- This article is about the meteorological phenomenon. For other uses of the term see Cyclone (disambiguation).
In meteorology, a cyclone refers to 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 hurricanes and tornadoes. Because of this, most weather forecasters avoid using the term cyclone without a qualifying term.
The center of a cyclone is a low-pressure region. Pressure gradient force, from high- to low-pressure reasons, causes high winds around these regions. Wind flow around a large cyclone is almost invariantly anticlockwise, in the northern hemisphere, and clockwise, in the southern hemisphere, due to the Coriolis effect. Anticyclonic large storms are extremely rare on Earth, though Jupiter's Great Red Spot storm is anticyclonic.
Types of Cylones
Tropical cyclones (also known as tropical storms, hurricanes and typhoons) are cyclones which form over warm ocean waters and draw their energy from the evaporation and condensation of that water. 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 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 if it through horizontal temperature gradients in the atmosphere. They move out of the extratropical regions towards the polar regions, bringing precipitation in the form of rain or drizzle. Nearly always called by the term low-pressure cells in North America, 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. Usually, a part of the cold front will develop into a warm front, giving the frontal zone a shape (when drawn on a weather chart) that is called a "wave". An old name for such a system is "warm wave" (in the United States).
In the beginning, the sea-level pressure is not very low; typically, it is about 1009 millibars (1009 hectopascals) (hPa) (SI). Intensification or "deepening" of the extratropical cyclone progresses slowly. Often, the sea-level pressure in the low-pressure cell fails to drop below 1000 millibars (1000 hectopascals) (hPa) (SI). Rapidly falling air pressure is very rare or non-existent, even after several days have passed and the storm has moved to higher latitudes.
Extratropical cyclones are generally mild storms with surface winds of 7-15 knots. The band of precipitation that is associated with the warm front is often extensive. They tend to move along a predictable path at a moderate rate of progress. They are much more common in the United States than in Europe which is located above the arid Sahara desert. The dry, arid air of the Sahara desert is not conducive to the formation of extratropical cyclones.
In the North Pacific ocean, the Kuroshio or Japan current moves warm water into the northern North Pacific ocean. In the North Atlantic ocean, the Gulf Stream moves warm water into the northern North Atlantic ocean.  (http://www.physicalgeography.net/fundamentals/8q_1.html) The warm water at high latitudes provides the energy that brings on the generation of subpolar cyclones or low-pressure cells that exhibit characteristics which differentiate them from extratropical cyclones.
- They are tall and often exceed 30,000 feet (c. 10km) in height.
- The air temperature near the center of the storm is colder than the air to the north, east, south, and west of it. For that reason, the storms are called "cold-core lows."
- At first, the storms move from high latitudes to lower latitudes, i.e., they travel towards the southeast over North America and Europe, then eastwards.
- As the storms move across land, the air pressure may fall rapidly to below 980 millibars (980 hectopascals) (hPa) (SI).
Similar storms may appear, at times, at very high latitudes. The very cold storms are called subarctic cyclones or low-pressure cells in the Northern Hemisphere and subantarctic cyclones or low pressure cells in the Southern Hemisphere.
In North America, tornados 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, tornados are comparatively small but extremely powerful. Tornados 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.
- Glossary definition of cyclogenesis (http://nsidc.org/arcticmet/glossary/cyclogenesis.html)
- Glossary definition of cyclolysis (http://nsidc.org/arcticmet/glossary/cyclolysis,html)