Cyclone

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

Radar image of a tropical cyclone in the northern hemisphere.

In meteorology, a cyclone is an area of low atmospheric pressure characterised by inward spiraling winds that spin counterclockwise in the northern hemisphere of the Earth, and clockwise in the southern hemisphere.<ref name="BBCCycDef"> BBC Weather Gloassary - Cyclone. BBC Weather. Retrieved on 2006-10-24. </ref><ref name="UCARCycDef"> UCAR Glossary - Cyclone. University Corporation for Atmospheric Research. Retrieved on 2006-10-24. </ref> Covering a wide variety of meteorological phenomenon such as tropical cyclones, extratropical cyclones and tornadoes, the generic term is rarely used by meteorologists without a qualifier.

1 Etymology
2 Structure
- 2.1 Categorisation
3 Cyclone as metaphor
4 See also
5 References
6 External links

[edit] Etymology

There are several theories on the origin of the term cyclone. The word appears to have been used first by Henry Piddington around 1840. It may have been derived from the Greek word kyklon, which means moving in a circle,<ref name="WeatherDocCycOrigin"> Keith C. Heidorn, PhD (2003-10-01). The Weather Doctor: Cyclone Origin. Home Page of Keith C. Heidorn (The Weather Doctor). Retrieved on 2006-10-24. </ref><ref name="BangPiddHenry"> Banglapedia: Piddington, Henry. Banglapedia. Retrieved on 2006-10-24. </ref>. 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. <ref name="CycRelDisasters"> Cyclone Related Disasters - What Are Cyclones?. India Meteorological department. Retrieved on 2006-10-24. </ref> 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'.<ref name="ClearExpCyc"> Richard Conan-Davies BSc Dip Ed (2003-05-14). ClearlyExplained.com - Cyclones. Clearly Explained.com. Retrieved on 2006-10-24. </ref>

[edit] 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.

[edit] Categorisation

Each type of cyclone has further characteristics which define it as a Polar cyclone, Polar low, Extratropical, Subtropical, Tropical, or Mesoscale (to name the six major types).

[edit] Polar cyclone

Main article: Polar cyclone

Polar or Arctic cyclones are vast areas of low pressure. They should not be confused with what are commonly referred to as polar lows, which behave like hurricanes. 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.

[edit] Polar low

Image:Polar low.jpg

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 less 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 comma cloud, mesocyclone, 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 gale force (17 m/s).

[edit] Extratropical

Image:UK-Cyclone.gif

Main article: Extratropical cyclone

An extratropical cyclone, sometimes inaccurately called a cyclone, is a synoptic scale low pressure weather system that has neither tropical or polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".<ref name="ExtraLessonMillUni"> Dr. DeCaria (2005-12-07). ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones. Department of Earth Sciences, Millersville University, Millersville, Pennsylvania. Retrieved on 2006-10-21. </ref>

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,<ref name="ExtraLessonMillUni">ExtraLessonMillUni</ref><ref name="ExtratropicalPhases">Robert Hart and Jenni Evans (2003). Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic TCs as Defined Within Cyclone Phase Space. American Meteorological Society. Retrieved on 2006-10-03.</ref> 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 distibution around the cyclone becomes fairly uniform with radius.

[edit] 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. They can form in a wide band of latitude, from the equator to 50°.

[edit] Tropical

Image:Cyclone Catarina from the ISS on March 26 2004.JPG

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.

[edit] Mesoscale

Main article: Mesocyclone

A mesocyclone is a large area of vertically spiraling air, usually associated with a supercell thunderstorm's updraft area. Storms with mesocyclones often feature strong surface winds and severe hail, and mesocyclones are directly associated to the formation of some tornadoes.

[edit] Cyclone as metaphor

Image:Puck cyclone cover.jpg

The cyclone has been used for over 100 years as a metaphor for political upheaval.<ref name="MoneyPoliticsOz"> Quentin P. Taylor (2004-12-02). Money and Politics in the Land of Oz. The Independant Institute. Retrieved on 2006-10-24. </ref> For example, according to political interpretations of The Wizard of Oz, the cyclone takes Dorothy to a utopia (Oz), and kills the Wicked Witch of the East who had oppressed the little people (the Munchkins).<ref name="MoneyPoliticsOz">Oz reference</ref> The storm cellar has also been used as a metaphor for seeking safety, as the 1894 cartoon shows. The caption reads, "In the cyclone cellar, waiting for fair weather."