Cyclone

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Cyclone :: Cyclones_1999

In meteorology, a cyclone is the rotation of a volume of air around an area of low atmospheric pressure. Cyclones are responsible for a wide variety of different meteorological phenomena such as tropical cyclones and tornadoes. Because of this, most weather forecasters avoid using the term cyclone without a qualifying term.

The terms hurricane and typhoon are regionally specific names for a strong tropical cyclone —— a non-frontal, synoptic-scale, warm-core low-pressure system with cyclonic surface wind circulation (Holland 1993).

Structure

The center of a cyclone is a low-pressure region. Near the center of the cyclone the pressure gradient force, from high- to low-pressure regions, and the Coriolis force must be in an approximate balance (or else the cyclone would collapse in on itself under the pressure gradient). The wind flow around a large cyclone is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect. Large anticyclonic storms are extremely rare on Earth, though Jupiter's Great Red Spot storm is anticyclonic.

Types of cyclones

Tropical cyclones

Tropical cyclones (also known as tropical storms, hurricanes and typhoons) are storm systems that form over warm (generally tropical) ocean waters and draw energy from condensation of moist air. A tropical cyclone's low-pressure center is at sea level; a strong tropical cyclone develops a high-pressure area over it high in the atmosphere. Tropical cyclones are associated with strong thunderstorms, high winds, and flooding from storm surge.hi

Extratropical cyclones

Extratropical cyclones (or low-pressure cells) have characteristics of tropical cyclones and mid-latitude cyclones, drawing some energy from evaporation and condensation over warm ocean waters, and some from atmospheric temperature gradients. They move away from tropical regions toward polar regions, bringing precipitation and high winds. They often form quickly along cold fronts that stall in regions of warm, moist air, which overruns the denser cold air along and behind a front and develops a shallow cyclonic circulation. Extratropical cyclones also evolve from tropical cyclones that move into non-tropical regions and lose strictly tropical characteristics

Subtropical cyclones

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°.

Mid-latitude cyclones

Mid-latitude cyclones are generated by the temperature difference between warm and cold air masses, with warm water at high latitudes generally providing that differential. These storms have a cold core (low-pressure center), unlike tropical and extratropical cyclones. Similar storms may appear at very high latitudes; these very cold storms are called subarctic (or subantarctic) cyclones.

Polar low

Polar lows are similar in behavior and size to tropical cyclones, although generally much shorter lived.

Polar lows are typically several hundred kilometers in diameter, generally have strong winds (usually less than hurricane intensity) and last one to two days on average. Unlike most typical cyclones, they develop extremely rapidly, reaching peak intensity within 24 hours. They generally form under cold upper-level lows when cold arctic air flows over a warm body of water.

On satellite imagery, a polar low appears very similar to a hurricane, with an eye and convective bands wrapping around the center. Research aircraft data suggests that these "arctic hurricanes" may be warm-core systems. Lack of weather data makes forecasting of polar lows difficult. Most predictions in this area are more subjective than the prediction of tropical cyclones.

Arctic cyclone

Arctic cyclones are vast areas of low pressure in polar regions that have a weak cyclonic rotation.

Mesocyclones

A mesocyclone is an area of vertical atmospheric rotation, typically 2-6 miles across. They are often found in the right-rear flank of supercell thunderstorms, and are visible as a hook echo on Doppler weather radar. The presence of a mesocyclone can only be truly verified by radar, although visual clues such as curved inflow bands may be present.

Mesocyclones form when strong changes of wind speed and/or direction with height (wind shear) sets the lower part of the atmosphere spinning horizontally. The updraft of a thunderstorm can then draw this area of spinning air from horizontal to vertical.

Tornadoes

In North America, tornadoes are sometimes described as cyclones because they involve powerful winds around a low-pressure vortex. However, they differ from other cyclones by their very local nature; most cyclones are massive storms, while tornadoes are comparatively small but extremely powerful. Tornadoes occur on too local a scale for the Coriolis effect to determine the direction of rotation; for this reason tornado winds sometimes flow anticyclonically, or opposite the direction dictated by the Coriolis effect.

Martian cyclones

On April 27, 1999, a rare cyclone 1,100 miles in diameter was detected by the Hubble Space Telescope in the northern polar region of Mars. It consisted of three cloud bands wrapped around a massive 200 mile diameter eye, and contained features similar to storms that have been detected in the poles of Earth. It was only observed briefly, as it seemed to be dissipating when it was imaged six hours later, and was not seen on later imaging passes. ^* (Dust devils have also been observed on Mars.)

Derivation of the term

The word "cyclone" appears to have been used first by Henry Piddington around 1840. The word is derived from the Greek word "kyklon" which means moving in a circle [http://banglapedia.search.com.bd/HT/P_0168.htm.

Cyclone as metaphor

The cyclone has been used for over 100 years as a metaphor for political upheaval. 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). 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."

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
Glossary Definition: Cyclonic rotation - StormWiki
Weather Facts: The Polar Low - Weather Online UK
^*- NOAA FAQ

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