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{{otheruses1|the meteorological phenomenon}} |
{{otheruses1|the meteorological phenomenon}} |
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[[Image:Polar low.jpg|thumb|250 px|Polar low over the [[Barents Sea]] on [[February 27]], [[1987]]]] |
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In [[meteorology]], a '''cyclone''' is an area of [[atmospheric pressure|low atmospheric pressure]] characterized by inward [[spiral]]ing [[wind]]s that rotate [[counter clockwise]] in the [[northern hemisphere]] and [[clockwise]] in the [[southern hemisphere]] of the Earth.<ref name="BBCCycDef"> {{cite web |
In [[meteorology]], a '''cyclone''' is an area of [[atmospheric pressure|low atmospheric pressure]] characterized by inward [[spiral]]ing [[wind]]s that rotate [[counter clockwise]] in the [[northern hemisphere]] and [[clockwise]] in the [[southern hemisphere]] of the Earth.<ref name="BBCCycDef"> {{cite web |
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| title = BBC Weather Glossary - Cyclone |
| title = BBC Weather Glossary - Cyclone |
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| publisher= [http://meted.ucar.edu/ University Corporation for Atmospheric Research] |
| publisher= [http://meted.ucar.edu/ University Corporation for Atmospheric Research] |
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| url = http://meted.ucar.edu/satmet/goeschan/glossary.htm#c |
| url = http://meted.ucar.edu/satmet/goeschan/glossary.htm#c |
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| accessdate = 2006-10-24 }} </ref> The generic term covers a wide variety of [[List of meteorological phenomena|meteorological phenomena]]. These include [[tropical cyclone]]s and [[extratropical cyclone]]s, so meteorologists rarely use "cyclone" without additional qualification. |
| accessdate = 2006-10-24 }} </ref> The generic term covers a wide variety of [[List of meteorological phenomena|meteorological phenomena]]. These include [[tropical cyclone]]s and [[extratropical cyclone]]s, so meteorologists rarely use "cyclone" without additional qualification. [[Cyclogenesis]] describes the process of development for extratropical cyclones,<ref name="Arc">Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/cyclogenesis.html Cyclogenesis.] Retrieved on 2006-[[December 4|12-04]].</ref> while [[tropical cyclogenesis]] describes the process of development of tropical cyclones. Tropical cyclones, [[polar low]]s, and [[mesocyclone]]s are smaller warm core systems which lie within the [[mesoscale]], while extratropical cyclones and polar cyclones are larger cold core systems which are on the [[synoptic scale]].<ref name="AnlFcastHelp">{{cite web |
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| title = Cyclone Phase Analysis and Forecast: Help Page |
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| author = Robert Hart |
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| publisher = [http://moe.met.fsu.edu Florida State University Department of Meteorology] |
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| date = [[2003-02-18]] |
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| url = http://moe.met.fsu.edu/cyclonephase/help.html |
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| accessdate = 2006-10-03 }}</ref><ref>Orlanski, I., 1975. A rational subdivision of scales for atmospheric processes. Bulletin of the American Meteorological Society, 56(5), 527-530.</ref> Cyclones have also been seen on other planets outside of the [[Earth]], such as [[Mars]] and [[Neptune]].<ref name="Brand">David Brand. [http://www.news.cornell.edu/releases/May99/mars.cyclone.deb.html Colossal cyclone swirling near Martian north pole is observed by Cornell-led team on Hubble telescope.] Retrieved on [[2008-06-15]].</ref><ref name="WIZ">[[NASA]]. [http://solarsystem.nasa.gov/educ/themes/display.cfm?Item=hurricane Historic Hurricanes.] Retrieved on [[2008-06-14]].</ref> |
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==Structure== |
==Structure== |
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There are a number of structural characteristics common to all cyclones. |
There are a number of structural characteristics common to all cyclones. As they are [[low pressure area]]s, their center is the area of lowest atmospheric pressure in the region, often known in mature tropical cyclones as the [[eye (cyclone)|eye]].<ref name="FAQ eye">{{cite web | author=[[Chris Landsea|Landsea, Chris]] and Sim Aberson. | title=What is the "eye"? | date=[[August 13]] [[2004]] | publisher=[[Atlantic Oceanographic and Meteorological Laboratory]] | accessdate=2006-06-14 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A11.html}}</ref> 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 effect|Coriolis force]] must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure.<ref>[[University of Aberdeen]]. [http://www.abdn.ac.uk/physics/meteo/met8sum.pdf The Atmosphere in Motion.] Retrieved on [[2008-06-15]].</ref> 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.) |
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==Formation== |
==Formation== |
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[[Image:Baroclinicleafphasecyclogenesiscropped.gif|thumb|right|250px|The initial low pressure area forms at the location of the red dot on the image. It is usually perpendicular (at a right angle to) the leaf-like cloud formation seen on satellite during the early stage of cyclogenesis. The location of the axis of the upper level [[jet stream]] is in light blue.]] |
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[[Image:Animated hurricane.gif|frame|right|[[Radar]] image of a [[tropical cyclone]] in the [[northern hemisphere]].]] |
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{{main|Cyclogenesis}} |
{{main|Cyclogenesis}} |
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Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area).<ref name="Arc">Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/cyclogenesis.html Cyclogenesis.] Retrieved on 2006-[[December 4|12-04]].</ref> 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.<ref name = "AOML FAQ A7">{{cite web | author = [[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division | title = Frequently Asked Questions: What is an extra-tropical cyclone? | publisher = [[NOAA]] | accessdate = 2007-03-23 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A7.html}}</ref> Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.<ref name="FoN">Forces of Nature. [http://library.thinkquest.org/C003603/english/tornadoes/themesocyclone.shtml Tornadoes : the mesocyclone.] Retrieved on [[2008-06-15]].</ref> [[Waterspout]]s can also form from mesocyclones, but more often develop from environments of high instability and low vertical [[wind shear]].<ref name="NWS">[[National Weather Service]] [[Key West]] summary of waterspout types: http://www.srh.noaa.gov/eyw/HTML/spoutweb.htm</ref> Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of [[high pressure area]]s—[[Anticyclogenesis]].<ref name="CyclogenesisDef"> {{cite web | title = American Meteorological Society Glossary - Cyclogenesis | publisher = [http://www.allenpress.com Allen Press Inc.] | date = [[2000-06]] | url = http://amsglossary.allenpress.com/glossary/search?p=1&query=cyclogenesis | accessdate = 2006-10-12 }}</ref> |
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Cold-core cyclones (most cyclone varieties) form due to the nearby presence of an upper level trough, which increases divergence over an area that induces upward motion and surface low pressure. Warm-core cyclones (such as tropical cyclones and many mesocyclones) can have their initial start due to a nearby upper trough, but after formation of the initial disturbance, depend upon a storm-relative upper level high to maintain or increase their strength. |
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==Categorization== |
==Categorization== |
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There are six main types of cyclones: [[Polar cyclone]]s, [[Polar low]]s, [[Extratropical cyclone]]s, [[Subtropical cyclone]]s, [[Tropical cyclone]]s, and [[Mesocyclone]]s. |
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===Polar cyclone=== |
===Polar cyclone=== |
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{{main|Polar cyclone}} |
{{main|Polar cyclone}} |
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Polar, sub-polar, or arctic cyclones (also known as the polar vortex)<ref name="glossvortex">Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=polar-vortex1 Polar vortex.] Retrieved on [[2008-06-15]].</ref> are vast areas of low pressure which stengthen in the winter and weaken in the summer.<ref>Halldór Björnsson. [http://andvari.vedur.is/~halldor/HB/Met210old/GlobCirc.html Global circulation.] Retrieved on [[2008-06-15]].</ref> A polar cyclone is a low pressure [[weather system]], usually spanning {{convert|1000|km|mi}} to {{convert|2000|km|mi}}, 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.<ref name="glossvortex">Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=polar-vortex1 Polar vortex.] Retrieved on [[2008-06-15]].</ref> In the southern hemisphere, it tends to be located near the edge of the [[Ross ice shelf]] near 160 west longitude.<ref>Rui-Rong Chen, Don L. Boyer, and Lijun Tao. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0469(1993)050%3C4058%3ALSOAMI%3E2.0.CO%3B2&ct=1 Laboratory Simulation of Atmospheric Motions in the Vicinity of Antarctica.] Retrieved on [[2008-06-15]].</ref> When the polar vortex is strong, westerly flow descends to the earth's surface. When the polar cyclone is weak, significant cold outbreaks occur.<ref>James E. Kloeppel. [http://www.news.uiuc.edu/scitips/01/12weather.html Stratosheric polar vortex influences winter cold, researchers say.] Retrieved on [[2008-06-15]].</ref> |
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'''Polar''' or '''Arctic cyclones''' are vast areas of low pressure. A polar cyclone is a low pressure [[weather system]], usually spanning 1,000–2,000 kilometers, in which the air circulates in a counterclockwise direction in the northern hemisphere, and a clockwise direction in the southern. |
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===Polar low=== |
===Polar low=== |
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[[Image:Polar low.jpg|thumb|Polar low over the [[Barents Sea]] on [[February 27]], [[1987]]]] |
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{{main|Polar low}} |
{{main|Polar low}} |
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A |
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 {{convert|1000|km|mi}} 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.<ref>Rasmussen,E.A. and Turner, J.(2003). Polar Lows: Mesoscale Weather Systems in the Polar Regions, Cambridge University Press, Cambridge, pp 612.</ref> |
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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. |
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===Extratropical=== |
===Extratropical=== |
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[[Image:UK-Cyclone.gif|thumb|right|250px|A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between [[isobars]] indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm [[Surface weather analysis|frontal boundaries]].]] |
[[Image:UK-Cyclone.gif|thumb|right|250px|A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between [[isobars]] indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm [[Surface weather analysis|frontal boundaries]].]] |
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{{main|Extratropical cyclone}} |
{{main|Extratropical cyclone}} |
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An |
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".<ref name="ExtraLessonMillUni"> {{cite web |
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| title = ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones |
| title = ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones |
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| author = Dr. DeCaria |
| author = Dr. DeCaria |
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| accessdate = 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-cyclone]]s, drive the weather over much of the Earth. |
| accessdate = 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-cyclone]]s, drive the weather over much of the Earth. |
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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 distribution around the cyclone becomes fairly uniform with radius. |
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 distribution around the cyclone becomes fairly uniform with radius.<ref>Ryan N. Maue. [http://www.coaps.fsu.edu/~maue/cyclone_ch3.html CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS.] Retrieved on [[2008-06-15]].</ref> |
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===Subtropical=== |
===Subtropical=== |
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{{main|Subtropical cyclone}} |
{{main|Subtropical cyclone}} |
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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.<ref name="A6">[[Chris Landsea]]. [http://www.aoml.noaa.gov/hrd/tcfaq/A6.html Subject: A6) What is a sub-tropical cyclone?] Retrieved on [[2008-06-14]].</ref> As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones, and used terms such as quasi-tropical and semi-tropical to describe the cyclone hybrids.<ref>David B. Spiegler. [http://docs.lib.noaa.gov/rescue/mwr/101/mwr-101-04-0380.pdf Reply.] Retrieved on [[2008-04-20]].</ref> By 1972, the [[National Hurricane Center]] officially recognized this cyclone category.<ref>R. H. Simpson and Paul J. Hebert. [http://www.aoml.noaa.gov/general/lib/lib1/nhclib/mwreviews/1972.pdf Atlantic Hurricane Season of 1972.] Retrieved on [[2008-06-14]].</ref> Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.<ref name="A6">[[Chris Landsea]]. [http://www.aoml.noaa.gov/hrd/tcfaq/A6.html Subject: A6) What is a sub-tropical cyclone?] Retrieved on [[2008-06-14]].</ref> They have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and exist in areas of weak to moderate temperature gradient.<ref name="A6">[[Chris Landsea]]. [http://www.aoml.noaa.gov/hrd/tcfaq/A6.html Subject: A6) What is a sub-tropical cyclone?] Retrieved on [[2008-06-14]].</ref> 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.<ref name="HistSubTropCyclones">{{cite web |
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A '''subtropical cyclone''' is a weather system that has some characteristics of a [[tropical cyclone]] and some characteristics of an [[extratropical cyclone]]. It can form in a wide band of [[latitude]], from the [[equator]] to 50°. |
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| title = A Fifty year History of Subtropical Cyclones |
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| author = David Mark Roth |
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| publisher = [http://www.hpc.ncep.noaa.gov/ Hydrometeorological Prediction Center] |
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| date = 2002-02-15 |
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| url = http://www.hpc.ncep.noaa.gov/research/roth/Subpreprint.pdf |
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| accessdate = 2006-10-04 }}</ref> This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season. |
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===Tropical=== |
===Tropical=== |
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[[Image:Cyclone Catarina from the ISS on March 26 2004.JPG|thumb|250px|[[Cyclone Catarina]], a rare [[South Atlantic tropical cyclone]] viewed from the [[International Space Station]] on [[March 26]] [[2004]]]] |
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{{main|Tropical cyclone}} |
{{main|Tropical cyclone}} |
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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 [[flood]]ing [[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'easter]]s, [[European windstorm]]s, and [[polar low]]s, leading to their classification as "warm core" storm systems.<ref name = "AOML FAQ A7">{{cite web | author = [[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division | title = Frequently Asked Questions: What is an extra-tropical cyclone? | publisher = [[NOAA]] | accessdate = 2007-03-23 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A7.html}}</ref> |
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The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in [[tropics|tropical]] regions of the globe, and their formation in [[Air mass#Classification|Maritime Tropical air masses]]. The term "cyclone" refers to such storms' [[cyclone|cyclonic]] nature, with [[Clockwise and counterclockwise|counterclockwise]] rotation in the [[Northern Hemisphere]] and clockwise rotation in the [[Southern Hemisphere]]. Depending on their location and strength, tropical cyclones are referred to by other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply as a cyclone.<ref name="NHC glossary">{{cite web | author = [[National Hurricane Center]] | url = http://www.nhc.noaa.gov/aboutgloss.shtml | year = 2005 | title = Glossary of NHC/TPC Terms | accessdate= 2006-11-29 | publisher = [[National Oceanic and Atmospheric Administration]]}}</ref> |
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A '''tropical cyclone''' is a storm system fueled by the heat released when moist air rises and the water vapor in it [[condensation|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'easter]]s, [[European windstorm]]s, and [[polar low]]s by the heat mechanism that fuels them, which makes them "warm core" storm systems. |
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While tropical cyclones can produce extremely powerful winds and torrential [[rain]], they are also able to produce high waves and damaging [[storm surge]].<ref name="oxfo">{{cite web|author=James M. Shultz, Jill Russell and Zelde Espinel|year=2005|title=Epidemiology of Tropical Cyclones: The Dynamics of Disaster, Disease, and Development|publisher=Oxford Journal|accessdate=2007-02-24|url=http://epirev.oxfordjournals.org/cgi/content/full/27/1/21}}</ref> They develop over large bodies of warm water,<ref name = "AOML FAQ A15">{{ cite web | author = [[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division | title = Frequently Asked Questions: How do tropical cyclones form? | publisher = [[NOAA]] | accessdate = 2006-07-26 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A15.html}}</ref> and lose their strength if they move over land.<ref>[[National Hurricane Center]]. [http://www.aoml.noaa.gov/hrd/tcfaq/C2.html Subject : C2) Doesn't the friction over land kill tropical cyclones?] Retrieved on [[2008-02-25]].</ref> 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 [[flood]]ing up to {{convert|40|km|mi}} from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve [[drought]] conditions.<ref name="2005 EPac outlook">[[National Oceanic and Atmospheric Administration]]. [http://www.cpc.ncep.noaa.gov/products/Epac_hurr/Epac_hurricane.html 2005 Tropical Eastern North Pacific Hurricane Outlook.] Retrieved on [[2006-05-02]].</ref> 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 temperature worldwide. |
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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'''. |
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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. |
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Tropical cyclones can produce extremely strong winds, [[tornado]]es, torrential rain, high waves, and [[storm surge]]s. The heavy rains and storm surges can produce extensive [[flood]]ing. 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]]. |
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===Mesoscale=== |
===Mesoscale=== |
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[[Image:Greensburg3 small.gif|thumb|A mesocyclone from the [[May 2007 Tornado Outbreak|Greensburg, Kansas tornado]] indicated on [[Doppler effect|Doppler]] [[weather radar]].]] |
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{{main|Mesocyclone}} |
{{main|Mesocyclone}} |
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A |
A mesocyclone is a [[vortex]] of air, approximately {{convert|2|km|mi}} to {{convert|10|km|mi}} in diameter (the [[mesoscale]] of meteorology), within a [[convection|convective]] [[storm]].<ref name="MesocyloneDef"> {{cite web| title = American Meteorological Society Glossary - Mesocyclone |
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| publisher = [http://www.allenpress.com Allen Press Inc.] |
| publisher = [http://www.allenpress.com Allen Press Inc.] |
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| date = 2000-06 |
| date = 2000-06 |
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| url = http://amsglossary.allenpress.com/glossary/search?id=mesocyclone1 |
| url = http://amsglossary.allenpress.com/glossary/search?id=mesocyclone1 |
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| accessdate = 2006-12-07 }}</ref> |
| accessdate = 2006-12-07 }}</ref> |
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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]].<ref>[[National Weather Service]] Forecast Office State College, Pennsylvania. [http://www.erh.noaa.gov/ctp/features/2006/07_10/ Splitting Storm and Anti-cyclonic Rotating Mesocyclone in a Thunderstorm over Elk County July 10th, 2006.] Retrieved on [[2008-06-15]].</ref> Such storms can feature strong surface winds and severe [[hail]]. Mesocyclones often occur together with updrafts in [[supercell]]s, where [[tornado]]es may form. About 1700 mesocyclones form annually across the [[United States]], but only half produce tornadoes.<ref name="FoN">Forces of Nature. [http://library.thinkquest.org/C003603/english/tornadoes/themesocyclone.shtml Tornadoes : the mesocyclone.] Retrieved on [[2008-06-15]].</ref> Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("[[wind shear]]") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column. Mesocyclones are normally relatively localized: they lie between the [[Synoptic scale meteorology|synoptic scale]] (hundreds of kilometers) and microscale (hundreds of meters). Radar imagery is used to identify these features.<ref>Roger Edwards. [http://www.spc.ncep.noaa.gov/faq/tornado/ The Online Tornado FAQ: Frequently Asked Questions About Tornadoes.] Retrieved on [[2008-06-14]].</ref> |
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==Extraterrestrial cyclones== |
==Extraterrestrial cyclones== |
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[[Image:Mars cyclone.jpg|thumb|right|Cyclone on Mars, imaged by the [[Hubble Space Telescope]]]] |
[[Image:Mars cyclone.jpg|thumb|right|Cyclone on Mars, imaged by the [[Hubble Space Telescope]]]] |
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Cyclones are not unique to Earth. Cyclonic storms are common on [[gas giant|Jovian planets]], like the [[Small Dark Spot]] on [[Neptune]]. [[Mars]] has also exhibited cyclonic storms. 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]].<ref name="HaydPlan"> {{cite web |
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.<ref>[[NASA]]. [http://solarsystem.nasa.gov/educ/themes/display.cfm?Item=hurricane Historic Hurricanes.] Retrieved on [[2008-06-14]].</ref> [[Mars]] has also exhibited cyclonic storms.<ref name="Brand">David Brand. [http://www.news.cornell.edu/releases/May99/mars.cyclone.deb.html Colossal cyclone swirling near Martian north pole is observed by Cornell-led team on Hubble telescope.] Retrieved on [[2008-06-15]].</ref> 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]].<ref name="HaydPlan"> {{cite web |
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| title = Jupiter's Great Red Spot |
| title = Jupiter's Great Red Spot |
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| author = Ellen Cohen |
| author = Ellen Cohen |
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==See also== |
==See also== |
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{{tcportal}} |
{{tcportal}} |
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*[[Anticyclone]] |
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*[[Cyclogenesis]] |
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*[[Eye (cyclone)]] |
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*[[Gyre]] |
*[[Gyre]] |
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*[[List of environment topics]] |
*[[List of environment topics]] |
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*[[Meteorology]] |
*[[Meteorology]] |
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*[[Mesocyclone]] |
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*[[Polar cyclone]] |
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*[[Polar low]] |
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*[[Extratropical cyclone]] |
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*[[Subtropical cyclone]] |
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*[[Tropical cyclone]] |
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*[[Tropical cyclogenesis]] |
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{{Cyclones}} |
{{Cyclones}} |
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*[http://www.myspace.com/kentcyclone Kent Cyclone] - Listen to 'Cyclone What?' here |
*[http://www.myspace.com/kentcyclone Kent Cyclone] - Listen to 'Cyclone What?' here |
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*[http://emdat.be The EM-DAT International Disaster Database] by the [[Centre for Research on the Epidemiology of Disasters]] |
*[http://emdat.be The EM-DAT International Disaster Database] by the [[Centre for Research on the Epidemiology of Disasters]] |
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*[[TaskForceCyclone|Task Force Cyclone]] |
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[[Category:Basic meteorological concepts and phenomena]] |
[[Category:Basic meteorological concepts and phenomena]] |
Revision as of 12:02, 15 June 2008
In meteorology, a cyclone is an area of low atmospheric pressure characterized by inward spiraling winds that rotate counter clockwise in the northern hemisphere and clockwise in the southern hemisphere of the Earth.[1][2] The generic term covers a wide variety of meteorological phenomena. These include tropical cyclones and extratropical cyclones, so meteorologists rarely use "cyclone" without additional qualification. Cyclogenesis describes the process of development for extratropical cyclones,[3] while tropical cyclogenesis describes the process of development of tropical cyclones. Tropical cyclones, polar lows, and mesocyclones are smaller warm core systems which lie within the mesoscale, while extratropical cyclones and polar cyclones are larger cold core systems which are on the synoptic scale.[4][5] Cyclones have also been seen on other planets outside of the Earth, such as Mars and Neptune.[6][7]
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.[8] 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.[9] 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. (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).[3] 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.[10] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[11] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[12] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high pressure areas—Anticyclogenesis.[13]
Categorization
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)[14] are vast areas of low pressure which stengthen in the winter and weaken in the summer.[15] A polar cyclone is a low pressure weather system, usually spanning 1,000 kilometres (620 mi) to 2,000 kilometres (1,200 mi), 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.[14] In the southern hemisphere, it tends to be located near the edge of the Ross ice shelf near 160 west longitude.[16] When the polar vortex is strong, westerly flow descends to the earth's surface. When the polar cyclone is weak, significant cold outbreaks occur.[17]
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 kilometres (620 mi) and exist for no more 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 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.[18]
Extratropical
An extratropical cyclone is a synoptic scale low pressure weather system that has neither tropical nor polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".[19]
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,[19][20] 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 distribution around the cyclone becomes fairly uniform with radius.[21]
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.[22] As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones, and used terms such as quasi-tropical and semi-tropical to describe the cyclone hybrids.[23] By 1972, the National Hurricane Center officially recognized this cyclone category.[24] Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.[22] They have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and exist in areas of weak to moderate temperature gradient.[22] 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.[25] This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season.
Tropical
A tropical cyclone is a storm system characterized by a 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.[10]
The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in Maritime Tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on their location and strength, tropical cyclones are referred to by other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply as a cyclone.[26]
While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge.[27] They develop over large bodies of warm water,[28] and lose their strength if they move over land.[29] 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 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions.[30] 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 temperature worldwide.
Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an 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.
Mesoscale
A mesocyclone is a vortex of air, approximately 2 kilometres (1.2 mi) to 10 kilometres (6.2 mi) in diameter (the mesoscale of meteorology), within a convective storm.[31] 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.[32] 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.[11] Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column. Mesocyclones are normally relatively localized: they lie between the synoptic scale (hundreds of kilometers) and microscale (hundreds of meters). Radar imagery is used to identify these features.[33]
Extraterrestrial cyclones
Cyclones are not unique to Earth. Cyclonic storms are common on 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.[34] Mars has also exhibited cyclonic storms.[6] 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.[35]
See also
References
- ^ "BBC Weather Glossary - Cyclone". BBC Weather. Retrieved 2006-10-24.
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- ^ "UCAR Glossary - Cyclone". University Corporation for Atmospheric Research. Retrieved 2006-10-24.
{{cite web}}
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- ^ a b Arctic Climatology and Meteorology. Cyclogenesis. Retrieved on 2006-12-04.
- ^ Robert Hart (2003-02-18). "Cyclone Phase Analysis and Forecast: Help Page". Florida State University Department of Meteorology. Retrieved 2006-10-03.
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- ^ Orlanski, I., 1975. A rational subdivision of scales for atmospheric processes. Bulletin of the American Meteorological Society, 56(5), 527-530.
- ^ a b David Brand. Colossal cyclone swirling near Martian north pole is observed by Cornell-led team on Hubble telescope. Retrieved on 2008-06-15.
- ^ NASA. Historic Hurricanes. Retrieved on 2008-06-14.
- ^ Landsea, Chris and Sim Aberson. (August 13 2004). "What is the "eye"?". Atlantic Oceanographic and Meteorological Laboratory. Retrieved 2006-06-14.
{{cite web}}
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(help) - ^ University of Aberdeen. The Atmosphere in Motion. Retrieved on 2008-06-15.
- ^ a b Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. "Frequently Asked Questions: What is an extra-tropical cyclone?". NOAA. Retrieved 2007-03-23.
- ^ a b Forces of Nature. Tornadoes : the mesocyclone. Retrieved on 2008-06-15.
- ^ National Weather Service Key West summary of waterspout types: http://www.srh.noaa.gov/eyw/HTML/spoutweb.htm
- ^ "American Meteorological Society Glossary - Cyclogenesis". Allen Press Inc. 2000-06. Retrieved 2006-10-12.
{{cite web}}
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(help)|publisher=
- ^ a b Glossary of Meteorology. Polar vortex. Retrieved on 2008-06-15.
- ^ Halldór Björnsson. Global circulation. Retrieved on 2008-06-15.
- ^ Rui-Rong Chen, Don L. Boyer, and Lijun Tao. Laboratory Simulation of Atmospheric Motions in the Vicinity of Antarctica. Retrieved on 2008-06-15.
- ^ James E. Kloeppel. Stratosheric polar vortex influences winter cold, researchers say. Retrieved on 2008-06-15.
- ^ Rasmussen,E.A. and Turner, J.(2003). Polar Lows: Mesoscale Weather Systems in the Polar Regions, Cambridge University Press, Cambridge, pp 612.
- ^ a b Dr. DeCaria (2005-12-07). "ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones". Department of Earth Sciences, Millersville University, Millersville, Pennsylvania. Retrieved 2006-10-21.
{{cite web}}
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- ^ Robert Hart and Jenni Evans (2003). "Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic TCs as Defined Within Cyclone Phase Space" (PDF). American Meteorological Society. Retrieved 2006-10-03.
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- ^ Ryan N. Maue. CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS. Retrieved on 2008-06-15.
- ^ a b c Chris Landsea. Subject: A6) What is a sub-tropical cyclone? Retrieved on 2008-06-14.
- ^ David B. Spiegler. Reply. Retrieved on 2008-04-20.
- ^ R. H. Simpson and Paul J. Hebert. Atlantic Hurricane Season of 1972. Retrieved on 2008-06-14.
- ^ David Mark Roth (2002-02-15). "A Fifty year History of Subtropical Cyclones" (PDF). Hydrometeorological Prediction Center. Retrieved 2006-10-04.
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- ^ National Hurricane Center (2005). "Glossary of NHC/TPC Terms". National Oceanic and Atmospheric Administration. Retrieved 2006-11-29.
- ^ James M. Shultz, Jill Russell and Zelde Espinel (2005). "Epidemiology of Tropical Cyclones: The Dynamics of Disaster, Disease, and Development". Oxford Journal. Retrieved 2007-02-24.
- ^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. "Frequently Asked Questions: How do tropical cyclones form?". NOAA. Retrieved 2006-07-26.
- ^ National Hurricane Center. Subject : C2) Doesn't the friction over land kill tropical cyclones? Retrieved on 2008-02-25.
- ^ National Oceanic and Atmospheric Administration. 2005 Tropical Eastern North Pacific Hurricane Outlook. Retrieved on 2006-05-02.
- ^ "American Meteorological Society Glossary - Mesocyclone". Allen Press Inc. 2000-06. Retrieved 2006-12-07.
{{cite web}}
: Check date values in:|date=
(help); External link in
(help)|publisher=
- ^ 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.
- ^ Roger Edwards. The Online Tornado FAQ: Frequently Asked Questions About Tornadoes. Retrieved on 2008-06-14.
- ^ NASA. Historic Hurricanes. Retrieved on 2008-06-14.
- ^ Ellen Cohen. "Jupiter's Great Red Spot". Hayden Planetarium. Retrieved 2007-11-16.
{{cite web}}
: External link in
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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'
- Kent Cyclone - Listen to 'Cyclone What?' here
- The EM-DAT International Disaster Database by the Centre for Research on the Epidemiology of Disasters