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, a rare South Atlantic Tropical Cyclone viewed from the International Space Station on March 26, 2004.]] In Meteorology , a Tropical Cyclone is a storm system with a closed circulation around a center of Low Pressure , fueled by the heat released when moist air rises and condenses. The name underscores their origin in the Tropics and their Cyclonic nature. They are distinguished from other cyclonic storms such as Nor'easter s and Polar Low s by the heat mechanism that fuels them, which makes them "warm core" storm systems. Depending on their strength and location, a tropical cyclone can be called a tropical depression, '''tropical storm''', '''hurricane''', or '''typhoon''', among other names. Tropical cyclones can produce extremely high winds, tornadoes, torrential rain (leading to mudslides and flash floods), and drive Storm Surge onto coastal areas. Though the effects on populations and ships can be catastrophic, tropical cyclones have been known to relieve drought conditions. They carry heat away from the tropics, an important mechanism of the global Atmospheric Circulation that maintains equilibrium in the environment. Mechanics of tropical cyclones . 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 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. , this defines the initial domain of the tropical cyclone. By contrast, Mid-latitude Cyclone s draw their energy mostly from pre-existing horizontal temperature Gradient s in the atmosphere. In order to continue to drive its Heat Engine , a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is 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. NHC Tropical Cyclone FAQ Subject C2 accessed March 31, 2006 as Hurricanes Katrina and Rita passed over. Each of these storms cooled water temperatures more than 4°C in places along their paths, and cooled the entire Gulf by about 1°.]] In addition, the passage of a tropical cyclone over the ocean can cause the upper ocean to cool substantially, which can influence subsequent cyclone development. Tropical cyclones cool the ocean by acting like "heat engines" that transfer heat from the ocean surface to the atmosphere through Evaporation . Cooling is also caused by upwelling of cold water from below due to the suction effect of the low-pressure center of the storm. Additional cooling may come from cold water from raindrops that remain on the ocean surface for a time. Cloud cover may also play a role in cooling the ocean by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days. Passing of Hurricanes Cools Entire Gulf accessed April 26, 2006 Scientists at the National Center For Atmospheric Research estimate that a hurricane releases heat energy at the rate of 50 to 200 Trillion Watt s. By comparison, this is about the amount of energy released by exploding a 10-megaton Nuclear Bomb every 20 minutes University Corporation For Atmospheric Research Hurricanes: Keeping an eye on weather's biggest bullies accessed March 31, 2006 or 200 times the total energy production capacity of the entire world. 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 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: # Water Temperatures of at least 26.5 °C (80°F) NHC Tropical Cyclone FAQ Subject A15 accessed March 30, 2006 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. NHC Tropical Cyclone FAQ Subject A16 accessed March 30, 2006 # Rapid cooling with height. This allows the release of Latent Heat , which is the source of energy in a tropical cyclone. # 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. # Low Wind Shear . When wind shear is high, the convection in a cyclone or disturbance will be disrupted, blowing the system apart. # 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. # A pre-existing system of disturbed weather. The system must have some sort of circulation as well as a low pressure center. Only specific weather disturbances can result in tropical cyclones. These include: # 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. # 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 . # 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 -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.1 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. 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 Ocean s, and the northern Indian Ocean. Worldwide, an average of 80 tropical cyclones form each year. NHC Tropical Cyclone FAQ Subject E10 accessed March 31, 2006
Unusual formation areas on October 9, 2005 at 2300 UTC near the Madeira Islands .]] The following areas spawn tropical cyclones only very rarely.
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. NHC Tropical Cyclone FAQ Subject G1 accessed March 31, 2006 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. Structure and classification A strong tropical cyclone consists of the following components.
Intensities of tropical cyclones , showing the lack of organization in tropical depressions when compared to stronger cyclones.]] 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 . 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 See Also: Tropical cyclone scales 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 U.S. National Hurricane Center classifies hurricanes of Category 3 and above as ''major hurricanes''. The U.S. 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 at a height of 10m. The U.S. weather service defines sustained winds based on 1-minute average speed measured 10 m (33 ft) above the surface.http://www.weather.gov/directives/sym/pd01006004curr.pdf2 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 near peak intensity, showing Clockwise rotation due to the 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 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. 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:
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 Front s 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 .Whipple, A. (1982, 1984)"Storm" p. 151 Time Life Books ISBN 0-8094-4312-0 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. NHC Tropical Cyclone FAQ Subject C5a accessed April 2, 2006 Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing Iceberg s 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. NHC Tropical Cyclone FAQ Subject C5f accessed April 2, 2006 However, it has been suggested by some that we can change the course of a storm during its early stages of formation, 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 's rainbands photographed at 7,000 feet.]] Intense tropical cyclones pose a particular observation challenge. As they are a dangerous oceanic phenomenon, Weather Station s 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 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 Dropsonde s 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.Bowman, L. "Drones defy heart of storm." South Mississippi Sun-Herald accessed March 30, 2006 Tropical cyclones far from land are tracked by Weather Satellite s 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 Florida Coastal Monitoring Program project overview accessed March 30, 2006 and the Wind Engineering Mobile Instrumented Tower Experiment. WEMITE homepage accessed March 30, 2006 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 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 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 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. NHC Tropical Cyclone FAQ Subject B1 accessed March 30, 2006 Bureau of Meteorology FAQ Question 13 accessed March 31, 2006 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. NHC Tropical Cyclones FAQ Subject J4 accessed March 31, 2006 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. 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. #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. #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. NHC Tropical Cyclone FAQ Subject E15 accessed March 30, 2006 #: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. NHC Tropical Storm Manuel Report accessed March 31, 2006 #: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." NHC Tropical Storm Larry Discussion Number 16 accessed March 31, 2006 #:Up to now, there has been no tropical cyclone retaining its name during the passage from Atlantic to Pacific, or vice versa. #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 Tropical Depression 10 -Tropical Depression 12 (which became Hurricane Katrina ) from 2005. #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 in 2000Padgett, G. Monthly Global Tropical Cyclone Summary for July 2000 accessed March 30, 2000 Effects A mature tropical cyclone can release heat at a rate upwards of 6x1014 watts. 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:
in Gulfport, Mississippi . Katrina was the costliest tropical cyclone in United States history.]] Often, the secondary effects of a tropical cyclone are equally damaging. These include:
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 Deficit s along much of its path.Christopherson, R. (1992) "Geosystems An Introduction to Physical Geography" pp 222-224. Macmillan Publishing Company New York. ISBN 0-02-322443-6 In addition, 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. 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 . 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. Kerry Emanuel's page on Tropical Cyclones accessed March 30, 2006 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 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." NHC Tropical Cyclone FAQ Subject G4 accessed March 30, 2006 Regarding strength, a similar conclusion was consensus until recently. This consensus is now questioned by Kerry Emanuel . In an article in '' Nature '', ''Nature'' Vol. 436, pp 686–688 accessed March 20, 2006 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." Preprint of a paper by Kerry Emanuel accessed March 20, 2006 Along similar lines, P.J. Webster and others published an article Webster Science 2005 Hurricanes accessed March 20, 2006 in '' Science '' Science. Volume 309, pp 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." 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. 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, Zfacts accessed March 20, 2006 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 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. Encarta Online accessed March 31, 2006 In the , which killed about 22,000 people in the Antilles . , Tropical Cyclone Tracy , and the United States.]] The most intense storm on record was , but Camille, like all tropical cyclones, was much larger and long-lived than any tornado. 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. National Weather Service Super Typhoon Paka's (1997) Surface Winds Over Guam accessed March 30, 2006 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 60 miles (100 km) wide. NHC Tropical Cyclone FAQ Subject E5 accessed March 30, 2006 . NHC Kenna Report accessed March 31, 2006 from 2004.]] 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 before 1960 but were not observed until Weather Satellite s began monitoring the Earth's oceans in that year. A tropical cyclone need not be particularly strong to cause memorable damage; caused the deaths of around 1,000 people in Central America due to the effects of its rainfall. American Meteorological Society "Eastern North Pacific Tropical Cyclones of 1982" May 1983 Monthly Weather Review accessed March 31, 2006 On , 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 . NHC Katrina Report accessed March 31, 2006 Regional storm terminology , Pacific Ocean, August 1985 .]] Terms used in weather reports for tropical cyclones that have surface winds over 64 Knot s (73.6 Mph ) or 32 M/s vary by region:
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:
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''. NHC Tropical Cyclone FAQ Subject B4 accessed April 15, 2006 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.Whipple, p. 53 Other storm systems See Also: Cyclone Many other forms of cyclone can form in nature. Several of these relate to the formation or dissipation of tropical cyclones. Extratropical cyclone 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; NHC Tropical Cyclone FAQ Subject A7 accessed March 31, 2006 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 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. NHC Tropical Cyclone FAQ Subject A6 accessed March 31, 2006 From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.Padgett, G. Monthly Global Tropical Cyclone Summary for December 2000 accessed March 31, 2006 European windstorms In the , Germany , and the United Kingdom in December 1999 , were named "Lothar" and "Martin." In British Shipping Forecast s, sustained ten minute average winds of force 12 on the Beaufort Scale are described as "hurricane force." See also Meteorology Forecasting and preparation Categories Notes External links Tracking and Warning
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