A 'tropical cyclone' is a meteorological term for a
storm system characterized by a
low pressure center and
thunderstorms that produces strong wind and flooding rain. A tropical cyclone feeds on the heat released when moist air rises and the water vapor it contains
condenses. They are fueled by a different heat mechanism than other cyclonic windstorms such as
nor'easters,
European windstorms, and
polar lows, leading to their classification as "warm core" storm systems.
The adjective "tropical" refers to both the geographic origin of these systems, which form almost exclusively in
tropical regions of the globe, and their formation in
Maritime Tropical air masses. The noun "cyclone" refers to such storms'
cyclonic nature, with
counterclockwise rotation in the
Northern Hemisphere and clockwise rotation in the
Southern Hemisphere. Depending on their location and strength, tropical cyclones are referred to by various other names, such as 'hurricane', 'typhoon', 'tropical storm', 'cyclonic storm', and 'tropical depression'.
While tropical cyclones can produce extremely powerful winds and torrential
rain, they are also able to produce high waves and damaging
storm surge. They develop over large bodies of warm water, and lose their strength if they move over land. This is the reason coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal
flooding up to 25
mi (40
km) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve
drought conditions. They also carry heat and energy away from the tropics and transport it towards
temperate latitudes, which makes them an important part of the global
atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's
troposphere, and to maintain a relatively stable and warm temperature worldwide.
Many tropical cyclones
develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. Others form when
other types of cyclones acquire tropical characteristics. Tropical systems are then moved by
steering winds in the
troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an
eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates.
Physical structure
Structure of a tropical cyclone
All tropical cyclones are areas 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.
[1] Tropical cyclones are characterized and driven by the release of large amounts of latent
heat of condensation, which occurs when moist air is carried upwards and its water vapor condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
[ Frequently Asked Questions: What is an extra-tropical cyclone? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
Banding
Rainbands are bands of showers and thunderstorms that spiral cyclonically toward the storm center. 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.
[2] Intense annular tropical cyclones are distinctive for their lack of rainbands; instead, they possess a thick circular area of disturbed weather around their low pressure center.
[3] While all surface low pressure areas require divergence aloft to continue deepening, the divergence over tropical cyclones is in all directions away from the center. The upper levels of a tropical cyclone feature winds directed away from the center of the storm with an
anticyclonic rotation, due to the
Coriolis effect.
Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to requiring a relative lack of vertical
wind shear to maintain the warm core at the center of the storm.
[4][5]
Eye and inner core
A strong tropical cyclone will harbor an area of sinking air at the center of circulation. If this area is strong enough, it can develop into an
eye. Weather in the eye is normally calm and free of clouds, though the sea may be extremely violent.
The eye is normally circular in shape, and may range in size from 3 to 370 km (2–230 miles) in diameter.
[6][7] Intense, mature hurricanes can sometimes exhibit an inward curving of the eyewall's top, making it resemble a football stadium; this phenomenon is thus sometimes referred to as the ''
stadium effect''.
[8]
There are other features that either surround the eye, or cover it. The
central dense overcast is the concentrated area of strong thunderstorm activity near the center of a tropical cyclone;
[9] in weaker tropical cyclones, the CDO may cover the center completely.
[ Frequently Asked Questions: What is a "CDO"? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] The
eyewall is a circle of strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a hurricane's eyewall passes over land.
Associated with eyewalls are
eyewall replacement cycles, which occur naturally 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 10–25 km (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
angular momentum. During this phase, the tropical cyclone weakens (i.e., the maximum winds die off somewhat and the central pressure goes up), but eventually the outer eyewall replaces the inner one completely. The storm can be of the same intensity as it was previously or, in some cases, it can be even stronger after the eyewall replacement cycle. Even if the cyclone is weaker at the end of the cycle, the storm may strengthen again as it builds a new outer ring for the next eyewall replacement.
[ Frequently Asked Questions: What are "concentric eyewall cycles" (or "eyewall replacement cycles") and why do they cause a hurricane's maximum winds to weaken? Atlantic Oceanographic and , Hurricane Research Division ]
Size
The size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed
isobar. If the radius is less than two
degrees of latitude (120 nm, 222 km), then the cyclone is "very small" or a "midget." Radii of 2–3 degrees (120–180 nm, 222–333 km) are considered "small." Radii between 3 and 6 latitude degrees (180–360 nm, 333–666 km) are considered "average sized." Tropical cyclones are considered "large" when the closed isobar radius is 6–8 degrees of latitude (360–480 nm, 666–888 km), while "very large" tropical cyclones have a radius of greater than 8 degrees (480 nm, 888 km).
[10] Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius of the central dense overcast.
Mechanics
Tropical cyclones form when the energy released by the condensation of moisture in rising air causes a
positive feedback loop over warm ocean waters.
A tropical cyclone's primary
energy source is the release of the
heat of condensation from water vapor
condensing at high altitudes, with
solar heating being the initial source for evaporation. Therefore, a tropical cyclone can be visualized as a giant vertical
heat engine supported by mechanics driven by physical forces such as the
rotation and
gravity of the
Earth.
[ Hurricanes... Unleashing Nature's Fury: A Preparedness Guide National Weather Service ] 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.
[11] This provides the system with enough energy to be self-sufficient and causes a
positive feedback loop that continues as long as the tropical cyclone can draw energy from its
thermal reservoir, the warm water at the surface of the ocean. 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.
What primarily distinguishes tropical cyclones from other meteorological phenomena is deep
convection as a driving force.
[12] Because convection is strongest in a
tropical climate, it 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 to maintain the positive feedback loop running. As a result, when a tropical cyclone passes over land, it is cut off from its heat source and its strength diminishes rapidly.
The passage of a tropical cyclone over the ocean can cause the upper layers of the ocean to cool substantially, which can influence subsequent cyclone development. Cooling is primarily caused by upwelling of cold water from deeper in the ocean due to the wind stresses the storm itself induces upon the sea surface. Additional cooling may come in the form of cold water from falling raindrops. 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.
[13]
Scientists at the
National Center 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 200 times the world-wide electrical generating capacity,
or to exploding a 10-
megaton nuclear bomb every 20 minutes.
[14]
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. These high cirrus clouds may be the first signs of an approaching tropical cyclone when seen from dry land.
Major basins and related warning centers
Main articles: Tropical cyclone basins,
Regional Specialized Meteorological Centre,
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 six
Tropical Cyclone Warning Centres (TCWCs) that provide information to smaller regions.
[15] The RSMCs and TCWCs, however, are not the only organizations that provide information about tropical cyclones to the public. The
Joint Typhoon Warning Center (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 and names for tropical cyclones that approach the
Philippines in the Northwestern Pacific. The
Canadian Hurricane Centre (CHC) issues advisories on hurricanes and their remnants when they affect Canada.
On
March 26,
2004,
Cyclone Catarina became the first recorded
South Atlantic cyclone and subsequently struck southern
Brazil as the equivalence of a Category 2 hurricane on the
Saffir-Simpson Hurricane Scale. As the cyclone formed outside of the authority of another warning center, Brazilian meteorologists initially treated the system as an
extratropical cyclone, though subsequently classified it as tropical.
[16]
Formation
Main articles: Tropical cyclogenesis
Times
Worldwide, tropical cyclone activity peaks in late
summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. 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.
[ Frequently Asked Questions: When is hurricane season? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
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.
[17] 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.
Factors
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. While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the following conditions. In most situations,
water temperatures of at least 26.5 °C (80 °F) are needed
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.
[ Frequently Asked Questions: Why do tropical cyclones require 80 °F (26.5 °C) ocean temperatures to form? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] Another factor is rapid cooling with height, which allows the release of the
heat of condensation that powers a tropical cyclone.
High humidity is needed, especially in the lower-to-mid
troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.
Low amounts of
wind shear are needed, as high shear is disruptive to the storm's circulation.
Tropical cyclones generally need to form more than 500 km (310 miles) or 5 degrees of
latitude away from the
equator, allowing the
Coriolis effect to deflect winds blowing towards the low pressure center and creating a circulation.
Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.
Locations
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. Another important source of atmospheric instability is found in
tropical waves, which cause about 85% of intense tropical cyclones in the Atlantic ocean,
[18] and become most of the tropical cyclones in the Eastern Pacific basin.
Tropical cyclones originate on the eastern side of oceans, but move west, intensifying as they move. Most of these systems form between 10 and 30 degrees away of the
equator, and 87% form no farther away than 20 degrees of latitude, north or south. 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.
[ Worldwide Tropical Cyclone Tracks 1979-88 Neumann, Charles J. ] However, it is possible for tropical cyclones to form within this boundary as
Tropical Storm Vamei did in 2001 and
Cyclone Agni in 2004.
Movement and track
Steering 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 analogized by Dr. Neil Frank, former director of the
National Hurricane Center, to "leaves carried along by a stream."
[ Frequently Asked Questions: What determines the movement of tropical cyclones? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
Tropical systems, while generally located
equatorward of the 20
th parallel, are steered primarily westward by the east-to-west winds on the equatorward side of the
subtropical ridge—a persistent high pressure area over the world's oceans.
In the tropical North Atlantic and Northeast Pacific oceans,
trade winds—another name for the westward-moving wind currents—steer
tropical waves westward from the
African coast and towards the Caribbean Sea, North America, and ultimately into the central Pacific ocean before the waves dampen out.
These waves are the precursors to many tropical cyclones within this region.
[18] In the Indian Ocean and Western Pacific (both 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.
[21]
Coriolis effect
The Earth's rotation imparts an acceleration known as the ''
Coriolis effect'', ''Coriolis acceleration'', or colloquially, ''Coriolis force''. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents.
[22] The poleward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more poleward. The westerly winds on the equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is poleward. Thus, tropical cyclones in the
Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the
Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis effect.
The Coriolis effect also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds - that force is the
heat of condensation.
Interaction with the mid-latitude westerlies
When a tropical cyclone crosses the
subtropical ridge axis, its general track around the high-pressure area is deflected significantly by winds moving towards the general low-pressure area to its north. When the cyclone track becomes strongly poleward with an easterly component, the cyclone has begun ''recurvature.''
[23] A typhoon moving through the Pacific Ocean towards
Asia, for example, will recurve offshore of
Japan to the north, and then to the northeast, if the typhoon encounters winds blowing northeastward toward a low-pressure system passing over
China or
Siberia. Many tropical cyclones are eventually forced toward the northeast by
extratropical cyclones, which move from west to east to the north of the subtropical ridge. An example of a tropical cyclone in recurvature was
Typhoon Ioke in 2006, which took a similar trajectory.
[24]
Landfall
Officially, ''
landfall'' is when a storm's center (the center of its circulation, not its edge) crosses the coastline.
Storm conditions may be experienced on the coast and inland hours before landfall; in fact, a tropical cyclone can launch its strongest winds over land, yet not make landfall; if this occurs, then it is said that the storm made a ''direct hit'' on the coast.
Due to this definition, the landfall area experiences half of a land-bound storm by the time the actual landfall occurs. For emergency preparedness, actions should be timed from when a certain wind speed or intensity of rainfall will reach land, not from when landfall will occur.
[25]
Interactions between typhoons
When two cyclones approach each other, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the
Fujiwhara effect, after Dr. Sakuhei Fujiwhara.
Dissipation
Factors
A tropical cyclone can cease to have tropical characteristics through several different ways. One such way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly losing strength. Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two, or evolve into
extratropical cyclones. While there is a chance a tropical cyclone could regenerate if it managed to get back over open warm water, if it remains over mountains for even a short time, it can rapidly lose its structure. Many storm fatalities occur in mountainous terrain, as the dying storm unleashes torrential rainfall, leading to deadly
floods and
mudslides, similar to those that happened with
Hurricane Mitch in 1998. Additionally, dissipation can occur if a storm remains in the same area of ocean for too long, mixing the upper 30 meters (100 feet) of water. This occurs because the cyclone draws up colder water from deeper in the sea through
upwelling, and causes the water surface to become too cool to support the storm. Without warm surface water, the storm cannot survive.
[26]
A tropical cyclone can dissipate when it moves over waters significantly below 26.5 °C. This will cause the storm to lose its tropical characteristics (i.e. thunderstorms near the center and warm core) and become a remnant low pressure area, which can persist for several days. This is the main dissipation mechanism in the Northeast Pacific ocean.
[27] Weakening or dissipation can occur if it experiences vertical
wind shear, causing the convection and heat engine to move away from the center; this normally ceases development of a tropical cyclone.
[28] Additionally, its interaction with the main belt of the Westerlies, by means of merging with a nearby frontal zone, can cause tropical cyclones to evolve into
extratropical cyclones. This transition can take 1–3 days.
[ Tropical Cyclone Intensity Terminology United States Naval Research Laboratory ] 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. In the
Pacific ocean and
Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach the west coast of
North America. These phenomena can also affect Europe, where they are known as ''
European windstorms'';
Hurricane Iris's extratropical remnants became one in 1995.
[29] Additionally, a cyclone can merge with another area of low pressure, becoming a larger area of low pressure. This can strengthen the resultant system, although it may no longer be a tropical cyclone.
Artificial dissipation
In the 1960s and 1970s, the
United States government attempted to weaken hurricanes through
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—a hurricane seeded in Project Stormfury—dropped as much as 30%, but Debby regained its 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.
[30] 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% 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.
[ Frequently Asked Questions: Why don't we try to destroy tropical cyclones by seeding them with silver iodide? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing
icebergs into the tropical oceans.
Other ideas range from covering the ocean in a substance that inhibits evaporation,
[ Frequently Asked Questions: Why don't we try to destroy tropical cyclones by placing a substance on the ocean surface? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] dropping large quantities of ice into the eye at very early stages of development (so that the
latent heat is absorbed by the ice, instead of being converted to kinetic energy that would feed the positive feedback loop),
[ Frequently Asked Questions: Why don't we try to destroy tropical cyclones by cooling the surface waters with icebergs or deep ocean water? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] or blasting the cyclone apart with nuclear weapons.
[ Frequently Asked Questions: Why don't we try to destroy tropical cyclones by nuking them? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] Project Cirrus even involved throwing dry ice on a cyclone.
[31] These approaches all suffer from one flaw above many others: tropical cyclones are simply too large for any of them to be practical.
[ Frequently Asked Questions: Why do not we try to destroy tropical cyclones by (fill in the blank)? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
Effects
Main articles: Effects of tropical cyclones
Tropical cyclones out at sea cause large waves, heavy rain, and high winds, disrupting international shipping and, at times, causing shipwrecks.
[32] Tropical cyclones stir up water, leaving a cool wake behind them,
which causes the region to be less favourable for subsequent tropical cyclones. On land, strong
winds can damage or destroy vehicles, buildings, bridges, and other outside objects, turning loose debris into deadly flying projectiles. The
storm surge, or the increase in sea level due to the cyclone, is typically the worst effect from landfalling tropical cyclones, historically resulting in 90% of tropical cyclone deaths.
[33]
The broad rotation of a landfalling tropical cyclone, and vertical wind shear at its periphery, spawns
tornadoes. Tornadoes can also be spawned
as a result of
eyewall mesovortices, which persist until landfall.
[ Frequently Asked Questions: Are TC tornadoes weaker than midlatitude tornadoes? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
Within the last two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million persons worldwide. Large areas of standing water caused by flooding lead to infection, as well as contributing to mosquito-borne illnesses. Crowded evacuees in
shelters increase the risk of disease propagation.
Tropical cyclones significantly interrupt infrastructure, leading to power outages, bridge destruction, and hamper reconstruction efforts.
Although cyclones take an enormous toll in lives and personal property, they may be important factors in the
precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.
[35] Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the
middle latitudes and polar regions.
[36] The storm surge 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. Tropical cyclone destruction spurs redevelopment, greatly increasing local property values.
[37]
Observation and forecasting
Observation
Main articles: Tropical cyclone observation
Intense tropical cyclones pose a particular observation challenge, as they are a dangerous oceanic phenomenon, and
weather stations, being relatively small, are rarely available on the site of the storm itself. Surface observations are generally available only if the storm is passing over an island or a coastal area, or if there is a nearby ship. Usually, real-time measurements are taken in the periphery of the cyclone, where conditions are less catastrophic and its true strength cannot be evaluated. For this reason, there are teams of meteorologists that move into the path of tropical cyclones to help evaluate their strength at the point of landfall.
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.
In-situ measurements, in real-time, can be taken by sending specially equipped reconnaissance flights into the cyclone. In the Atlantic basin, these flights are regularly flown by United States government
hurricane hunters.
[38] 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. A similar mission was also completed successfully in the western Pacific ocean. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare.
[39]
A general decrease in error trends in tropical cyclone path prediction is evident since the 1970s
Forecasting
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. The deep layer mean flow is considered to be the best tool in determining track direction and speed. If storms are significantly sheared, use of wind speed measurements at a lower altitude, such as at the 700
hpa pressure surface (3000 meters or 10000 feet above sea level) will produce better predictions. Tropical forecasters also consider smoothing out short-term wobbles of the storm center in order
a best practice, as it allows them to determine a more accurate trajectory.
[40] High-speed computers and sophisticated simulation software allow forecasters to produce
computer models that predict tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. Combining forecast models with increased understanding of the forces that act on tropical cyclones, as well as with a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades.
[41] However, scientists say they are less skillful at predicting the intensity of tropical cyclones.
[42] 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
Main articles: Tropical cyclone scales
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. For example, if a
tropical storm in the Northwestern Pacific reaches hurricane-strength winds on the
Beaufort scale, it is referred to as a ''typhoon''; if a tropical storm passes the same benchmark in the
Northeast Pacific Basin, or in
the Atlantic, it is called a ''hurricane''.
Neither "hurricane" nor "typhoon" is used in the South Pacific.
Additionally, as indicated in the table below, each basin uses a separate
system of terminology, making comparisons between different basins difficult. In the Pacific Ocean, hurricanes from the Central North Pacific sometimes cross the
International Date Line into the Northwest Pacific, becoming typhoons (such as
Hurricane/Typhoon Ioke in 2006); on rare occasions, the reverse will occur.
[43] It should also be noted that typhoons with sustained winds greater than 130
knots (240
km/h or 150
mph) are called ''Super Typhoons'' by the Joint Typhoon Warning Center.
[44]
A 'tropical depression' is an organized system of clouds and thunderstorms with a defined, closed 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. However, it is already a low-pressure system, hence the name "depression."
The practice of the
Philippines is to name tropical depressions from their own naming convention when the depressions are within the Philippines' area of responsibility.
[45]
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, although an eye is not usually present. Government weather services, other than the Philippines, first assign 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 of at least 33 m/s (64 kt, 74 mph, or 118 km/h).
A cyclone of this intensity 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 16–80
km (10–50
mi) wide in which the strongest
thunderstorms and winds circulate around the storm's center. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 85 m/s (165 kt, 190 mph, 305 km/h).
| 'Tropical Cyclone Classifications (all winds are 10-minute averages)' |
|---|
| Beaufort scale | 10-minute sustained winds (knots) | N Indian Ocean
IMD | SW Indian Ocean
MF | Australia
BOM | SW Pacific
FMS | NW Pacific
JMA | NW Pacific
JTWC | NE Pacific &
N Atlantic
NHC & CPHC |
|---|---|---|---|---|---|---|---|---|
| 0–6 | <28 | Depression | Trop. Disturbance | Tropical Low | Tropical Depression | Tropical Depression | Tropical Depression | Tropical Depression |
| 7 | 28–29 | Deep Depression | Depression |
| 30–33 | Tropical Storm | Tropical Storm |
| 8–9 | 34–47 | Cyclonic Storm | Moderate Tropical Storm | Trop. Cyclone (1) | Tropical Cyclone | Tropical Storm |
| 10 | 48–55 | Severe Cyclonic Storm | Severe Tropical Storm | Tropical Cyclone (2) | Severe Tropical Storm |
| 11 | 56–63 | Typhoon | Hurricane (1) |
| 12 | 64–72 | Very Severe Cyclonic Storm | Tropical Cyclone | Severe Tropical Cyclone (3) | Typhoon |
| 73–85 | Hurricane (2) |
| 86–89 | Severe Tropical Cyclone (4) | Major Hurricane (3) |
| 90–99 | Intense Tropical Cyclone |
| 100–106 | Major Hurricane (4) |
| 107–114 | Severe Tropical Cyclone (5) |
| 115–119 | Very Intense Tropical Cyclone | Super Typhoon |
| >120 | Super Cyclonic Storm | Major Hurricane (5) |
Origin of storm terms
The word ''typhoon'', used today in the Northwest Pacific, has two possible and equally plausible origins. The first is from the
Chinese 大風 (
Cantonese:
daaih fūng;
Mandarin:
dà fēng) which means "great
wind."
[ Hurricanes: The Greatest Storms on Earth Earth Observatory ] (The Chinese term as 颱風 or 台风
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).
[46] Alternatively, the word may be derived from
Urdu,
Persian and
Arabic ''ţūfān''
(طوفان), which in turn originates from
Greek ''
tuphōn'' (Τυφών), a monster in
Greek mythology responsible for hot winds.
[47] The related
Portuguese word ''tufão'', used in Portuguese for any tropical cyclone, is also derived from Greek ''tuphōn''.
The word ''hurricane'', used in the North Atlantic and Northeast Pacific, is derived from the name of a native
Caribbean Amerindian storm
god,
Huracan, via
Spanish ''huracán''.
[ Frequently Asked Questions: What is the origin of the word "hurricane"? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] (Huracan is also the source of the word ''Orcan'', another word for the
European windstorm. These events should not be confused.)
Naming
Main articles: Tropical cyclone naming,
Lists of tropical cyclone names
Storms reaching tropical storm strength were initially given names to eliminate confusion when there are multiple systems in any individual basin at the same time which assists in warning people of the coming storm.
[48] In most cases, a tropical cyclone retains its name throughout its life; however, under
special circumstances, tropical cyclones may be renamed while active. 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.
Notable tropical cyclones
Main articles: List of notable tropical cyclones,
List of notable Atlantic hurricanes,
List of notable Pacific hurricanes
Tropical cyclones that cause extreme destruction are rare, though when they occur, they can cause great amounts of damage or thousands of fatalities.
The
1970 Bhola cyclone is the deadliest tropical cyclone on record, killing over 300,000 people
[49] and potentially as many as 1 million
[50] after striking the densely populated
Ganges Delta region of
Bangladesh on
November 13,
1970. Its powerful storm surge was responsible for the high death toll.
The
North Indian cyclone basin has historically been the deadliest basin, with several cyclones since 1900 killing over 100,000 people, all in Bangladesh.
[51][52] Elsewhere,
Typhoon Nina killed 29,000 in
China due to a
2000-year flood which caused 62 dams including the
Banqiao Dam to fail; another 145,000 died during the subsequent famine and epidemic.
[53] The
Great Hurricane of 1780 is the deadliest
Atlantic hurricane on record, killing about 22,000 people in the
Lesser Antilles.
[ The Deadliest Atlantic Tropical Cyclones, 1492-1996 National Hurricane Center ] A tropical cyclone does need not be particularly strong to cause memorable damage, primarily if the deaths are from rainfall or mudslides.
Tropical Storm Thelma in November 1991 killed thousands in the
Philippines,
[54] while in 1982, the unnamed tropical depression that eventually became
Hurricane Paul killed around 1,000 people in
Central America.
[55]
Hurricane Katrina is estimated as the costliest tropical cyclone worldwide,
[56] causing $81.2 billion in property damage (2005 USD)
[57] with overall damage estimates exceeding $100 billion (2005 USD).
Katrina killed at least 1,836 people after striking
Louisiana and
Mississippi as a
major hurricane in August 2005.
The
Galveston Hurricane of 1900 is the deadliest natural disaster in the
United States, killing an estimated 6,000 to 12,000 people in
Galveston, Texas.
Hurricane Iniki in 1992 was the most powerful storm to strike
Hawaii in recorded history, hitting
Kauai as a Category 4 hurricane, killing six people, and causing U.S. $3 billion in damage.
[58] Other destructive Eastern
Pacific hurricanes include
Pauline and
Kenna, both causing severe damage after striking
Mexico as major hurricanes.
[59][60] In March 2004,
Cyclone Gafilo struck northeastern
Madagascar as a powerful cyclone, killing 74, affecting more than 200,000, and becoming the worst cyclone to affect the nation for over 20 years.
[61]
The most intense storm on record was
Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of 870
mbar (25.69
inHg) and maximum sustained wind speeds of 165 knots (190 mph, 305 km/h).
[62] Tip, however, does not solely hold the record for fastest sustained winds in a cyclone.
Typhoon Keith in the Pacific and Hurricanes
Camille and
Allen in the North Atlantic currently share this record with Tip.
[63] Camille was the only storm to actually strike land while at that intensity, making it, with 165 knots (190 mph, 305 km/h) sustained winds and 210 mph (335 km/h) gusts, the strongest tropical cyclone on record at landfall.
[64] Typhoon Nancy in 1961 had recorded wind speeds of 185 knots (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 storm with the highest wind speeds on record.
[ Frequently Asked Questions: Which is the most intense tropical cyclone on record? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] Similarly, a surface-level gust caused by
Typhoon Paka on
Guam was recorded at 205 knots (235 mph, 380 km/h). Had it been confirmed, it would be the strongest non-
tornadic wind ever recorded on the
Earth's surface, but the reading had to be discarded since the
anemometer was damaged by the storm.
[65]
In addition to being the most intense tropical cyclone on record, Tip was the largest cyclone on record, with tropical storm-force winds 2,170 km (1,350 miles) in diameter. The smallest storm on record,
Cyclone Tracy, was roughly 100 km (60 miles) wide before striking
Darwin,
Australia in 1974.
[ Frequently Asked Questions: Which are the largest and smallest tropical cyclones on record? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ]
Hurricane John is the longest-lasting tropical cyclone on record, lasting 31 days in
1994. Prior to the advent of satellite imagery in 1961, however, many tropical cyclones were underestimated in their durations.
[66] John is the second longest-tracked tropical cyclone in the Northern Hemisphere on record, behind
Typhoon Ophelia of 1960 which had a path of 8,500 miles (12,500 km). Reliable data for Southern Hemisphere cyclones is unavailable.
[67]
Long term activity trends
Atlantic Multidecadal Cycle since 1950, using accumulated cyclone energy (ACE)
:
While the number of storms in the Atlantic has increased since 1995, there is no obvious global trend; the annual number of tropical cyclones worldwide remains about 87 ± 10. However, the ability of climatologists to make long-term data analysis in certain basins is limited by the lack of reliable historical data in some basins, primarily in the Southern Hemisphere.
[68] In spite of that, there is some evidence that the intensity of hurricanes is increasing.
Kerry Emanuel stated, "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."
[69]
Atlantic storms are 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 a greater degree, the number of people living in susceptible coastal areas, 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.
[70]
The number and strength of Atlantic hurricanes may undergo a 50-70 year cycle, also known as the
Atlantic Multidecadal Oscillation. Although more common since 1995, few above-normal hurricane seasons occurred during 1970-1994.
[71] Destructive hurricanes struck frequently from 1926-60, including many major New England hurricanes. A record 21 Atlantic tropical storms formed in
1933, a record only recently exceeded in
2005, which saw 28 storms. Tropical hurricanes occurred infrequently during the seasons of 1900-1925; however, many intense storms formed during 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. Some historical weather experts say these storms may have been as high as
Category 4 in strength.
[72]
These active hurricane seasons predated satellite coverage of the Atlantic basin. Before the satellite era began in 1960, tropical storms or hurricanes went undetected unless a ship reported a voyage through the storm or a storm hit land in a populated area.
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
:
The
U.S. National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory performed a simulation to determine if there is a
statistical trend in the frequency or strength of cyclones over time. The simulation 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."
[73]
In an article in ''
Nature'',
Kerry Emanuel stated that potential hurricane destructiveness, a measure combining hurricane strength, duration, and frequency, "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." Emanuel predicted "a substantial increase in hurricane-related losses in the twenty-first century."
[74]. Similarly, P.J. Webster and others published an article in ''
Science'' examining the "changes in tropical cyclone number, duration, and intensity" over the last 35 years, the period when satellite data has been available. Their main finding was although the number of cyclones decreased throughout the planet excluding the north
Atlantic Ocean, there was a great increase in the number and proportion of very strong cyclones.
[75]
The strength of the reported effect is surprising in light of modeling studies
[76] that predict only a one half category increase in storm intensity as a result of a ~2 °C global warming. Such an response would have predicted only a ~10% increase in Emanuel's potential destructiveness index during the twentieth century rather than the ~75-120% increase he reported.
Secondly, after adjusting for changes in population and inflation, and despite a more than 100% increase in Emanuel's potential destructiveness index, no statistically significant increase in the monetary damages resulting from Atlantic hurricanes has been found.
[77]
Both Emanuel and Webster et al. consider
sea surface temperatures to be vital in the development of cyclones. Though neither study can directly link hurricanes with global warming, the increase in sea surface temperatures is believed to be due to both global warming and nature variability, e.g. the hypothesized
Atlantic Multidecadal Oscillation (AMO), though an exact attribution has not been defined.
[ However, recent temperatures are the warmest ever observed for many ocean basins.]
In February 2007, the United Nations Intergovernmental Panel on Climate Change released its fourth assessment report on climate change. The report noted many observed changes in the climate, including atmospheric composition, global average temperatures, ocean conditions, among others. The report concluded the observed increase in tropically intensity is larger than climate models predict. Additionally, the report considered that it is likely that storm intensity will continue to increase through the 21st century, and declared it more likely than not that there has been some human contribution to the increases in tropical cyclone intensity.[78] However, there is no universal agreement about the magnitude of the effects anthropogenic global warming has on tropical cyclone formation, track, and intensity. For example, critics such as Chris Landsea assert that man-made effects would be "quite tiny compared to the observed large natural hurricane variability."[ Frequently Asked Questions: What may happen with tropical cyclone activity due to global warming? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] A statement by the American Meteorological Society on February 1, 2007 stated that trends in tropical cyclone records offer "evidence both for and against the existence of a detectable anthropogenic signal" in tropical cyclogenesis.[79] Albeit many aspects of a link between tropical cyclones and global warming are still being "hotly debated",
Related cyclone types
In addition to tropical cyclones, there are two other classes of cyclones within the spectrum of cyclone types. These kinds of cyclones, known as extratropical cyclones and subtropical cyclones, can be stages a tropical cyclone passes through during its formation or dissipation.[82]
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;[ Frequently Asked Questions: What is an extra-tropical cyclone? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] 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 when their low-pressure centers cause powerful winds and very high seas.
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 latitudes, from the equator to 50°. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.[ Frequently Asked Questions: What is a sub-tropical cyclone? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.[ Monthly Global Tropical Cyclone Summary for December 2000 Padgett, Gary ]
Tropical cyclones in popular culture
Main articles: Tropical cyclones in popular culture
In popular culture, tropical cyclones have made appearances in different types of media, including films, books, television, music, and electronic games. The media can have tropical cyclones that are entirely fictional, or can be based on real events.[ Frequently Asked Questions: What fictional books, plays, and movies have been written involving tropical cyclones? Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division ] For example, George Rippey Stewart's ''Storm'', a best-seller published in 1941, is thought to have influenced meteorologists into giving female names to Pacific tropical cyclones.[83] Another example is the hurricane in ''The Perfect Storm'', which describes the sinking of the ''Andrea Gail'' by the 1991 Halloween Nor'easter.[84] Also, hypothetical hurricanes have been featured in parts of the plots of series such as ''The Simpsons'', ''Invasion'', ''Family Guy'', ''Seinfeld'', ''CSI Miami'', and ''Dawson's Creek''. The 2004 film ''The Day After Tomorrow'' includes several mentions of actual tropical cyclones as well as featuring fantastical "hurricane-like" non-tropical arctic storms.
See also
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