Cold front - Wikipedia

A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of ... Coldfront FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Leadingedgeofacoolermassofair Forotheruses,seeColdFront. NottobeconfusedwithColdwave. Acoldfrontistheleadingedgeofacoolermassofairatgroundlevelthatreplacesawarmermassofairandlieswithinapronouncedsurfacetroughoflowpressure.Itoftenformsbehindanextratropicalcyclone(tothewestintheNorthernHemisphere,totheeastintheSouthern),attheleadingedgeofitscoldairadvectionpattern—knownasthecyclone'sdry"conveyorbelt"flow.Temperaturedifferencesacrosstheboundarycanexceed30 °C(54 °F)fromonesidetotheother.Whenenoughmoistureispresent,raincanoccuralongtheboundary.Ifthereissignificantinstabilityalongtheboundary,anarrowlineofthunderstormscanformalongthefrontalzone.Ifinstabilityisweak,abroadshieldofraincanmoveinbehindthefront,andevaporativecoolingoftheraincanincreasethetemperaturedifferenceacrossthefront.Coldfrontsarestrongerinthefallandspringtransitionseasonsandareweakestduringthesummer.Thesymbolofacoldfront:abluelinewithtrianglespointinginthedirectionoftravel AcoldfrontovertheeasternandcentralregionoftheUnitedStatesofAmerica Contents 1Developmentofcoldfronts 2Clouds 3Precipitation 3.1Frontogeneticalcirculation 4Temperaturechanges 5Characteristicsofboundariesaroundanextratropicalcyclone 6Seealso 7References 8Externallinks Developmentofcoldfronts[edit] AnincomingcoldfrontinnorthernOhio(2016) Acoldfrontoccurswhenamassofcomparativelycolderairmovesintowherewarmerairispresent.Thedrier,colderairformsasteeplyslopingboundaryunderthewarmer,moisterairatthesurfaceandliftsthatair.Thisoftencausescloudformationswithastrongverticaldevelopment,whichmaymanifestasalineofshowersandthunderstormswhenenoughmoistureispresent.[1]Onweathermaps,thesurfacepositionofthecoldfrontismarkedwiththesymbolofabluelineoftriangles/spikes(pips)pointinginthedirectionoftravel.Acoldfront'slocationisattheleadingedgeofthetemperaturedropoff,whichinanisothermanalysiswouldshowupastheleadingedgeoftheisothermgradient,anditnormallylieswithinasharpsurfacetrough.[2]Coldfrontsmovefasterthanwarmfrontsandcanproducesharperchangesinweather.Sincecoldairisdenserthanwarmair,itrapidlyreplacesthewarmairprecedingtheboundary.[3] Inthenorthernhemisphere,acoldfrontusuallycausesashiftofwindfromsouthwesttonorthwestclockwise,alsoknownasveering,andinthesouthernhemisphereashiftfromnorthwesttosouthwest(counterclockwise,backing).Atmosphericpressuresteadilydecreaseswiththeapproachofacoldfront.Withfrontalpassage,thepressurerisessharplyandthenstabilizes.Normally,coldfrontscanbemarkedbythesecharacteristics:[4][1] Weatherphenomenon Priortothepassingofthefront Whilethefrontispassing Afterthepassingofthefront Temperature Warm Coolingsuddenly Steadilycooling Atmosphericpressure Decreasingsteadily Lowest,thensuddenincrease Increasingsteadily Winds Southwesttosoutheast(northernhemisphere) Northwesttonortheast(southernhemisphere) Gusty;shifting Northtowest,usuallynorthwest(northernhemisphere) Southtowest,usuallysouthwest(southernhemisphere) Precipitation/conditions* Lightpatchyraincanbeproducedbystratocumulusorstratusinthewarmsector.Insummer,sometimesthunderstormsifaprecedingsqualllineispresent.Inwintersnowsquallsorshowersmayoccur.[5] Prolongedrain(nimbostratus)orthunderstorms(cumulonimbus):dependsonconditions. Showers,thenclearing Clouds* Oftenprecededbycirrus,cirrostratusthenaltostratuslikeawarmfront(butusuallywithsmalleramountsoftheseclouds).Areasofcirrocumulusandaltocumuluswithincirrostratusandaltostratusmorecommonlyseenthanatawarmfront.Largercumuluscloudsunderthehighercloudtypesthanatawarmfront,wherestratocumulusandcumulushumilisusuallyoccur.Someofthesecumuluscloudsmayproduceshowersaheadofthefront. Cumulonimbusandcumuluscongestusproducingfrequentshowers,withasheetofupperaltostratus,throughwhichthesuncansometimesbeseen.Lesscommonlynimbostratusoccurswithcontinuousrain. Patchyaltocumulusorstratocumulusandhighercirruscloudsalongwithfastmovingstratusfractustheneventuallyscatteredcumulusandsometimescumulonimbus. Visibility* Fairtopoorinhaze Poor,butimproving Good,exceptinshowers DewPoint High,steady Suddendrop Falling *providedthereissufficientmoisture. Stratocumuluscloudsafteracoldfront Clouds[edit] Altocumuluscloudsoftenareasignofanenteringcoldfront. Ifthecoldfrontishighlyunstable,cumulonimbuscloudsproducingthunderstormscommonlyformalongthefront.Anvilcirruscloudsmayspreadaconsiderabledistancedownwindfromthethunderstorms.[6]Theothercloudtypesassociatedwithacoldfrontdependonatmosphericconditionssuchasairmassstabilityandwindshear.[7]Asthefrontapproaches,middle-étagegiveswaytoaltostratusandlow-levelstratocumuluswithintermittentlightprecipitationifthewarmairmassbeingdisplacedbythecoldfrontismostlystable.Withsignificantairmassinstability,verticallydevelopedcumulusorcumulonimbuswithshowersandthunderstormswillformalongthefront. Afterthepassageofthecoldfront,theskyusuallyclearsashighpressurebuildsinbehindthesystem,althoughsignificantamountsofcumulusorstratocumulus,oftenintheformoflongbandscalledcloudstreetsmaypersistiftheairmassbehindthefrontremainshumid.[8]Smallandunchangingamountsofcumulusorcirruscloudsinanotherwiseclearskyareusuallyindicationsofcontinuingfairweatheraslongasthebarometricpressureremainscomparativelyhigh. Altocumulusundulatuscloudsseveralhoursafteracoldfront Precipitation[edit] AcoldfrontasitappearedontheNationalWeatherServiceWichita,KansasWSR-88DonApril3,2011.Thethinbluelinelabeled"coldfront"isthefront,withseverethunderstormsseendevelopingbehindthefront,whichismovingtowardsthebottomright. StrongthunderstormassociatedwithacoldfrontinNorthernMexico Acoldfrontcommonlybringsanarrowbandofprecipitationthatfollowsalongtheleadingedgeofthecoldfront.Thesebandsofprecipitationareoftenverystrong,[9]andcanbringseverethunderstorms,hailstorms,[10]snowsqualls,[5]and/ortornadoes.Inthespring,thesecoldfrontscanbeverystrong,andcanbringstrongwindswhenthepressuregradientishigherthannormal.Duringthewintermonths,coldfrontssometimescomethroughanareawithlittleornoprecipitation.Widerrainbandscanoccurbehindcoldfrontswhichtendtohavemorestratiform,andlessconvective,precipitation.[11]Theserainstormssometimesbringflooding,andcanmoveveryslowlywhenthestormsteeringitisstrongandembeddedwithinameridionalflowpattern(withmorepoletoequatormotionratherthanwesttoeastmotion).Inthewinter,coldfrontscanbringcoldspells,andoccasionallysnow.Inthespringorsummerintemperatelatitudes,hailmayoccasionallyfallalongwiththerain.Ifmoistureisnotsufficient,suchaswhenasystemhaspreviouslymovedacrossamountainbarrier,coldfrontscanpasswithoutcloudiness. Frontogeneticalcirculation[edit] Frontogenesisistheprocessofcreatingorsteepeningthetemperaturegradientofafront.Duringthisprocesstheatmospherereactsinanattempttorestorebalance,theconsequenceisacircularmotionalongthefrontwhereairisbeingliftedup,alongthecoldfrontanddroppingdownward,behindthefrontalboundary.Thisistheactualforceofupwardmotionalongafrontthatisresponsibleforcloudsandprecipitation. Asthetemperaturegradientsteepensduringfrontogenesis,thethermalwindbecomesimbalanced.Tomaintainbalance,thegeostrophicwindaloftandbelowadjust,suchthatregionsofdivergence/convergenceform.Masscontinuitywouldrequireaverticaltransportofairalongthecoldfrontwherethereisdivergence(loweredpressure).Althoughthiscirculationisdescribedbyaseriesofprocesses,theyareactuallyoccurringatthesametime,observablealongthefrontasathermallydirectcirculation.Thereareseveralfactorsthatinfluencethefinalshapeandtiltofthecirculationaroundthefront,ultimatelydeterminingthekindandlocationofcloudsandprecipitation.[7][12] Temperaturechanges[edit] Coldfrontsaretheleadingedgeofcoolerairmasses,hencethename"coldfront".Theyhavestrongertemperaturechangesduringthefall(autumn)andspringandduringthemiddleofwinter.Temperaturechangesassociatedwithcoldfrontscanbeasmuchas30 °C(54 °F).Whencoldfrontscomethrough,thereisusuallyaquick,yetstronggustofwind,thatshowsthatthecoldfrontispassing.Insurfaceweatherobservations,aremarkknownasFROPAiscodedwhenthisoccurs.[13]Theeffectsfromacoldfrontcanlastfromhourstodays.Theairbehindthefrontiscoolerthantheairitisreplacingandthewarmairisforcedtorise,soitcools.Asthecooleraircannotholdasmuchmoistureaswarmair,cloudsformandrainoccurs.[4] Characteristicsofboundariesaroundanextratropicalcyclone[edit] Occludedcycloneexample.Thetriplepointistheintersectionofthecold,warm,andoccludedfronts. Coldfrontsformwhenacoolerairmassmovesintoanareaofwarmerairinthewakeofadevelopingextratropicalcyclone.Thewarmerairinteractswiththecoolerairmassalongtheboundary,andusuallyproducesprecipitation.Coldfrontsoftenfollowawarmfrontorsquallline.Verycommonly,coldfrontshaveawarmfrontaheadbutwithaperpendicularorientation.Inareaswherecoldfrontscatchuptothewarmfront,theoccludedfrontdevelops.Occludedfrontshaveanareaofwarmairaloft.Whensuchafeatureformspolewardofanextratropicalcyclone,itisknownasatrowal,whichisshortforTRoughOfWarmAiraLoft.[14]Acoldfrontisconsideredawarmfrontifitbeginstoretreataheadofthenextextratropicalcyclonealongthefrontalboundary,andcalledastationaryfrontifitstalls. Seealso[edit] Airmass BlueNorther(weather) Coldwave Surfaceweatheranalysis Weatherfront References[edit] ^abWhiteman,C.David(2000).Mountainmeteorology :fundamentalsandapplications.NewYork:OxfordUniversityPress.pp. 77–79.ISBN 978-0-19-803044-7.OCLC 428735924. ^DavidRoth(2006-12-14)."UnifiedSurfaceAnalysisManual"(PDF).HydrometeorologicalPredictionCenter.Retrieved2012-01-09. ^PaulM.Markowski;YvetteP.Richardson(2011-09-20).MesoscaleMeteorologyinMidlatitudes.JohnWileyandSons.p. 120.ISBN 978-1-119-96667-8.Retrieved2012-01-09. ^ab"ColdFront:transitionzonefromwarmairtocoldair".ww2010.atmos.uiuc.edu.Retrieved2019-11-09. ^abDonald,Ahrens,C.(2007).Meteorologytoday :anintroductiontoweather,climate,andtheenvironment(8th ed.).Belmont,Calif.:Thomson/Brooks/Cole.pp. 298–300.ISBN 978-0495011620.OCLC 66911677. ^LeeM.Grenci;JonM.Nese(2001).AWorldofWeather:FundamentalsofMeteorology:AText/LaboratoryManual(3rd ed.).Kendall/HuntPublishingCompany.pp. 207–212.ISBN 978-0-7872-7716-1.OCLC 51160155. ^abHolton,JamesR.(2004).AnIntroductiontoDynamicMeteorology.AcademicPress.p. 277.ISBN 978-0-12-354015-7. ^Weston,K.J.(1980)."Anobservationalstudyofconvectivecloudstreets".Tellus.32(5):433–438.Bibcode:1980TellA..32..433W.doi:10.1111/j.2153-3490.1980.tb00970.x. ^GlossaryofMeteorology(2009).Prefrontalsquallline.Archived2007-08-17attheWaybackMachineRetrievedon2008-12-24. ^Schemm,S.;L.Nisi,A.Martinov;D.Leuenberg&O.Martius(2016)."OnthelinkbetweencoldfrontsandhailinSwitzerland".AtmosphericScienceLetters.17(5):315–325.Bibcode:2016AtScL..17..315S.doi:10.1002/asl.660. ^K.A.BrowningandRobertJ.Gurney(1999).GlobalEnergyandWaterCycles.Retrievedon2008-12-26. ^Carlson,TobyN.(1991).Mid-latitudeWeatherSystems.HarperCollinsAcademic.p. 435.ISBN 978-0-04-551115-0. ^NavCanada(January2005).AviationWeatherServicesGuide.p. 36. ^St.LouisUniversity(2003-08-04)."WhatisaTROWAL?viatheInternetWaybackMachine".Archivedfromtheoriginalon2006-09-16.Retrieved2006-11-02. 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