Cyclone

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This article is about the meteorological phenomenon. For other uses, see Cyclone (disambiguation).

An extratropical cyclone near Iceland
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synoptic scale). Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale.[5] Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale.[6]

Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the

Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream
.

Weather fronts mark the boundary between two masses of air of different

precipitation and fog. Warm fronts move poleward
ahead of the cyclone path. Occluded fronts form late in the cyclone life cycle near the center of the cyclone and often wrap around the storm center.

hurricane (from the name of the ancient Central American deity of wind, Huracan), in the Indian and south Pacific oceans it is called a cyclone, and in the northwestern Pacific it is called a typhoon.[15]
The growth of instability in the vortices is not universal. For example, the size, intensity, moist-convection, surface evaporation, the value of potential temperature at each potential height can affect the nonlinear evolution of a vortex.[16][17]

Nomenclature

The Journal of the Asiatic Society. He also coined the term cyclone, meaning the coil of a snake. In 1842, he published his landmark thesis, Laws of the Storms.[18]

Structure

Comparison between extratropical and tropical cyclones on surface analysis

There are a number of structural characteristics common to all cyclones. A cyclone is a

Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure.[20]

Because of the

Coriolis effect, the wind flow around a large cyclone is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.[21] In the Northern Hemisphere, the fastest winds relative to the surface of the Earth therefore occur on the eastern side of a northward-moving cyclone and on the northern side of a westward-moving one; the opposite occurs in the Southern Hemisphere.[22] In contrast to low-pressure systems, the wind flow around high-pressure systems are clockwise (anticyclonic
) in the northern hemisphere, and counterclockwise in the southern hemisphere.

Formation

The initial extratropical low-pressure area forms at the location of the red dot on the image. It is usually perpendicular (at a right angle to) the leaf-like cloud formation seen on satellite during the early stage of cyclogenesis. The location of the axis of the upper level jet stream is in light blue.
positive feedback loop over warm ocean waters.[23]
Main articles: Cyclogenesis and Tropical cyclogenesis

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere.[9] Cyclogenesis is an umbrella term for several different processes that all result in the development of some sort of cyclone.[24] It can occur at various scales, from the microscale to the synoptic scale.

Extratropical cyclones begin as waves along

warm seclusion
occurs.

Tropical cyclones form as a result of significant convective activity, and are warm core.[11] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[13] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[14] Cyclolysis is the opposite of cyclogenesis, and is the high-pressure system equivalent, which deals with the formation of high-pressure areas—Anticyclogenesis.[25]

A surface low can form in a variety of ways. Topography can create a surface low.

trowal.[28]

Tropical cyclogenesis is the development and strengthening of a tropical cyclone.[29] The mechanisms by which tropical cyclogenesis occurs are distinctly different from those that produce mid-latitude cyclones. Tropical cyclogenesis, the development of a warm-core cyclone, begins with significant convection in a favorable atmospheric environment. There are six main requirements for tropical cyclogenesis:

  1. sufficiently warm sea surface temperatures,[30]
  2. atmospheric instability,
  3. high humidity in the lower to middle levels of the troposphere
  4. enough Coriolis force to develop a low-pressure center
  5. a preexisting low-level focus or disturbance
  6. low vertical wind shear.[31]

An average of 86 tropical cyclones of tropical storm intensity form annually worldwide,

Saffir–Simpson hurricane scale).[33]

Synoptic scale

isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm frontal boundaries
.

The following types of cyclones are identifiable in synoptic charts.

Surface-based types

See also: Low-pressure area

There are three main types of surface-based cyclones: Extratropical cyclones, Subtropical cyclones and Tropical cyclones

Extratropical cyclone

Main article: Extratropical cyclone

An extratropical cyclone is a

gradients (rather than vertical) in temperature and dew point otherwise known as "baroclinic zones".[35]

"

Extratropical" is applied to cyclones outside the tropics, in the middle latitudes. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" when a tropical cyclone has moved (extratropical transition) beyond the tropics.[35][36] They are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena that, along with anticyclones
, drive weather over much of the Earth.

Although extratropical cyclones are almost always classified as

barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.[37] An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters sufficient to warm its core, and as a result develops central convection.[38] A particularly intense type of extratropical cyclone that strikes during winter is known colloquially as a nor'easter
.

Polar low
Main article: Polar low
A polar low over the Sea of Japan in December 2009

A polar low is a small-scale, short-lived atmospheric low-pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. Polar lows were first identified on the meteorological satellite imagery that became available in the 1960s, which revealed many small-scale cloud vortices at high latitudes. The most active polar lows are found over certain ice-free maritime areas in or near the Arctic during the winter, such as the Norwegian Sea, Barents Sea, Labrador Sea and Gulf of Alaska. Polar lows dissipate rapidly when they make landfall. Antarctic systems tend to be weaker than their northern counterparts since the air-sea temperature differences around the continent are generally smaller [citation needed]. However, vigorous polar lows can be found over the Southern Ocean. During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach −45 °C (−49 °F) move over open waters, deep convection forms, which allows polar low development to become possible.[39] The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s.[40]

Subtropical

Subtropical Storm Alex in the north Atlantic Ocean in January 2016
Main article: Subtropical cyclone

A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form between the equator and the 50th parallel.[41] As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones, and used terms such as quasi-tropical and semi-tropical to describe the cyclone hybrids.[42] By 1972, the National Hurricane Center officially recognized this cyclone category.[43] Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.[41] They have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and exist in areas of weak to moderate temperature gradient.[41]

Since they form from extratropical cyclones, which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required is around 23 degrees Celsius (73 °F) for their formation, which is three degrees Celsius (5 °F) lower than for tropical cyclones.[44] This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.[45]

Tropical

Main article: Tropical cyclone
2017 Atlantic hurricane season summary map

A tropical cyclone is a

water vapour contained in the moist air.[46] 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.[46][11]

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

The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in

tropical depression, or simply as a cyclone.[47]

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and a damaging

latitudes,[47] 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.[47]

Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable.[47] 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. 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.[11] A tropical cyclone is usually not considered to become subtropical during its extratropical transition.[52]

Upper level types

Polar cyclone

Main article:
Polar cyclone

A polar, sub-polar, or Arctic cyclone (also known as a

Ross ice shelf near 160 west longitude.[56] When the polar vortex is strong, its effect can be felt at the surface as a westerly wind (toward the east). When the polar cyclone is weak, significant cold outbreaks occur.[57]

TUTT cell

Main article: Upper tropospheric cyclonic vortex

Under specific circumstances, upper level cold lows can break off from the base of the tropical upper tropospheric trough (TUTT), which is located mid-ocean in the Northern Hemisphere during the summer months. These upper tropospheric cyclonic vortices, also known as TUTT cells or TUTT lows, usually move slowly from east-northeast to west-southwest, and their bases generally do not extend below 20,000 feet (6,100 m) in altitude. A weak inverted surface trough within the

trade wind is generally found underneath them, and they may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of a surface vortex. In rare cases, they become warm-core tropical cyclones. Upper cyclones and the upper troughs that trail tropical cyclones can cause additional outflow channels and aid in their intensification. Developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet emanating from the developing tropical disturbance/cyclone.[58][59]

Mesoscale

The following types of cyclones are not identifiable in synoptic charts.

Mesocyclone

Main article: Mesocyclone

A mesocyclone is a vortex of air, 2.0 kilometres (1.2 mi) to 10 kilometres (6.2 mi) in diameter (the mesoscale of meteorology), within a convective storm.[60] Air rises and rotates around a vertical axis, usually in the same direction as low-pressure systems[61] in both northern and southern hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a supercell.[61][62] Such storms can feature strong surface winds and severe hail.[61] Mesocyclones often occur together with updrafts in supercells, where tornadoes may form.[61] About 1,700 mesocyclones form annually across the United States, but only half produce tornadoes.[13]

Tornado

Main article: Tornado

A tornado is a violently rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or,[63] in rare cases, the base of a cumulus cloud. Also referred to as twisters, a colloquial term in America, or cyclones, although the word cyclone is used in meteorology, in a wider sense, to name any closed low-pressure circulation.

Dust devil

Main article: Dust devil

A dust devil is a strong, well-formed, and relatively long-lived whirlwind,[64] ranging from small (half a metre wide and a few metres tall) to large (more than 10 metres wide and more than 1000 metres tall).[64] The primary vertical motion is upward.[64] Dust devils are usually harmless, but can on rare occasions grow large enough to pose a threat to both people and property.[64]

Waterspout

Main article: Waterspout

A waterspout is a columnar vortex forming over water that is, in its most common form, a non-

mesocyclones
do occur.

Steam devil

Main article: Steam devil

A gentle vortex over calm water or wet land made visible by rising water vapour.

Fire whirl

Main article: Fire whirl

A fire whirl – also colloquially known as a fire devil, fire tornado, firenado, or fire twister – is a whirlwind induced by a fire and often made up of flame or ash.

Other planets

Main article: Extraterrestrial vortex
Cyclone on Mars, imaged by the Hubble Space Telescope

Cyclones are not unique to Earth. Cyclonic storms are common on Jovian planets, such as the Small Dark Spot on Neptune.[65] It is about one third the diameter of the Great Dark Spot and received the nickname "Wizard's Eye" because it looks like an eye. This appearance is caused by a white cloud in the middle of the Wizard's Eye.[8] Mars has also exhibited cyclonic storms.[7] Jovian storms like the Great Red Spot are usually mistakenly named as giant hurricanes or cyclonic storms. However, this is inaccurate, as the Great Red Spot is, in fact, the inverse phenomenon, an anticyclone.[66]

See also

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External links

Wikimedia Commons has media related to Cyclones.
Look up cyclone in Wiktionary, the free dictionary.
Wikivoyage has a travel guide for Cyclones.
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