Tropical cyclone

Hurricane Isabel (2003) as seen from orbit during Expedition 7 of the International Space Station. The eye, eyewall, and surrounding rainbands, characteristics of tropical cyclones in the narrow sense, are clearly visible in this view from space.

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane (n/),[1][2][3] typhoon (n/), tropical storm, cyclonic storm, tropical depression, and simply cyclone.[4] A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; while in the south Pacific or Indian Ocean, comparable storms are referred to simply as “tropical cyclones” or “severe cyclonic storms”.[4]

“Tropical” refers to the geographical origin of these systems, which form almost exclusively over tropical seas. “Cyclone” refers to their winds moving in a circle,[5] whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and blowing clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter.

The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator.[6] Tropical cyclones are almost unknown in the South Atlantic due to a consistently strong wind shear and a weak Intertropical Convergence Zone.[7] Also, the African easterly jet and areas of atmospheric instability which gives rise to cyclones in the Atlantic Ocean and Caribbean Sea, along with the Asian monsoon and Western Pacific Warm Pool, are features of the Northern Hemisphere and Australia.

Coastal regions are particularly vulnerable to the impact of a tropical cyclone, compared to inland regions. The primary energy source for these storms is warm ocean waters, therefore these forms are typically strongest when over or near water, and weaken quite rapidly over land. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones also draw in air from a large area—which can be a vast area for the most severe cyclones—and concentrate the precipitation of the water content in that air (made up from atmospheric moisture and moisture evaporated from water) into a much smaller area. This continual replacement of moisture-bearing air by new moisture-bearing air after its moisture has fallen as rain, may cause extremely heavy rain and river flooding up to 40 kilometres (25 mi) from the coastline, far beyond the amount of water that the local atmosphere holds at any one time.

Though their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate.

Physical structure

Diagram of a Northern hemisphere hurricane

Tropical cyclones are areas of relatively low pressure in the troposphere, with the largest pressure perturbations occurring at low altitudes near the surface. On Earth, the pressures recorded at the centers of tropical cyclones are among the lowest ever observed at sea level.[8] The environment near the center of tropical cyclones is warmer than the surroundings at all altitudes, thus they are characterized as “warm core” systems.[9]

Wind field

The near-surface wind field of a tropical cyclone is characterized by air rotating rapidly around a center of circulation while also flowing radially inwards. At the outer edge of the storm, air may be nearly calm; however, due to the Earth’s rotation, the air has non-zero absolute angular momentum. As air flows radially inward, it begins to rotate cyclonically (counter-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere) in order to conserve angular momentum. At an inner radius, air begins to ascend to the top of the troposphere. This radius is typically coincident with the inner radius of the eyewall, and has the strongest near-surface winds of the storm; consequently, it is known as the radius of maximum winds.[10] Once aloft, air flows away from the storm's center, producing a shield of cirrus clouds.[11]

The previously mentioned processes result in a wind field that is nearly axisymmetric: Wind speeds are low at the center, increase rapidly moving outwards to the radius of maximum winds, and then decay more gradually with radius to large radii. However, the wind field often exhibits additional spatial and temporal variability due to the effects of localized processes, such as thunderstorm activity and horizontal flow instabilities. In the vertical direction, winds are strongest near the surface and decay with height within the troposphere.[12]

Eye and center

Thunderstorm activity in the eyewall of Cyclone Bansi as seen from the International Space Station, on January 12, 2015

At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear “eye”. Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent.[13] The eye is normally circular in shape, and is typically 30–65 km (19–40 mi) in diameter, though eyes as small as 3 km (1.9 mi) and as large as 370 km (230 mi) have been observed.[14][15]

The cloudy outer edge of the eye is called the “eyewall”. The eyewall typically expands outward with height, resembling an arena football stadium; this phenomenon is sometimes referred to as the stadium effect.[15] The eyewall is where the greatest wind speeds are found, air rises most rapidly, clouds reach to their highest altitude, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[13]

In a weaker storm, the eye may be obscured by the central dense overcast, which is the upper-level cirrus shield that is associated with a concentrated area of strong thunderstorm activity near the center of a tropical cyclone.[16]

The eyewall may vary over time in the form of eyewall replacement cycles, particularly in intense tropical cyclones. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward, which is believed to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the tropical cyclone weakens temporarily. The outer eyewall eventually replaces the primary one at the end of the cycle, at which time the storm may return to its original intensity.[17]

Rapid deepening

On occasion, tropical cyclones may undergo a process known as rapid deepening, a period in which the minimum sea-level pressure of a tropical cyclone decreases by 42mb in a 24-hour period.[18] In order for rapid deepening to occur, several conditions must be in place. Water temperatures must be extremely high (near or above 30 °C, 86 °F), and water of this temperature must be sufficiently deep such that waves do not upwell cooler waters to the surface. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well—for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.[19]

Size

Size descriptions of tropical cyclones
ROCI (Diameter) 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[20]

There are a variety of metrics commonly used to measure storm size. The most common metrics include the radius of maximum wind, the radius of 34-knot wind (i.e. gale force), the radius of outermost closed isobar (ROCI), and the radius of vanishing wind.[21][22] An additional metric is the radius at which the cyclone's relative vorticity field decreases to 1×10−5 s−1.[15]

On Earth, tropical cyclones span a large range of sizes, from 100–2,000 kilometres (62–1,243 mi) as measured by the radius of vanishing wind. They are largest on average in the northwest Pacific Ocean basin and smallest in the northeastern Pacific Ocean basin.[23] If the radius of outermost closed isobar is less than two degrees of latitude (222 km (138 mi)), then the cyclone is "very small" or a "midget". A radius of 3–6 latitude degrees (333–670 km (207–416 mi)) is considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees (888 km (552 mi)).[20] Observations indicate that size is only weakly correlated to variables such as storm intensity (i.e. maximum wind speed), radius of maximum wind, latitude, and maximum potential intensity.[22][23]

Size plays an important role in modulating damage caused by a storm. All else equal, a larger storm will impact a larger area for a longer period of time. Additionally, a larger near-surface wind field can generate higher storm surge due to the combination of longer wind fetch, longer duration, and enhanced wave setup.[24]

The upper circulation of strong hurricanes extends into the tropopause of the atmosphere, which at low latitudes is 15,000–18,000 metres (50,000–60,000 ft).[25]

Other Languages
Afrikaans: Tropiese sikloon
aragonés: Ciclón tropical
asturianu: Ciclón tropical
Avañe'ẽ: Aravai jere
azərbaycanca: Tropik siklon
Bân-lâm-gú: Jia̍t-tāi soân-hong
башҡортса: Тропик циклон
беларуская: Трапічны цыклон
беларуская (тарашкевіца)‎: Трапічны цыклён
Bikol Central: Bagyo
Bislama: Bigwin
bosanski: Tropski ciklon
Чӑвашла: Çил-тăвăл
Cebuano: Bagyo
Cymraeg: Corwynt
dansk: Orkan
eesti: Orkaan
Esperanto: Uragano
فارسی: توفند
føroyskt: Ódnir
français: Cyclone tropical
Frysk: Orkaan
galego: Furacán
한국어: 열대 저기압
Hawaiʻi: Makani Pāhili
hrvatski: Tropska oluja
Ido: Uragano
Bahasa Indonesia: Siklon tropis
interlingua: Cyclon tropic
íslenska: Fellibylur
Basa Jawa: Siklon tropis
ಕನ್ನಡ: ಚಂಡಮಾರುತ
Kiswahili: Kimbunga
Kreyòl ayisyen: Siklòn twopikal
kurdî: Barove
latviešu: Viesuļvētra
lietuvių: Uraganas
Limburgs: Orkaan
македонски: Тропски циклон
मराठी: हरिकेन
Bahasa Melayu: Taufan
Mirandés: Ciclone tropical
Nederlands: Tropische cycloon
日本語: 熱帯低気圧
norsk nynorsk: Tropisk syklon
oʻzbekcha/ўзбекча: Tropik siklon
پنجابی: چکھڑ
Papiamentu: Tormenta tropikal
português: Ciclone tropical
română: Ciclon tropical
Runa Simi: Hatun pillunkuy
संस्कृतम्: प्रभञ्जनः
shqip: Uragani
Simple English: Tropical cyclone
slovenčina: Tropická cyklóna
slovenščina: Tropski ciklon
српски / srpski: Ураган
srpskohrvatski / српскохрватски: Orkan
Basa Sunda: Siklon Tropis
Tagalog: Bagyo
татарча/tatarça: Гарасат
Tsetsêhestâhese: Mahpe hevovetäso
Türkçe: Kasırga
українська: Тропічний циклон
اردو: طوفان
文言: 熱帶氣旋
Winaray: Bagyo
粵語: 熱帶氣旋
中文: 熱帶氣旋