relate to the formation or dissipation of tropical cyclones.

Extratropical cyclone

Template:Hurricane main 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;<ref name=NHCA7>NHC Tropical Cyclone FAQ Subject A7 accessed March 31, 2006</ref> Infrequently, an extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone. 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.

Subtropical storm

Template:Hurricane main A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wide band of latitude, from the equator to 50°. Although subtropical storms rarely attain hurricane-force winds, they may become tropical in nature as their core warms.<ref name=NHCA6>NHC Tropical Cyclone FAQ Subject A6 accessed March 31, 2006</ref> From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.<ref name=PadgetDecember2000>Padgett, G. Monthly Global Tropical Cyclone Summary for December 2000 accessed March 31, 2006</ref>

European windstorms

Template:Hurricane main In the United Kingdom and Europe, some severe northeast Atlantic cyclonic depressions are referred to incorrectly as "hurricanes," even though they rarely originate in the tropics. These European windstorms can generate hurricane-force winds. The Free University of Berlin names all areas of high and low pressure over Europe. Two powerful extratropical cyclones that ravaged France, Germany, and the United Kingdom in December 1999, were named "Lothar" and "Martin". In British Shipping Forecasts, sustained ten minute average winds of force 12 on the Beaufort scale are described as "hurricane force."

See also

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Notes

<references/>

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ar:هوريكان bg:Тропическа буря ca:Huracà cs:Tropická cyklóna cy:Teiffŵn da:Orkan de:Tropischer Wirbelsturm et:Orkaan eo:Uragano es:Huracán fi:Trooppinen hirmumyrsky fr:Cyclone tropical he:סופה טרופית id:Siklon tropis it:Ciclone tropicale ja:台風 ko:열대저기압 la:Typhon marinus lv:Viesuļvētra mk:Тропски циклон nl:Orkaan pl:Cyklon tropikalny pt:Furacão ru:Тропический циклон sl:Tropski ciklon sv:Tropisk cyklon zh:熱帶氣旋 zh-min-nan:Hong-thai