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Tropical cyclone

2007 Schools Wikipedia Selection. Related subjects: Storms

   Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from
   the International Space Station on March 26, 2004
   Enlarge
   Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from
   the International Space Station on March 26, 2004

   A ''tropical cyclone'' is a storm system fueled by the heat released
   when moist air rises and the water vapor in it condenses. The term
   describes the storm's origin in the tropics and its cyclonic nature,
   which means that its circulation is counterclockwise in the northern
   hemisphere and clockwise in the southern hemisphere. Tropical cyclones
   are distinguished from other cyclonic windstorms such as nor'easters,
   European windstorms, and polar lows by the heat mechanism that fuels
   them, which makes them "warm core" storm systems.

   Depending on their location and strength, there are various terms by
   which tropical cyclones are known, such as hurricane, typhoon, tropical
   storm, cyclonic storm and tropical depression.

   Tropical cyclones can produce extremely strong winds, tornadoes,
   torrential rain, high waves, and storm surges. They are born and
   sustained over large bodies of warm water and lose their strength over
   land; this explains why coastal regions can receive much damage while
   inland regions are relatively safe. The heavy rains and storm surges
   can produce extensive flooding. Although their effects on human
   populations can be devastating, tropical cyclones also can have
   beneficial effects by relieving drought conditions. They carry heat
   away from the tropics, an important mechanism of the global atmospheric
   circulation that maintains equilibrium in the earth's troposphere.

Mechanics of tropical cyclones

   Tropical cyclones form when the energy released by the condensation of
   moisture in rising air causes a positive feedback loop over warm ocean
   waters.
   Tropical cyclones form when the energy released by the condensation of
   moisture in rising air causes a positive feedback loop over warm ocean
   waters.

   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 being 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. In another way, tropical cyclones could be viewed as a
   special type of Mesoscale Convective Complex, which continues to
   develop over a vast source of relative warmth and moisture.
   Condensation leads to higher wind speeds, as a tiny fraction of the
   released energy is converted into mechanical energy; the faster winds
   and lower pressure associated with them in turn cause increased surface
   evaporation and thus even more condensation. Much of the released
   energy drives updrafts that increase the height of the storm clouds,
   speeding up condensation. 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 rotation 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. 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. To continue to drive its heat engine, a tropical cyclone
   must remain over warm water, which provides the needed atmospheric
   moisture. 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.
   Chart displaying the drop in surface temperature in the Gulf of Mexico
   as Hurricanes Katrina and Rita passed over
   Enlarge
   Chart displaying the drop in surface temperature in the Gulf of Mexico
   as Hurricanes Katrina and Rita passed over

   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. 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.

   Scientists at the National Centre for Atmospheric Research estimate
   that a tropical cyclone releases heat energy at the rate of 50 to 200
   trillion joules per day. For comparison, this rate of energy release is
   equivalent to exploding a 10-megaton nuclear bomb every 20 minutes or
   200 times the world-wide electrical generating capacity per day.

   While the most obvious motion of clouds is toward the centre, 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 centre. The high cirrus clouds may be the first signs of an
   approaching tropical cyclone.

Physical structure

   Structure of a hurricane
   Structure of a hurricane

   A strong tropical cyclone consists of the following components:
     * Surface low: All tropical cyclones rotate around an area of low
       atmospheric pressure near the Earth's surface. The pressures
       recorded at the centers of tropical cyclones are among the lowest
       that occur on Earth's surface at sea level.
     * Warm core: Tropical cyclones are characterized and driven by the
       release of large amounts of latent heat of condensation as moist
       air is carried upwards and its water vapor condenses. This heat is
       distributed vertically, around the centre 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 the
       shield of cirrus clouds produced by the eyewall thunderstorms.
       Typically, these are the highest and coldest clouds in the cyclone.
     * 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). 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 centre, 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.
     * Rainbands: Bands of showers and thunderstorms that spiral
       cyclonically toward the storm centre. High wind gusts and heavy
       downpours often occur in individual rainbands, with relatively calm
       weather between bands. Tornadoes often form in the rainbands of
       landfalling tropical cyclones. Annular hurricanes are distinctive
       for their lack of rainbands.
     * Outflow: The upper levels of a tropical cyclone feature winds
       headed away from the centre 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 centre of the
       storm.

Formation

Factors in formation

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

   The formation of tropical cyclones is the topic of extensive ongoing
   research and is still not fully understood. Six factors appear to be
   generally necessary, although tropical cyclones may occasionally form
   without meeting all of these conditions:
    1. Water temperatures of at least 26.5 °C (80°F) 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.
    2. Rapid cooling with height. This allows the release of latent heat,
       which is the source of energy in a tropical cyclone.
    3. High humidity, especially in the lower-to-mid troposphere. When
       there is a great deal of moisture in the atmosphere, conditions are
       more favourable for disturbances to develop.
    4. Low wind shear. When wind shear is high, the convection in a
       cyclone or disturbance will be disrupted, preventing formation of
       the feedback loop.
    5. Distance from the equator. This allows the Coriolis force to
       deflect winds blowing towards the low pressure centre, causing a
       circulation. The minimum distance is about 500 km (310 miles) or 5
       degrees from the equator.
    6. A pre-existing system of disturbed weather. The system must have
       some sort of circulation as well as a low pressure centre.

   This TRMM image shows the height of rain columns within Hurricane
   Irene.
   Enlarge
   This TRMM image shows the height of rain columns within Hurricane
   Irene.

   Generally, tropical cyclones generally form from four different types
   of systems: monsoon troughs, tropical waves, non-tropical lows, and
   decaying frontal boundaries. Monsoon troughs, which are broad areas of
   converging winds from both hemispheres, are the main trigger of
   tropical cyclone formation worldwide. When they strengthen, either due
   to strengthening high pressure poleward of the trough or by increased
   flow passing through the equator from the opposite hemisphere,
   thunderstorm activity increases and tropical cyclogenesis can occur.

   Another common mechanism for tropical cyclone formation are tropical
   waves, also called easterly waves, which are westward-moving areas of
   convergent winds. These generate most of the hurricanes in the Atlantic
   and northeast Pacific basins. Tropical waves often carry with them
   clusters of thunderstorms, which can then develop into tropical
   cyclones. A similar phenomenon to tropical waves are West African
   disturbance lines, which are squalls that form over Africa and move
   into the Atlantic, often as a part of the Intertropical Convergence
   Zone. Tropical cyclones also frequently form from upper tropospheric
   troughs, which are cold-core upper-level lows. A warm-core tropical
   cyclone may result when one of these works down to the lower levels of
   the atmosphere and produces deep convection. Off-season tropical
   cyclones most often form in this manner. Finally, 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.
   Cumulative tracks of all cyclones from 1985 to 2005
   Enlarge
   Cumulative tracks of all cyclones from 1985 to 2005

Locations of formation

   Most tropical cyclones form in a worldwide band of thunderstorm
   activity called by several names: the Intertropical Discontinuity
   (ITD), the Intertropical Convergence Zone (ITCZ), or the monsoon
   trough.

   Most 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, tropical cyclones rarely form
   or move within about 5 degrees of the equator, where the Coriolis
   effect is weakest. However, it is possible for tropical cyclones to
   form within this boundary as did Typhoon Vamei in 2001 and Cyclone Agni
   in 2004.

Major basins

   Traditionally, areas of tropical cyclone formation are divided into
   seven basins. These include the north Atlantic Ocean, the eastern and
   western parts of the Pacific Ocean (considered separately because
   tropical cyclones rarely form in the central Pacific), the southwestern
   Pacific, the southwestern and southeastern Indian Oceans, and the
   northern Indian Ocean. The North Atlantic is the most studied of the
   basins, while the Western Pacific is the most active and the North
   Indian the least active. An average of 86 tropical cyclones of tropical
   storm intensity form annually worldwide, with 47 reaching
   hurricane/typhoon strength, and 20 becoming intense tropical cyclones
   (at least of Category 3 intensity).
                 Basins and WMO Monitoring Institutions
           Basin                  Responsible RSMCs and TCWCs
   Northern Atlantic     National Hurricane Centre
   Northeastern Pacific  National Hurricane Centre
   North central Pacific Central Pacific Hurricane Centre
   Northwestern Pacific  Japan Meteorological Agency
   Northern Indian       Indian Meteorological Department
   Southwestern Indian   Météo-France
   South and
   Southwestern Pacific  Fiji Meteorological Service
                         Meteorological Service of New Zealand^†
                         Papua New Guinea National Weather Service^†
                         Bureau of Meteorology^† (Australia)
   Southeastern Indian   Bureau of Meteorology^† (Australia)
            ^†: Indicates a Tropical Cyclone Warning Centre

   There are six Regional Specialised Meteorological Centres (RSMCs)
   worldwide. These organizations are designated by the World
   Meteorological Organization and are responsible for tracking and
   issuing bulletins, warnings, and advisories about tropical cyclones in
   their designated areas of responsibility. Additionally, there are five
   Tropical Cyclone Warning Centres (TCWCs) that provide information to
   smaller regions. The RSMCs and TCWCs, however, are not the only
   organizations that provide information about tropical cyclones to the
   public. The Joint Typhoon Warning Centre (JTWC) issues informal
   advisories in all basins except the Northern Atlantic and Northeastern
   Pacific. The Philippine Atmospheric, Geophysical and Astronomical
   Services Administration (PAGASA) issues informal advisories, as well as
   names, for tropical cyclones that approach the Philippines in the
   Northwestern Pacific. The Canadian Hurricane Centre (CHC) issues
   advisories on hurricanes and their remnants that affect Canada.
     * Northern Atlantic Ocean: 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 with an average of
       around 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.
       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. Rarely, a hurricane can reach western Europe,
       including Hurricane Lili, which dissipated over the British Isles
       in October 1996, and Tropical Storm Vince, which made landfall on
       the southwestern coast of Spain in September 2005.
     * Northeastern 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,
       Texas, Hawaii, northern Central America, California, Arizona, and
       on rare occasions, Japan. No hurricane included in the modern
       database has reached California; however, historical records from
       1858 speak of a storm that struck San Diego with winds over 75
       m.p.h./65 kts, above hurricane force, though it is not known if the
       storm actually made landfall. Since 1900, only one system of
       tropial storm strength has made landfall in California.
     * Northwestern Pacific Ocean: Tropical storm activity in this region
       frequently affects China, Japan, Hong Kong, the Philippines, and
       Taiwan, but also many other countries in Southeast Asia, such as
       Vietnam, South Korea, and parts of 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
       coast of China sees the most landfalling tropical cyclones
       worldwide. The Philippines receives an average 18 typhoon landings
       per year. Rarely does a typhoon or an extratropical storm reach
       northward to Siberia, Russia.
     * 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.(see
       data, ) Tropical cyclones 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.
     * Southwestern Pacific Ocean: Tropical activity in this region
       largely affects Australia and Oceania. On rare occasions, tropical
       storms reach the vicinity of Brisbane, Australia and into New
       Zealand, usually during or after extratropical transition.
     * Southeastern Indian Ocean: Tropical activity in this region affects
       Australia and Indonesia. According to the Australian Bureau of
       Meteorology, the most frequently hit portion of Australia is
       between Exmouth and Broome in Western Australia.
     * Southwestern Indian Ocean: This basin is the least understood, due
       to a lack of historical data. Cyclones forming here impact
       Madagascar, Mozambique, Mauritius, Reunion, Comoros, Tanzania, and
       Kenya.

Times of formation

   Worldwide, tropical cyclone activity peaks in late summer when water
   temperatures are 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.

   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 time frame 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.
   Season Lengths and Seasonal Averages
   Basin Season Start Season End Tropical Storms (>34 knots) Tropical
   Cyclones (>63 knots) Category 3+ Tropical Cyclones (>95 knots)
   Northwest Pacific – – 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

Movement and track

Large-scale winds

   Although tropical cyclones are large systems generating enormous
   energy, their movements over the earth's surface are controlled by
   large-scale winds—the streams in the earth's atmosphere. The path of
   motion is referred to as a tropical cyclone's track, and has been
   compared by Dr. Neil Frank, former director of the National Hurricane
   Centre, as "leaves carried along by a stream."

   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
   westward from the African coast and towards the Caribbean and North
   America. These waves are the precursors to many tropical cyclones and
   are the main source of Atlantic hurricanes during most seasons, and
   also play a significant role in the formation of tropical cyclones in
   the Eastern Pacific.

   In the Indian Ocean and western Pacific (north and south of the
   equator), tropical cyclogenesis is strongly influenced by the seasonal
   movement of the Intertropical Convergence Zone and the monsoon trough,
   rather than by easterly waves. In these basins as well, tropical
   cyclone paths are broadly determined by synoptic scale features.

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. These speeds
   are due to the conservation of angular momentum - air is drawn in from
   an area much larger than the cyclone such that the tiny rotational
   speed (originally imparted by the Coriolis acceleration) is magnified
   greatly as the air is drawn in to the low pressure centre.

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.

Landfall

   Officially, " landfall" is when a storm's center (the centre 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 tropical cyclones, 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.

   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, such as the extratropical remnants of
   Hurricane Iris in 1995.

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 in 1947,
   disaster struck when a hurricane east of Jacksonville, Florida promptly
   changed its course after being seeded, and smashed into Savannah,
   Georgia. Because there was so much uncertainty about the behaviour 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, greatly reducing the number of
   possible test storms. 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.

   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. Project Cirrus even involved throwing dry ice on
   a cyclone. These approaches all suffer from the same flaw: tropical
   cyclones are simply too large for any of them to be practical.

Effects

   Pie graph of American tropical cyclone casualties by cause from
   1970-1999
   Enlarge
   Pie graph of American tropical cyclone casualties by cause from
   1970-1999

   A mature tropical cyclone can release heat at a rate upwards of 6x10^14
   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:
     * 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.
     * Tornado activity - The broad rotation of a hurricane often spawns
       tornadoes. Also, tornadoes can be spawned as a result of eyewall
       mesovortices, which persist until landfall. While these tornadoes
       are normally not as strong as their non-tropical counterparts, they
       can still cause tremendous damage.

   The aftermath of Hurricane Katrina in Gulfport, Mississippi. Katrina
   was the costliest tropical cyclone in United States history.
   Enlarge
   The aftermath of Hurricane Katrina 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:
     * 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 be important factors in the precipitation regimes of places
   they impact and bring much-needed precipitation to otherwise dry
   regions. Hurricanes in the eastern north Pacific often supply moisture
   to the Southwestern United States and parts of Mexico. Japan receives
   over half of its rainfall from typhoons. Hurricane Camille averted
   drought conditions and ended water deficits along much of its path,
   though it also killed 259 people and caused $9.14 billion (2005 USD) in
   damage.

   Hurricanes also help to maintain global heat balance by moving warm,
   moist tropical air to the mid-latitudes and polar regions. Were it not
   for the movement of heat poleward (through other means as well as
   hurricanes), the tropical regions would be unbearably hot. The storm
   surges and winds of hurricanes may be destructive to human-made
   structures, but they also stir up the waters of coastal estuaries,
   which are typically important fish breeding locales.

   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 after
   Hurricane Camille. Of course, many former residents and businesses do
   relocate to inland areas away from the threat of future hurricanes as
   well.

   At sea, tropical cyclones can stir up water, leaving a cool wake behind
   them. This can cause the region to be less favourable for a subsequent
   tropical cyclone. On rare occasions, tropical cyclones may actually do
   the opposite. 2005's Hurricane Dennis blew warm water behind it,
   contributing to the unprecedented intensity of the close-following
   Hurricane Emily.

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 numerical global trend; the annual
   global number of tropical cyclones remains about 90 ± 10. However,
   there is some evidence that the intensity of hurricanes is increasing.
   "Records of hurricane activity worldwide show an upswing of both the
   maximum wind speed in and the duration of hurricanes. The energy
   released by the average hurricane (again considering all hurricanes
   worldwide) seems to have increased by around 70% in the past 30 years
   or so, corresponding to about a 15% increase in the maximum wind speed
   and a 60% increase in storm lifetime."

   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 be attributed to the increased
   intensity and duration of hurricanes striking North America and to the
   number of people living in susceptible coastal area following increased
   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 may have gone unmeasured in some instances. 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, could miss 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. The question is thus whether a
   statistical trend in frequency or strength of cyclones exists. The U.S.
   National Oceanic and Atmospheric Administration Geophysical Fluid
   Dynamics Laboratory performed a simulation that concluded "the
   strongest hurricanes in the present climate may be upstaged by even
   more intense hurricanes over the next century as the earth's climate is
   warmed by increasing levels of greenhouse gases in the atmosphere." .

   In an article in Nature, Kerry Emanuel states that the potential
   hurricane destructiveness, a measure which combines strength, duration,
   and frequency of hurricanes, "is highly correlated with tropical sea
   surface temperature, reflecting well-documented climate signals,
   including multidecadal oscillations in the North Atlantic and North
   Pacific, and global warming." K. Emanuel further predicts "a
   substantial increase in hurricane-related losses in the twenty-first
   century."

   Along similar lines, P.J. Webster and others published an article in
   Science 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 global warming and the hypothesized
   Atlantic Multidecadal Oscillation (AMO), a possible 50–70 year pattern
   of temperature variability. Emanuel, however, found the recent
   temperature increase was outside the range of previous sea surface
   temperature peaks. So, both global warming and a natural variation
   (such as the AMO) 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 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.

   Under the assumption that the six basins are statistically independent
   except for the effect of global warming, 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.

Observation and forecasting

Observation

   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 if it has overtaken an unfortunate ship. Even in
   these cases, real-time measurements are 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 United States
   government hurricane hunters. The aircraft used are WC-130 Hercules and
   WP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft
   fly directly into the 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.

   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 and the Wind
   Engineering Mobile Instrumented Tower Experiment. 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 Centre
   uses the data to evaluate conditions at landfall and to verify
   forecasts.

Forecasting

   Hurricane Epsilon organized and strengthened despite extremely
   unfavorable conditions.
   Enlarge
   Hurricane Epsilon organized and strengthened despite extremely
   unfavorable conditions.

   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.

Classifications, Terminology and Naming

Intensity classifications

   Three tropical cyclones at different stages of development. The
   youngest, on the left, demonstrates only the most basic circular shape.
   The storm at the top right, a few days older, demonstrates spiral
   banding and increased centralization, while the storm in the lower
   right, the oldest, has developed a cyclonic eye.
   Enlarge
   Three tropical cyclones at different stages of development. The
   youngest, on the left, demonstrates only the most basic circular shape.
   The storm at the top right, a few days older, demonstrates spiral
   banding and increased centralization, while the storm in the lower
   right, the oldest, has developed a cyclonic eye.

   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." The Philippines will name tropical
   depressions from their own naming convention within their sphere of
   influence.

   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, outside of the Philippines,
   assign first names to systems that reach this intensity (thus the term
   named storm).

   A hurricane or typhoon (sometimes simply referred to as a tropical
   cyclone, as opposed to a depression or storm) is 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 centre.

   The circulation of clouds around a cyclone's centre 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. Even if the cyclone is weaker at the end of the
   eyewall replacement cycle, the fact that it has just undergone one and
   will not undergo another one soon will allow it to strengthen further,
   if other conditions allow it to do so.

Categories and ranking

                                            Saffir-Simpson Hurricane Scale
                                       TD TS 1 2 3 4                     5

   Hurricanes are ranked according to their maximum winds using the
   Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest
   maximum winds (74-95 mph, 119-153 km/h), a Category 5 hurricane has the
   highest (> 155 mph, 249 km/h). The U.S. National Hurricane Centre
   classifies hurricanes of Category 3 and above as major hurricanes.

   The U.S. Joint Typhoon Warning Centre classifies West Pacific typhoons
   as tropical cyclones with winds greater than 73 mph (118 km/h).
   Typhoons with wind speeds of at least 150 mph (67 m/s or 241 km/h,
   equivalent to a strong Category 4 hurricane) are dubbed Super Typhoons.

   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. A
   category 1 storm features gusts less than 126 km/h (78 mph), while
   gusts in a category 5 cyclone are at least 280 km/h (174 mph).

   Meteorologists in the United States use maximum 1-minute average
   sustained winds 10 meters above the ground to determine tropical
   cyclone strength. Other countries use the maximum 10-minute average, as
   suggested by the World Meteorological Organization. Maximum wind speeds
   are typically about 12% lower with the 10-minute method than with the
   1-minute method.

   The rankings are not absolute in terms of damage and other effects,
   since it only based on windspeed. 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 minimal strength can produce significant
   damage and human casualties from flooding and landslides, particularly
   if they are slow-moving or very large in size.

Regional terminology

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

   Depending on their current basin and intensity, tropical cyclones may
   be referred to using one of many different terms, and each basin uses a
   separate system of terminology, making comparison difficult. Tropical
   cyclones which cross from one basin into another may then be referred
   to as a tropical cyclone of the type in the new basin rather than its
   original basin. This, however, is only common in the Pacific Ocean,
   where hurricanes from the Central North Pacific sometimes cross into
   the Northwest Pacific and are called typhoons. On very rare occasions,
   a typhoon will cross into the Central Pacific and become known as a
   hurricane. No other types of basin crossings that would result in a
   change of term have been recorded. Additionally, most basins use a
   10-minute average of sustained wind speeds to determine intensity, as
   recommended by the WMO, but this is not the case in the North Atlantic
   and Northeastern Pacific, where 1-minute averages, almost always
   higher, are used.

   In the North Atlantic and Northeastern Pacific, as well as the Central
   North Pacific, tropical cyclones with sustained winds of less than
   39 miles per hour (63 km/h) are referred as tropical depressions. If a
   tropical system acquires wind speeds of 39 miles per hour or greater,
   it becomes a tropical storm. Tropical storms which attain wind speeds
   of 74 miles per hour (119 km/h) or higher, or hurricane-force on the
   Beaufort wind scale, are then referred to as hurricanes. All
   measurements in the North Atlantic, Northeastern Pacific and North
   Central pacific use 1-minute average sustained wind speeds.

   In the Northwestern Pacific, tropical cyclones with sustained winds of
   less than 63 kilometres per hour (39 mph) are referred to as tropical
   depressions, as in the North Atlantic, Northeastern Pacific and Central
   Pacific, though measurements are made using 10-minute averages for
   sustained winds. Tropical systems with sustained winds that reach
   63 kilometres per hour (39 mph) are called tropical storms, again using
   the same terminology, but if a tropical storm in the Northwestern
   Pacific reaches sustained winds of 89 kilometres per hour (55 mph), it
   becomes referred to as a severe tropical storm. Systems that have
   sustained winds measured at hurricane-strength on the Beaufort scale
   are referred to as typhoons. It is also of note that typhoons with
   sustained winds greater than 239 kilometres per hour (150 mph) are
   called super typhoons by Joint Typhoon Warning Centre. This term is not
   defunct in official usage in WMO typhoon committee, both before and
   after 2000.

   In the Southwestern Indian Ocean: (1) a "tropical depression" is a
   tropical disturbance in which the maximum of the average wind speed is
   28 to 33 knots (51 to 62 km/h); (2) a "moderate tropical storm" is a
   tropical disturbance in which the maximum of the average wind speed is
   34 to 47 knots (63 to 88 km/h); (3) a "severe tropical storm" is a
   tropical disturbance in which the maximum of the average wind speed is
   48 to 63 knots (89 to 117 km/h); (4) a "tropical cyclone" is a tropical
   disturbance in which the maximum of the average wind speed is 64 to 89
   knots (118 to 165 km/h); (5) an "intense tropical cyclone" is a
   tropical disturbance in which the maximum of the average wind speed is
   90 to 115 knots (166 to 212 km/h); and (6) a "very intense tropical
   cyclone" is a tropical disturbance in which the maximum of the average
   wind speed is greater than 115 knots (greater than 212 km/h).

   There are many regional names for tropical cyclones, including ^†
   baguio and spelt in the vernacular, 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 taifū in Japanese, has an independent origin
            traceable variously to 風颱, 風篩 or 風癡 hongthai, going back to
            Song 宋 (960-1278) and Yuan 元(1260-1341) dynasties. The first
            record of the character 颱 appeared in 1685's edition of
            Summary of Taiwan 臺灣記略).
          + From Urdu, Persian or Arabic ţūfān (طوفان) < Greek tuphōn
            (Τυφών).
          + Portuguese tufão is also related to typhoon. See Typhon for
            more information.
     * The word hurricane is derived from the name of a native Caribbean
       Amerindian storm god, Huracan, via Spanish huracán.
     * The word cyclone 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. Tropical cyclones are then circular
       wind storms that form in the tropics.

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 are
   any) are "retired" and new names are chosen to take their place.

Naming schemes

   In the North Atlantic and Northeastern Pacific regions, feminine and
   masculine names are alternated in alphabetic order during a given
   season. The gender of the season's first storm also alternates year to
   year. 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 North Atlantic; only "Q" and "U" are omitted in the
   Northeastern Pacific. This allows for 21 names in the North Atlantic
   and 24 names in Northeastern Pacific. 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 language 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. This was first necessary during the
   2005 Atlantic 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 Centre in Honolulu, Hawaii. Four lists of
   Hawaiian names are selected and used in sequential order without regard
   to year.

   In the Northwestern 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. Names are retired from the lists upon request. The most
   common reason is to memorize the extensive damage caused by the storm.
   When names are retired, the contributing member should propose new
   names. A possible way to do so is through local name nomination
   contest, which was done in Hong Kong and China.

   The Australian Bureau of Meteorology maintains three lists of names,
   one for each of the Western, Northern and Eastern Australian regions.
   These lists are in alphabetical order and alternate gender, but are
   used sequentially rather than switched each year. There are also Fiji
   region and Papua New Guinea region names agreed upon WMO RA V Tropical
   Cyclone Committee members.

   The RA I Tropical Cyclone Committee for the South-West Indian Ocean
   creates the lists of names for the Southwestern Indian Ocean. The
   committee adopted two separate lists of names for the 2006-07 and
   2007-08 tropical cyclone seasons at its October 2005 meeting in
   Gaborone, Botswana. Nominations for the lists were submitted by
   Mauritius, Malawi, Mozambique, Namibia, Seychelles, South Africa,
   Swaziland, Zimbabwe, Tanzania, Botswana, Comoros, Lesotho, and
   Madagascar. If a tropical disturbance reaches "moderate tropical storm"
   status west of 55 degrees east longitude, then the Sub-regional
   Tropical Cyclone Advisory Centre in Madagascar assigns the appropriate
   name to the storm. If a tropical disturbance reaches "moderate tropical
   storm" status between 55 and 90 degrees east longitude, then the
   Sub-regional Tropical Cyclone Advisory Centre in Mauritius assigns the
   appropriate name to the storm.

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, Météo-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 Centre (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 Centre (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.
                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.
                In 2003, when Larry was about to move across Mexico, NHC
                attempted to provide greater clarity:

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

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

    3. Uncertainties of the continuation

                When the remnants of a tropical cyclone redevelop, the
                redeveloping system will be treated as a new tropical
                cyclone if there are uncertainties of the continuation,
                even though the original system may contribute to the
                forming of the new system. One example is Tropical
                Depression 10-Tropical Depression 12 (which became
                Hurricane Katrina) 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 and
                Tropical Storm Upana-Chanchu in 2000

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. If a second storm struck on the same saint's day later, it
   would be referred to as segundo (Spanish for "the second"), as with
   Hurricane San Felipe Segundo.

   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.

   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 helped to
   popularize the concept of giving names to tropical cyclones.

   From 1950 to 1954, names from the Joint Army/Navy Phonetic Alphabet
   were used for storms in the North Atlantic. 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
   Centre. Naming is now maintained by the World Meteorological
   Organization.

   In keeping with the common English language practice of referring to
   named 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 which the storms typically affect. In the
   southern hemisphere, male names were given to cyclones starting in
   1975.

Notable cyclones

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

   The deadliest tropical cyclone on record hit the densely populated
   Ganges Delta region of 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.

   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. 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. The deadliest Atlantic storm on
   record was the Great Hurricane of 1780, which killed about 22,000
   people in the Antilles.
   The relative sizes of Typhoon Tip, Tropical Cyclone Tracy, and the
   United States.
   The relative sizes of Typhoon Tip, Tropical Cyclone Tracy, and the
   United States.

   The most intense storm on record was Typhoon Tip in the northwestern
   Pacific Ocean in 1979, which had a minimum pressure of only 870 mbar
   and maximum sustained wind speeds of 190 mph (305 km/h). It weakened
   before striking Japan. Tip does not hold the record for fastest
   sustained winds in a cyclone alone; Typhoon Keith in the Pacific, and
   Hurricane Camille and Hurricane Allen in the North Atlantic currently
   share this record as well, although recorded wind speeds that fast are
   suspect since most monitoring equipment is likely to be destroyed by
   such extreme 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 centre 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. 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.

   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.

   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. Other destructive Pacific hurricanes
   include Pauline and Kenna.
   2004 image of Cyclone Catarina, the first identified hurricane-strength
   system in the South Atlantic.
   Enlarge
   2004 image of Cyclone Catarina, the first identified hurricane-strength
   system in the South Atlantic.

   On March 26, 2004, Cyclone Catarina became the first recorded South
   Atlantic cyclone (cyclone is the southern hemispheric term for
   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 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. 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. In addition, Hurricane Jeanne in 2004 caused
   the majority of its damage in Haiti, including approximately 3,000
   deaths, while just a tropical depression.

   On August 29, 2005, Hurricane Katrina made landfall in Louisiana and
   Mississippi. The U.S. National Hurricane Centre, in its August review
   of the tropical storm season stated that Katrina was probably the worst
   natural disaster in U.S. history. Currently, its death toll is at least
   1,836, mainly from flooding and the aftermath in New Orleans, Louisiana
   and the Mississippi Gulf Coast. It is also estimated to have caused
   $81.2 billion in property damage. 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.

Other storm systems

   Many other forms of cyclone can form in nature. Two 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;
   Additionally, although not as frequently, 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 cyclone

   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. From an
   operational standpoint, a tropical cyclone is usually not considered to
   become subtropical during its extratropical transition.

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