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Cyclone
In meteorology, a '''cyclone''' refers to an area of closed, circular fluid motion rotating in the same direction as the Earth's rotation|Earth[ ][
]. This is usually characterized by inward spiraling winds that rotate counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth.
Large-scale cyclonic circulations are almost always centred on areas of atmospheric pressure|low atmospheric pressure[ ][ ]. The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size.[Orlanski, I., 1975. A rational subdivision of scales for atmospheric processes. Bulletin of the American Meteorological Society, 56(5), 527-530.] Cyclones have also been seen on other planets outside of the Earth, such as Mars and Neptune.[NASA. Historic Hurricanes. Retrieved on 2008-06-14.]
Cyclogenesis describes the process of cyclone formation and intensification [Arctic Climatology and Meteorology. The cyclone god Peter is also thought to have created cyclones with his minion Marco and magical imp kelly!!!!!!!!!!rg/arcticmet/glossary/cyclogenesis.html Cyclogenesis.] Retrieved on 2006-12-04.]. Cyclogenesis|Extratropical cyclones form as waves in large regions of enhanced midlatitude temperature contrasts called baroclinity|baroclinic zones. These zones contract to form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, cyclones occluded front|occlude as cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the polar or subtropical jetstream.
Weather fronts separate two air mass|masses of air of different density|densities and are associated with the most prominent meteorological phenomenon|meteorological phenomena. Air masses separated by a front may differ in temperature or humidity. Strong cold fronts typically feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. They form west of the circulation center and generally move from west to east. Warm fronts form east of the cyclone center and are usually preceded by stratiform Precipitation (meteorology)|precipitation and fog. They move Geographical pole|poleward ahead of the cyclone path. Occluded fronts form late in the cyclone life cycle near the enter of the cyclone and often wrap around the storm center.
Tropical cyclogenesis describes the process of development of tropical cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core.Structure
There are a number of structural characteristics common to all cyclones. As they are low pressure areas, their center is the area of lowest atmospheric pressure in the region, often known in mature tropical cyclones as the eye (cyclone)|eye.[University of Aberdeen.The Atmosphere in Motion.]] The wind flow around a large cyclone is Clockwise and counterclockwise|counterclockwise in the northern hemisphere and clockwise in the southern hemisphere as a result of the Coriolis effect. (An anticyclone, on the other hand, rotates clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.)
Formation
Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area).[Arctic Climatology and Meteorology. Cyclogenesis. Retrieved on 2006-12-04.] Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. It can occur at various scales, from the microscale to the synoptic scale. Extratropical cyclones form as waves along weather fronts before occluding later in their life cycle as cold core cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core.[National Weather Service Key West summary of waterspout types: http://www.srh.noaa.gov/eyw/HTML/spoutweb.htm] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high pressure areas—Anticyclogenesis.[ ]
The surface low has a variety of ways of forming. Topography can force a surface low when dense low-level high pressure system ridges in east of a north-south mountain barrier.[COMET Program Flow Interaction With Topography] Mesoscale Convective System|Mesoscale convective systems can spawn surface lows which are initially warm core.[ Raymond D. Menard1, and J.M. Fritsch A Mesoscale Convective Complex-Generated Inertially Stable Warm Core Vortex] The disturbance can grow into a wave-like formation along the Surface weather analysis|front and the low will be positioned at the crest. Around the low, flow will become cyclonic, by definition. This rotational flow will push polar air equatorward west of the low via its trailing cold front, and warmer air with push poleward low via the warm front. Usually the cold front will move at a quicker pace than the warm front and “catch up” with it due to the slow erosion of higher density airmass located out ahead of the cyclone and the higher density airmass sweeping in behind the cyclone, usually resulting in a narrowing warm sector.[The Physics Factbook Density of Air] At this point an occluded front forms where the warm air mass is pushed upwards into a trough of warm air aloft, which is also known as a trowal.[St. Louis University What is a trowal?]
Tropical cyclogenesis is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere.[Chris Landsea. Subject: A15) How do tropical cyclones form ? Retrieved on 2008-06-08.] An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir-Simpson Hurricane Scale).[Roger Edwards. The Online Tornado FAQ: Frequently Asked Questions About Tornadoes. Retrieved on 2008-06-14.]
Types
There are six main types of cyclones: Polar cyclones, Polar lows, Extratropical cyclones, Subtropical cyclones, Tropical cyclones, and Mesocyclones
Polar cyclone
Polar, sub-polar, or Arctic cyclones (also known as the polar vortex)[Halldór Björnsson. Global circulation. Retrieved on 2008-06-15.] A polar cyclone is a low pressure weather system, usually spanning to , in which the air circulates in a counterclockwise direction in the northern hemisphere, and a clockwise direction in the southern hemisphere. In the Northern Hemisphere, the polar cyclone has two centers on average. One center lies near Baffin Island and the other over northeast Siberia.[Glossary of Meteorology. Polar vortex. Retrieved on 2008-06-15.] In the southern hemisphere, it tends to be located near the edge of the Ross ice shelf near 160 west longitude.[Rui-Rong Chen, Don L. Boyer, and Lijun Tao. Laboratory Simulation of Atmospheric Motions in the Vicinity of Antarctica. Retrieved on 2008-06-15.] When the polar vortex is strong, westerly flow descends to the earth's surface. When the polar cyclone is weak, significant cold outbreaks occur.[James E. Kloeppel. Stratosheric polar vortex influences winter cold, researchers say. Retrieved on 2008-06-15.]
Polar low
A polar low is a small-scale, short-lived atmospheric low pressure area|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 and exist for no more than a couple of days. They are part of the larger class of mesoscale meteorology|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.[Rasmussen,E.A. and Turner, J.(2003). Polar Lows: Mesoscale Weather Systems in the Polar Regions, Cambridge University Press, Cambridge, pp 612.]
Extratropical
An extratropical cyclone is a Synoptic scale meteorology|synoptic scale Low pressure area|low pressure weather system that has neither tropical cyclone|tropical nor polar cyclone|polar characteristics, being connected with Surface weather analysis|fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".[ ]
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 cyclone#Extratropical transition|extratropical transition has occurred,[ExtraLessonMillUni][Ryan N. Maue. CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS. Retrieved on 2008-06-15.] An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters and develops central convection, which warms its core.Subtropical
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 between the equator and the 50th parallel.[David B. Spiegler. Reply. Retrieved on 2008-04-20.] By 1972, the National Hurricane Center officially recognized this cyclone category.[R. H. Simpson and Paul J. Hebert. Atlantic Hurricane Season of 1972. Retrieved on 2008-06-14.] Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.[Chris Landsea. Subject: A6) What is a sub-tropical cyclone? Retrieved on 2008-06-14.]
Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold by three degrees Celsius, or five degrees Fahrenheit, lying around 23 degrees Celsius.Tropical
A tropical cyclone is a storm|storm system characterized by a low pressure system|low pressure center and numerous thunderstorms that produce strong winds and flooding rain. A tropical cyclone feeds on heat released when moist air rises, resulting in condensation of water vapour contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows, leading to their classification as "warm core" storm systems.[National Hurricane Center. Subject : C2) Doesn't the friction over land kill tropical cyclones? Retrieved on 2008-02-25.] This is the reason coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions.[National Oceanic and Atmospheric Administration. 2005 Tropical Eastern North Pacific Hurricane Outlook. Retrieved on 2006-05-02.] They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere, and to maintain a relatively stable and warm.
Many tropical cyclones tropical cyclogenesis|develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. Others form when #Related cyclone types|other types of cyclones acquire tropical characteristics. Tropical systems are then moved by #Steering winds|steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye (cyclone)|eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses;Mesoscale
A mesocyclone is a vortex of air, approximately to in diameter (the mesoscale of meteorology), within a convection|convective storm.[ ]
Air rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe thunderstorm.[National Weather Service Forecast Office State College, Pennsylvania. Splitting Storm and Anti-cyclonic Rotating Mesocyclone in a Thunderstorm over Elk County July 10th, 2006. Retrieved on 2008-06-15.] Such storms can feature strong surface winds and severe hail. Mesocyclones often occur together with updrafts in supercells, where tornadoes may form. About 1700 mesocyclones form annually across the United States, but only half produce tornadoes.[Forces of Nature. Tornadoes : the mesocyclone. Retrieved on 2008-06-15.]
Extraterrestrial cyclones
Cyclones are not unique to Earth. Cyclonic storms are common on gas giant|Jovian planets, like the Small Dark Spot on Neptune. Also known as the Wizard's Eye, it is about one third the diameter of the Great Dark Spot. It received the name "Wizard's Eye" because it looks like an eye. This appearance is caused by a white cloud in the middle of the Wizard's Eye.[David Brand. Colossal cyclone swirling near Martian north pole is observed by Cornell-led team on Hubble telescope. Retrieved on 2008-06-15.] Jovian storms like the Great Red Spot are usually mistakenly named as giant hurricanes or cyclonic storms. However, this is inaccurate, as the Great Red Spot is, in fact, the inverse phenomenon, an anticyclone.[ ]
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'
- The EM-DAT International Disaster Database by the Centre for Research on the Epidemiology of Disasters
Category:Basic meteorological concepts and phenomena
Category:Tropical cyclone meteorology
Category:Types of cyclone|*
Category:Weather hazards
Category:Vortices
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