Encyclopedia
In
meteorology, a
tropical cyclone is a storm system fueled by the heat released when moist air rises and condenses. The name underscores their origin in the
tropics and their
cyclonic nature, which is that its circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. They are distinguished from other cyclonic windstorms such as
nor'easters, European windstorms, and
polar lows by the heat mechanism that fuels them, which makes them "warm core" storm systems.
Depending on their location and strength, there are various terms by which tropical cyclones are known, such as
hurricane,
typhoon,
tropical storm,
cyclonic storm and
tropical depression.
Tropical cyclones can produce extremely strong winds,
tornadoes, torrential
rain, and huge waves swamping coastal areas called
storm surges. The heavy rains and storm surges create giant floods. Although the effects on human populations can be catastrophic, tropical cyclones have also been known to relieve drought conditions because they transport enormous amounts of moisture. They carry heat away from the tropics, an important mechanism of the global
atmospheric circulation that maintains equilibrium in the earth's troposphere.
Mechanics of tropical cyclones
Structurally, a tropical cyclone is a large, rotating system of
clouds,
wind, and
thunderstorms. Its primary
energy source is the release of the heat of condensation from water vapor
condensing at high altitudes, the heat ultimately derived from the
sun. Therefore, a tropical cyclone can be thought of as a giant vertical
heat engine supported by mechanics driven by physical forces such as the
rotation and
gravity of the
earth. In another way, tropical cyclones could be viewed as a special type of
Mesoscale Convective Complex, which continues to develop over a vast source of relative warmth and moisture.
Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy; the faster winds and lower pressure associated with them in turn cause increased surface evaporation and thus even more condensation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation. This gives rise to factors that provide the system with enough energy to be self-sufficient and cause a positive feedback loop where it can draw more energy as long as the source of heat, warm water, remains. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The rotation of the earth causes the system to spin, an effect known as the
Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.
The factors to form a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist and allow it to create a feedback loop by maximizing the energy intake possible, for example, such as high winds to increase the rate of evaporation, they can combine to produce the violent winds, incredible waves, torrential rains, and
floods associated with this phenomenon.
Condensation as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena. Because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast,
mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature
gradients in the atmosphere.
The passage of a tropical cyclone over the ocean can cause the upper ocean to cool substantially, which can influence subsequent cyclone development. Tropical cyclones cool the ocean by acting like "heat engines" that transfer heat from the ocean surface to the atmosphere through
evaporation. Cooling is also caused by upwelling of cold water from below. Additional cooling may come from cold water from raindrops that remain on the ocean surface for a time. Cloud cover may also play a role in cooling the ocean by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.
Scientists at the
National Center for Atmospheric Research estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 trillion joules per day. down to a depth of at least 50 m . Waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.
- Rapid cooling with height. This allows the release of latent heat, which is the source of energy in a tropical cyclone.
- High humidity, especially in the lower-to-mid troposphere. When there is a great deal of moisture in the atmosphere, conditions are more favourable for disturbances to develop.
- Low wind shear. When wind shear is high, the convection in a cyclone or disturbance will be disrupted, blowing the system apart.
- Distance from the equator. This allows the Coriolis force
...
to deflect winds blowing towards the low pressure center, causing a circulation. The minimum distance is 500 km or about 5 degrees from the equator.
- A pre-existing system of disturbed weather. The system must have some sort of circulation as well as a low pressure center.
Generally, tropical cyclones can only form from three different types of systems: tropical waves,
non-tropical lows, and decaying
frontal boundaries. Tropical cyclones form most often from tropical waves, also called easterly waves, which, as mentioned above, are westward moving areas of convergent winds. Tropical waves often carry with them thunderstorms, which can develop into tropical cyclones. A similar phenomenon to tropical waves are West African disturbance lines, which are squalls that form over
Africa and move into the Atlantic, often as a part of the
Intertropical Convergence Zone. Tropical cyclones also frequently form from upper
tropospheric troughs, which are cold-core upper level lows. A warm-core tropical cyclone may result when one of these works down to the lower levels and produces deep
convection. Off-season tropical cyclones most often form in this manner. Finally, decaying frontal boundaries may occasionally stall over warm waters and produce lines of active convection. If a low-level circulation forms under this convection, it may develop into a tropical cyclone.
Locations of formation
Most tropical cyclones form in a worldwide band of thunderstorm activity called the Intertropical Discontinuity , also called the
Intertropical Convergence Zone .
Most of these systems form between 10 and 30 degrees of the
equator and 87% form within 20 degrees of it. Because the
Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5 degrees of the equator, where the Coriolis effect is weakest. However, it is possible for tropical cyclones to form within this boundary as did
Typhoon Vamei in 2001 and
Cyclone Agni in 2004.
Major basins
Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the north
Atlantic Ocean, the eastern and western parts of the
Pacific Ocean , the southwestern Pacific, the southwestern and southeastern
Indian Oceans, and the northern Indian Ocean. The North Atlantic is the most studied of the basins, while the Western Pacific is the most active and the North Indian the least active. Worldwide, an average of 80 tropical cyclones form each year.
Additionally, the Joint Typhoon Warning Center issues informal advisories in all basins except the Northern Atlantic and Northeastern Pacific. The Philippine Atmospheric, Geophysical and Astronomical Services Administration issues informal advisories, as well as names, for tropical cyclones that approach the
Philippines in the Northwestern Pacific. The Canadian Hurricane Centre issues advisories on hurricanes and their remnants that affect Canada.
...
, and the
Gulf of Mexico. Tropical cyclone formation here varies widely from year to year, ranging from over twenty to one per year with an average of around ten. The
United States Atlantic coast,
Mexico,
Central America, the
Caribbean Islands, and
Bermuda are frequently affected by storms in this basin. Venezuela, the south-east of Canada and Atlantic
"Macaronesian" islands are also occasionally affected. Many of the more intense Atlantic storms are
Cape Verde-type hurricanes, which form off the west coast of
Africa near the
Cape Verde islands. Rarely, a hurricane can reach western
Europe, including
Hurricane Lili, which dissipated over the
British Isles in October 1996, and
Tropical Storm Vince, which made landfall on the southwestern coast of
Spain in September 2005.
- Northeastern Pacific Ocean: This is the second most active basin in the world, and the most dense . Storms that form here can affect western Mexico, Hawaii, northern Central America, and on extremely rare occasions, California and Arizona. There is no record of a hurricane ever reaching California; however, to some meteorologists, historical records in 1858 spoke of a storm that struck San Diego with winds over 75 m.p.h., above hurricane force.
- Northwestern Pacific Ocean: Tropical storm activity in this region frequently affects China, Japan, Hong Kong, the Philippines, and Taiwan, but also many other countries in Southeast Asia, such as Vietnam, South Korea, and parts of Indonesia, plus numerous Oceanian islands. This is by far the most active basin, accounting for one-third of all tropical cyclone activity in the world. The coast of China sees the most landfalling tropical cyclones worldwide. The Philippines receives an average 18 typhoon landings per year. Rarely does a typhoon or an extratropical storm reach northward to Siberia, Russia.
- Northern Indian Ocean: This basin is divided into two areas, the Bay of Bengal and the Arabian Sea, with the Bay of Bengal dominating . This basin's season has an interesting double peak; one in April and May before the onset of the monsoon, and another in October and November just after. Tropical cyclones which form in this basin have historically cost the most lives — most notably, the 1970 Bhola cyclone killed 200,000. Nations affected by this basin include India, Bangladesh, Sri Lanka, Thailand, Myanmar, and Pakistan. Rarely, a tropical cyclone formed in this basin will affect the Arabian Peninsula.
- Southwestern Pacific Ocean: Tropical activity in this region largely affects Australia and Oceania. On rare occasions, tropical storms reach the vicinity of Brisbane, Australia and into New Zealand, usually during or after extratropical transition.
- Southeastern Indian Ocean: Tropical activity in this region affects Australia and Indonesia. According to the Australian Bureau of Meteorology, the most frequently hit portion of Australia is between Exmouth and Broome in Western Australia.
- Southwestern Indian Ocean: This basin is the least understood, due to a lack of historical data. Cyclones forming here impact Madagascar, Mozambique, Mauritius, Reunion, Comoros, Tanzania, and Kenya.
Unusual formation areas
The following areas spawn tropical cyclones only very rarely.
- Temperate subtropics: Areas farther than thirty degrees from the equator are not normally conducive to tropical cyclone formation or strengthening, and areas more than forty degrees from the equator are very hostile to such development. The primary limiting factor is water temperatures, although higher shear at increasing latitudes is also a factor. These areas are sometimes frequented by cyclones moving poleward from tropical latitudes. On rare occasions, such as in 1988 and 1975 storms may form or strengthen in this region.
- Low latitudes: Areas within approximately ten degrees latitude of the equator do not experience a significant Coriolis Force
...
, a vital ingredient in tropical cyclone formation. However, in December 2001,
Typhoon Vamei formed in the Southern South China Sea and made landfall in
Malaysia. It formed from a thunderstorm formation in
Borneo that moved into the South China Sea.
- Southeastern Pacific: Tropical cyclone formation is rare in this region; when they do form, it is frequently linked to El Niño episodes. Most of the storms that enter this region formed farther west in the Southwest Pacific. They affect the islands of Polynesia in exceptional instances. During the 1982/83 El Niño event, French Polynesia was affected by six tropical cyclones in five months. In addition, there are no records of a tropical cyclone hitting western South America.
- South Atlantic: A combination of wind shear and a lack of tropical disturbances from the Intertropical Convergence Zone makes it very difficult for the South Atlantic to support tropical activity. However, three tropical cyclones have been observed here — a weak tropical storm in 1991 off the coast of Africa, Cyclone Catarina , which made landfall in Brazil in 2004 at Category 1 strength, and a smaller storm in January 2004, east of Salvador, Brazil. The January storm is thought to have reached tropical storm intensity based on scatterometer winds.
- Mediterranean Sea: Storms that appear similar to tropical cyclones in structure sometimes occur in the Mediterranean basin. Examples of these "Mediterranean tropical cyclones" formed in September 1947, September 1969, January 1982, September 1983, and January 1995. However, there is debate on whether these storms were tropical in nature.
- The Great Lakes: A storm system that appeared similar to a tropical cyclone formed in 1996 on Lake Huron. It formed an eye-like structure in its center, and it may have briefly been a tropical cyclone. The Great Lakes has a long history of rare but eventful cyclonic storms.
Times of formation
Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active.
In the North
Atlantic, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the North Atlantic hurricane season is September 10. The Northeast Pacific has a broader period of activity, but in a similar time frame to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November.
In the
Southern Hemisphere, tropical cyclone activity begins in late October and ends in May. Southern Hemisphere activity peaks in mid-February to early March.
| Season Lengths and Seasonal Averages |
|---|
| Basin | Season Start | Season End | Tropical Storms | Tropical Cyclones | Category 3+ Tropical Cyclones |
|---|
| Northwest Pacific | – | – | 26.7 | 16.9 | 8.5 |
| South Indian | October | May | 20.6 | 10.3 | 4.3 |
| Northeast Pacific | May | November | 16.3 | 9.0 | 4.1 |
| North Atlantic | June | November | 10.6 | 5.9 | 2.0 |
| Australia Southwest Pacific | October | May | 10.6 | 4.8 | 1.9 |
| North Indian | April | December | 5.4 | 2.2 | 0.4 |
Movement and track
Large-scale winds
Although tropical cyclones are large systems generating enormous energy, their movements over the earth's surface are controlled by large-scale winds—the streams in the earth's atmosphere. The path of motion is referred to as a tropical cyclone's
track, and has been compared by Dr. Neil Frank, former director of the
National Hurricane Center, as "leaves carried along by a stream."
The major force affecting the track of tropical systems in all areas are winds circulating around high-pressure areas. Over the north
Atlantic Ocean, tropical systems are steered generally westward by the east-to-west winds on the south side of the "Bermuda High", a persistent high-pressure area over the north Atlantic. Also, in the area of the North Atlantic where hurricanes form, trade winds, which are prevailing westward-moving wind currents, steer tropical waves westward from the
African coast and towards the Caribbean and North America. These waves are the precursors to many tropical cyclones and are the main source of
Atlantic hurricanes during most seasons, and also play a significant role in the formation of tropical cyclones in the Eastern Pacific.
In the Indian Ocean and western Pacific , tropical cyclogenesis is strongly influenced by the seasonal movement of the Intertropical Convergence Zone, rather than by easterly waves. In these basins as well, tropical cyclone paths are broadly determined by
synoptic scale features.
Coriolis effect
The earth's rotation also imparts an acceleration . This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents . Thus, tropical cyclones in the Northern Hemisphere, which commonly move west in the beginning, normally turn north , and cyclones in the Southern Hemisphere are deflected south, if no strong pressure systems are counteracting the Coriolis acceleration. The Coriolis acceleration also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds. These speeds are due to the conservation of
angular momentum - air is drawn in from an area much larger than the cyclone such that the tiny rotational speed is magnified greatly as the air is drawn in to the low pressure center.
Interaction with high and low pressure systems
Finally, when a tropical cyclone moves into higher latitude, its general track around a high-pressure area can be deflected significantly by winds moving toward a low-pressure area. Such a track direction change is termed
recurve. A hurricane moving from the Atlantic toward the
Gulf of Mexico, for example, will recurve to the north and then northeast if it encounters winds blowing northeastward toward a low-pressure system passing over North America. Many tropical cyclones along the East Coast and in the Gulf of Mexico are eventually forced toward the northeast by low-pressure areas which move from west to east over North America.
Landfall
Officially, "landfall" is when a storm's center reaches land. Naturally, storm conditions may be experienced on the coast and inland well before landfall. In fact, for a storm moving inland, the landfall area experiences half the storm before the actual landfall. For emergency preparedness, actions should be timed from when a certain wind speed will reach land, not from when landfall will occur.
For a list of notable and unusual landfalling tropical cyclones, see
list of notable tropical cyclones.
Dissipation
A tropical cyclone can cease to have tropical characteristics in several ways:
- It moves over land, thus depriving it of the warm water it needs to power itself, and quickly loses strength. Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two. There is, however, a chance they could regenerate if they manage to get back over open warm water. If a storm is over mountains for even a short time, it can rapidly lose its structure. However, many storm fatalities occur in mountainous terrain, as the dying storm unleashes torrential rainfall which can lead to deadly floods and mudslides.
- It remains in the same area of ocean for too long, drawing heat off of the ocean surface until it becomes too cool to support the storm. Without warm surface water, the storm cannot survive.
- It experiences wind shear, causing the convection to lose direction and the heat engine to break down.
- It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms.
- It enters colder waters. This does not necessarily mean the death of the storm, but the storm will lose its tropical characteristics. These storms are extratropical cyclones.
Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force winds and drop several inches of rainfall. When a tropical cyclone reaches higher latitudes or passes over land, it may merge with
weather fronts or develop into a
frontal cyclone, also called
extratropical cyclone. In the
Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach Europe as a European windstorm, such as the extratropical remnants of
Hurricane Iris in 1995.
Artificial dissipation
In the 1960s and 1970s, the United States government attempted to weaken hurricanes in its
Project Stormfury by
seeding selected storms with
silver iodide. It was thought that the seeding would cause supercooled water in the outer rainbands to freeze, causing the inner eyewall to collapse and thus reducing the winds. The winds of Hurricane Debbie dropped as much as 30 percent, but then regained their strength after each of two seeding forays. In an earlier episode in 1947, disaster struck when a hurricane east of
Jacksonville, Florida promptly changed its course after being seeded, and smashed into
Savannah, Georgia. Because there was so much uncertainty about the behavior of these storms, the federal government would not approve seeding operations unless the hurricane had a less than 10 percent chance of making landfall within 48 hours, greatly reducing the number of possible test storms. The project was dropped after it was discovered that eyewall replacement cycles occur naturally in strong hurricanes, casting doubt on the result of the earlier attempts. Today, it is known that silver iodide seeding is not likely to have an effect because the amount of supercooled water in the rainbands of a tropical cyclone is too low.
Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing
icebergs into the tropical oceans, dropping large quantities of ice into the eye at very early stages so that latent heat is absorbed by ice at the entrance instead of heat energy being converted to kinetic energy at high altitudes vertically above, covering the ocean in a substance that inhibits evaporation, or blasting the cyclone apart with nuclear weapons. Project Cirrus even involved throwing dry ice on a cyclone. These approaches all suffer from the same flaw: tropical cyclones are simply too large for any of them to be practical.
Effects
A mature tropical cyclone can release heat at a rate upwards of 6x10
14 watts.
Often, the secondary effects of a tropical cyclone are equally damaging. These include:
- Disease - The wet environment in the aftermath of a tropical cyclone, combined with the destruction of sanitation facilities and a warm tropical climate, can induce epidemics of disease which claim lives long after the storm passes. One of the most common post-hurricane injuries is stepping on a nail in storm debris, leading to a risk of tetanus or other infection. Infections of cuts and bruises can be greatly amplified by wading in sewage-polluted water. Large areas of standing water caused by flooding also contribute to mosquito-borne illnesses.
- Power outages - Tropical cyclones often knock out power to tens or hundreds of thousands of people , prohibiting vital communication and hampering rescue efforts.
- Transportation difficulties - Tropical cyclones often destroy key bridges, overpasses, and roads, complicating efforts to transport food, clean water, and medicine to the areas that need it.
Beneficial effects of tropical cyclones
Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact and bring much-needed precipitation to otherwise dry regions. Hurricanes in the eastern north Pacific often supply moisture to the
Southwestern United States 
