Everything about Tropical Cyclones totally explained
A
tropical cyclone is a
storm system characterized by a
low pressure center and numerous
thunderstorms that produce strong winds and
flooding
rain. A tropical cyclone feeds on heat released when moist
air rises, resulting in
condensation of
water vapour contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as
nor'easters,
European windstorms, and
polar lows, leading to their classification as "warm core" storm systems.
The term "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 term "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 other names, such as
hurricane,
typhoon,
tropical storm,
cyclonic storm,
tropical depression and simply
cyclone.
While tropical cyclones can produce extremely powerful winds and torrential
rain, they're 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 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 toward
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
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. 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.
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.
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. 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.
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, although the sea may be extremely violent. Intense, mature tropical cyclones 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.
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; in weaker tropical cyclones, the CDO may cover the center completely. The
eyewall is a circle of strong thunderstorms that surrounds the eye; here's where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.
| Size descriptions of tropical cyclones |
| ROCI |
Type |
| Less than 2 degrees latitude |
Very small/midget |
| 2 to 3 degrees of latitude |
Small |
| 3 to 6 degrees of latitude |
Medium/Average |
| 6 to 8 degrees of latitude |
Large |
| Over 8 degrees of latitude |
Very large |
Size
One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. If the radius is less than two degrees of latitude or, then the cyclone is "very small" or a "midget". Radii between 3 and 6 latitude degrees or to are considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees or . Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius at which its relative vorticity field decreases to 1×10-5 s-1 from its center.
Mechanics
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. 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 positive feedback loop continues for as long as conditions are favorable for tropical cyclone development. 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. 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.
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.
Scientists at the US National Center for Atmospheric Research estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day,
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.
Major basins and related warning centers
There are six Regional Specialized 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. The RSMCs and TCWCs are not the only organizations that provide information about tropical cyclones to the public. The Joint Typhoon Warning Center (JTWC) issues advisories in all basins except the Northern Atlantic for the purposes of the United States Government. The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues advisories and names for tropical cyclones that approach the Philippines in the Northwestern Pacific to protect the life and property of its citizens. The Canadian Hurricane Centre (CHC) issues advisories on hurricanes and their remnants for Canadian citizens when they affect Canada.
On March 26, 2004, Cyclone Catarina became the first recorded South Atlantic cyclone and subsequently struck southern Brazil with winds equivalent to Category 2 on the Saffir-Simpson Hurricane Scale. As the cyclone formed outside the authority of another warning center, Brazilian meteorologists initially treated the system as an extratropical cyclone, although subsequently classified it as tropical.
Formation
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.
In the Northern Atlantic Ocean, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March 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.
|
| Basin |
Season start |
Season end |
Tropical Storms
(>34 knots) |
Tropical Cyclones
(>63 knots) |
Category 3+ TCs
(>95 knots) |
| Northwest Pacific |
April |
January |
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 |
Factors
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 are needed down to a depth of at least ; waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the
heat of condensation that powers a tropical cyclone. 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, and become most of the tropical cyclones in the Eastern Pacific basin.
Tropical cyclones move westward equatorward of the subtropical ridge, 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. However, it's 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".
Tropical systems, while generally located
equatorward of the 20th 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. These waves are the precursors to many tropical cyclones within this region.
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. 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 isn't the driving force that brings this rotation to high speeds – that force is the
heat of condensation. 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 southwesterly winds (blowing northeastward) around a low-pressure system passing over
China or
Siberia. Many tropical cyclones are eventually forced toward the northeast by
extratropical cyclones in this manner, 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.
Landfall
Officially,
landfall is when a storm's center (the center of its circulation, not its edge) crosses the coastline.
Multiple storm interaction
When two cyclones approach one another, 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's 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, weakening will accelerate. 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 of water, dropping sea surface temperatures more than . Without warm surface water, the storm can't survive.
A tropical cyclone can dissipate when it moves over waters significantly below . This will cause the storm to lose its tropical characteristics (for example 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. 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. 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. Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane/typhoon 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 or typhoon-force wind speeds when they reach the west coast of
North America. These phenomena can also affect Europe, where they're known as
European windstorms;
Hurricane Iris's extratropical remnants are an example of such a windstorm from 1995. 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. The winds of
Hurricane Debbie—a hurricane seeded in Project Stormfury—dropped as much as 31%, 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. Because there was so much uncertainty about the behavior of these storms, the federal government wouldn't 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's known that silver iodide seeding isn't 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 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), or blasting the cyclone apart with nuclear weapons. Project Cirrus even involved throwing dry ice on a cyclone. These approaches all suffer from one flaw above many others: tropical cyclones are simply too large for any of the weakening techniques to be practical.
Effects
Tropical cyclones out at sea cause large waves, heavy rain, and high winds, disrupting international shipping and, at times, causing shipwrecks. Tropical cyclones stir up water, leaving a cool wake behind them,
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.
Over the past 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.
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. Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the
middle latitudes and polar regions. 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.
Observation and forecasting
Observation
Intense tropical cyclones pose a particular observation challenge, as they're a dangerous oceanic phenomenon, and
weather stations, being relatively sparse, 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's a nearby ship. Usually, real-time measurements are taken in the periphery of the cyclone, where conditions are less catastrophic and its true strength can't 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 by showing a storm's location and intensity every several minutes.
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. 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.
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, or average wind through the depth of the
troposphere, is considered 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 (above sea level) will produce better predictions. Tropical forecasters also consider smoothing out short-term wobbles of the storm as it allows them to determine a more accurate long-term trajectory. 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. However, scientists are less skillful at predicting the intensity of tropical cyclones. The lack of improvement in intensity forecasting is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development.
Classifications, terminology, and naming
Intensity classifications
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's referred to as a
typhoon; if a tropical storm passes the same benchmark in the
Northeast Pacific Basin, or in
the Atlantic, it's called a
hurricane. It should also be noted that typhoons with sustained winds greater than or are called
Super Typhoons by the Joint Typhoon Warning Center.
A
tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and
maximum sustained winds of less than or . It has no
eye and doesn't typically have the organization or the spiral shape of more powerful storms. However, it's already a low-pressure system, hence the name "depression".
A
tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between and . At this point, the distinctive cyclonic shape starts to develop, although an eye isn't usually present. Government weather services, other than the Philippines, first assign names to systems that reach this intensity (thus the term
named storm).
|-
!
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,
CHC &
CPHC
|-
| 0–6
| <28
| Depression
| Trop. Disturbance
|rowspan="3" | Tropical Low
|rowspan="3" | Tropical Depression
|rowspan="3" | Tropical Depression
|rowspan="2" | Tropical Depression
|rowspan="2" | Tropical Depression
|-
|rowspan="2" | 7
| 28–29
|rowspan="2" | Deep Depression
|rowspan="2" | Depression
|-
| 30–33
|rowspan="3" | Tropical Storm
|rowspan="3" | Tropical Storm
|-
| 8–9
| 34–47
| Cyclonic Storm
| Moderate Tropical Storm
| Trop. Cyclone (1)
|rowspan="11" | Tropical Cyclone
| Tropical Storm
|-
| 10
| 48–55
|rowspan="2" | Severe Cyclonic Storm
|rowspan="2" | Severe Tropical Storm
|rowspan="2" | Tropical Cyclone (2)
|rowspan="2" | Severe Tropical Storm
|-
| 11
| 56–63
|rowspan="7" | Typhoon
|rowspan="2" | Hurricane (1)
|-
|rowspan="8" | 12
| 64–72
|rowspan="7" | Very Severe Cyclonic Storm
|rowspan="3" | Tropical Cyclone
|rowspan="2" | Severe Tropical Cyclone (3)
|rowspan="8" | Typhoon
|-
| 73–85
| Hurricane (2)
|-
| 86–89
|rowspan="3" | Severe Tropical Cyclone (4)
|rowspan="2" | Major Hurricane (3)
|-
| 90–99
|rowspan="3" | Intense Tropical Cyclone
|-
| 100–106
|rowspan="3" | Major Hurricane (4)
|-
| 107–114
|rowspan="3" | Severe Tropical Cyclone (5)
|-
| 115–119
|rowspan="2" | Very Intense Tropical Cyclone
|rowspan="2" | Super Typhoon
|-
| >120
| Super Cyclonic Storm
| Major Hurricane (5)
|}
Origin of storm terms
The word
typhoon, used today in the Northwest Pacific, 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. The related
Portuguese word
tufão, used in Portuguese for typhoons, 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. (Huracan is also the source of the word
Orcan, another word for the
European windstorm. These events shouldn't be confused.) Huracan became the Spanish term for hurricanes.
Naming
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. 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 that vary from region to region and are drafted a few years ahead of time. The lists are decided on, 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
Tropical cyclones that cause extreme destruction are rare, although 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 more than 300,000 people and potentially as many as 1 million 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. Elsewhere,
Typhoon Nina killed nearly 100,000 in
China due to a
2000-year flood that caused 62 dams including the
Banqiao Dam to fail. The
Great Hurricane of 1780 is the deadliest
Atlantic hurricane on record, killing about 22,000 people in the
Lesser Antilles. 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, while in 1982, the unnamed tropical depression that eventually became
Hurricane Paul killed around 1,000 people in
Central America.
Hurricane Katrina is estimated as the costliest tropical cyclone worldwide, causing $81.2 billion in property damage (2005 USD) with overall damage estimates exceeding $100 billion (2005 USD).
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. Other destructive Eastern
Pacific hurricanes include
Pauline and
Kenna, both causing severe damage after striking
Mexico as major hurricanes. 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 more than 20 years.
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 or . Tip, however, doesn't 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. Camille was the only storm to actually strike land while at that intensity, making it, with or sustained winds and or gusts, the strongest tropical cyclone on record at landfall.
Typhoon Nancy in 1961 had recorded wind speeds of or, 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. Similarly, a surface-level gust caused by
Typhoon Paka on
Guam was recorded at or . 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.
In addition to being the most intense tropical cyclone on record, Tip was the largest cyclone on record, with tropical storm-force winds in diameter. The smallest storm on record,
Cyclone Tracy, was roughly wide before striking
Darwin,
Australia in 1974.
Hurricane John is the longest-lasting tropical cyclone on record, lasting 31 days in
1994. Before the advent of satellite imagery in 1961, however, many tropical cyclones were underestimated in their durations. 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.
Long-term activity trends
»
While the number of storms in the Atlantic has increased since 1995, there's 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. In spite of that, there's 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."
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,
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 1970–94. Destructive hurricanes struck frequently from 1926–60, including many major New England hurricanes. Twenty-one 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 1900–25; however, many intense storms formed during 1870–99. 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 19th century, 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.
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 reconnaissance aircraft encountered one, a ship reported a voyage through the storm, or a storm hit land in a populated area.
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". Similarly, P.J. Webster and others published an article in
Science examining the "changes in tropical cyclone number, duration, and intensity" over the past 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.
The strength of the reported effect is surprising in light of modeling studies that predict only a one half category increase in storm intensity as a result of a ~2 °C (3.6 °F) global warming. Such a response would have predicted only a ~10% increase in Emanuel's potential destructiveness index during the 20th century rather than the ~75–120% increase he reported.
Sufficiently warm
sea surface temperatures are considered vital to the development of tropical cyclones. Although 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, for example the hypothesized
Atlantic Multidecadal Oscillation (AMO), although an exact attribution hasn't been defined. However, there's 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". 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. Although 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.
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 when their low-pressure centers cause powerful winds and 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. From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.
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. 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. Another example is the hurricane in
The Perfect Storm, which describes the sinking of the
Andrea Gail by the
1991 Halloween Nor'easter. 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.
Further Information
Get more info on 'Tropical Cyclones'.
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