Cyclone
In meteorology, a cyclone is the rotation of a volume of air about 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.
Structure
The center of a cyclone is a low-pressure region. Near the center of the 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 anticlockwise 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 cyclones which form over warm (generally tropical) ocean waters and draw their energy from evaporation and condensation. They are characterized by a strong area of low pressure at the surface and an area of higher pressure aloft. Tropical cyclones are associated with strong thunderstorms, high winds, and flooding.
Extratropical cyclones
Extratropical cyclones (or low-pressure cells) lie somewhere in between tropical cyclones and mid-latitude cyclones, drawing a portion of their energy through the evaporation and condensation of ocean water, and some through horizontal temperature gradients in the atmosphere. They move out of the tropical regions towards the polar regions, bringing precipitation in the form of rain or drizzle. They often form quickly along cold fronts that have stagnated after moving into an area where warm, moist air exists. The warm, moist air is less dense, therefore it overruns the more dense cold air at and behind the cold front. A cyclonic motion is imparted to the ascending air, naturally, forming a shallow cyclone. Extratropical cyclones are also formed from tropical cyclones when they move into non-tropical regions and lose 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, 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 behaviour and size to tropical cyclones, although generally much shorter lived.
Polar lows are typically several hundred kilometres in diameter, generally have strong winds (although generally not at 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 Polar Lows appear very similar to hurricanes with an eye and convective bands wrapping the storm in the counter-clockwise fashion. Research aircraft data suggests that these 'arctic hurricanes' may be warm-cored systems. Polar Lows are currently difficult to predict due to scarcity of data. 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
- Main article: Tornado
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 tornados 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. [1] (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 [2] [3].
Cyclone as metaphor
The cyclone has been used by cartoonists 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 at right shows.