cyclone

 

atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effect, causes air to circulate about the core of lowest pressure in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Near the surface of the earth, the frictional drag on the air moving over land or water causes it to spiral gradually inward toward lower pressures. This inward movement of air is compensated for by rising currents near the center, which are cooled by expansion when they reach the lower pressures of higher altitudes. The cooling, in turn, greatly increases the relative humidity of the air, so that “lows” are generally characterized by cloudiness and high humidity; they are thus often referred to simply as storms.

According to the theory first proposed by the Norwegian physicist Vilhelm Bjerknes, the extratropical, or middle-latitude, cyclone originates as a wave, or perturbation, in the polar front separating the cold polar easterly winds from the warmer prevailing winds farther toward the equator. This wave, once induced by the opposing air currents, is accentuated by the rotational sense of the circulation, which pumps warm, moist air toward the pole around the eastern side of the cyclone center and cold, dry air toward the equator to the west of the center. Such wave cyclones often intensify, expanding the radius of the affected area to 500 mi (805 km) or more, while reducing atmospheric pressure, especially toward the center.

Tropical cyclones, formed over warm tropical oceans, are not associated with fronts, as are the middle-latitude wave cyclones, nor are they as large as the latter. A tropical cyclone that has matured to a severe intensity is called a hurricane when it occurs in the Atlantic Ocean or adjacent seas, a typhoon when it occurs in the Pacific Ocean or adjacent seas, or simply a cyclone or tropical cyclone when it occurs in the Indian Ocean region.

Cyclones in middle latitudes move generally from west to east along with the prevailing winds and cover 500 to 1,000 mi (800–1,610 km) each day; tropical cyclones usually move toward the west with the flow of the trade winds during their formative stages, then curve toward the poles around subtropical anticyclones.

Bibliography

See D. Longshore, Encyclopedia of Hurricanes, Typhoons, and Cyclones (1998).

In meteorology, a cyclone is an area of low atmospheric pressure characterized by inward spiraling winds that rotate counter clockwise in the northern hemisphere and clockwise in the southern hemisphere of the Earth.^[1][2] Since the generic term covers a wide variety of meteorological phenomena, such as tropical cyclones, extratropical cyclones, and tornadoes, meteorologists rarely use it without additional qualification.

Etymology

The word cyclones appears to have been coined by a Captain Henry Piddington, who used it to refer to the storm that blew a freighter in circles in Mauritius in February of 1845.^[3] Tropical cyclones are then circular wind storms that form in the tropics. It may have been derived from the Greek word kyklon, which means moving in a circle.^[4][5] Some say that Piddington derived the word from the Greek word cyclos, meaning "coils of a snake", in comparison with the tropical storms in the Bay of Bengal and in the Arabian Sea.^[6] Others argue that the word was derived from Cyclops, a creature in Greek mythology with one eye, as it is 'turning wind with one eye'.^[7]

Structure

There are a number of structural characteristics common to all cyclones. Their center is the area of lowest atmospheric pressure, often known in mature tropical and subtropical cyclones as the eye. Near the center, the pressure gradient force (from the pressure in the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure. The wind flow around a large cyclone is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere as a result of the Coriolis effect.

Formation

Main article: Cyclogenesis

Cold-core cyclones (most cyclone varieties) form due to the nearby presence of an upper level trough, which increases divergence aloft over an area that induces upward motion and surface low pressure. Warm-core cyclones (such as tropical cyclones and many mesocyclones) can have their initial start due to a nearby upper trough, but after formation of the initial disturbance, depend upon a storm-relative upper level high to maintain or increase their strength.

Categorisation

Each of the six main types of cyclone has further characteristics which define it as either a Polar cyclone, Polar low, Extratropical, Subtropical, Tropical, or Mesoscale.

Polar cyclone

Main article: Polar cyclone

Polar or Arctic cyclones are vast areas of low pressure. . A polar cyclone is a low pressure weather system, usually spanning 1,000–2,000 kilometers, in which the air circulates in a counterclockwise fashion in the northern hemisphere.

Polar low

Main article: Polar low

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. The systems usually have a horizontal length scale of less than 1,000 km 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.

Extratropical

Main article: Extratropical cyclone

An extratropical cyclone, sometimes inaccurately called a cyclone, is a synoptic scale low pressure weather system that has neither tropical nor polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".^[8]

The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where extratropical transition has occurred,^[8][9] and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with anti-cyclones, drive the weather over much of the Earth.

Although extratropical cyclones are almost always classified as baroclinic since they form along zones of temperature and dewpoint gradient, they can sometimes become barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.

Subtropical

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. It can form in a wide band of latitude, from the equator to 50°.

Tropical

Main article: Tropical cyclone

A tropical cyclone is a storm system fueled by the heat released when moist air rises and the water vapor in it condenses. The term describes the storm's origin in the tropics and its cyclonic nature, which means that its circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Tropical cyclones 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, high waves, and storm surges. The heavy rains and storm surges can produce extensive flooding. Although their effects on human populations can be devastating, tropical cyclones also can have beneficial effects by relieving drought conditions. They carry heat away from the tropics, an important mechanism of the global atmospheric circulation that maintains equilibrium in the Earth's troposphere.

Mesoscale

Main article: Mesocyclone

A mesocyclone is a cyclonic vortex of air, between approximately 2 and 10 km diameter within a convective storm. They can often be found in association with updrafts in supercells, where tornadoes may form. The term refers only to mesoscale cyclones found within convective storms, and does not apply to other cyclones on the mesoscale.^[10] Storms with mesocyclones can feature strong surface winds and severe hail.

Extraterrestrial cyclones

Main article: Extraterrestrial cyclones

Cyclones are not unique to Earth. Cyclonic storms are common on Jovian planets, like the Small Dark Spot on Neptune. Mars has also exhibited cyclonic storms. Jovian storms like the Great Red Spot are usually mistakenly named as giant hurricanes or cyclonic storms. This is wrong, as they are the total opposite, anticyclones.

See also

• Anticyclone
• Cyclogenesis
• Eye
• List of environment topics
• Meteorology
• Mesocyclone
• Polar cyclone
• Polar low
• Extratropical cyclone
• Subtropical cyclone
• Tropical cyclone
• Tropical cyclogenesis

Cyclones and Anticyclones of the world
Extratropical - Meso-scale - Polar - Polar low - Subtropical - Tropical

References

External links

• Fundamental of Physical Geography: The Mid-Latitude Cyclone - Dr. Michael Pidwirny, University of British Columbia, Okanagan
• Glossary Definition: Cyclogenesis - The National Snow and Ice Data Center
• Glossary Definition: Cyclolysis - The National Snow and Ice Data Center
• Weather Facts: The Polar Low - Weather Online UK
• NOAA FAQ
• Cyclones 'ClearlyExplained'
• Kent Cyclone - Listen to 'Cyclone What?' here

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