关于天气的英语作文
Theweatherisasetofallthephenomenainagivenatmosphereatagiventime.Italsoincludesinteractionswiththehydrosphere.Thetermusuallyreferstotheactivityofthesephenomenaovershortperiods(hoursordays),asopposedtothetermclimate,whichreferstotheaverageatmosphericconditionsoverlongerperiodsoftime.Whenusedwithoutqualification,"weather"isunderstoodtobetheweatherofEarth.
Weathermostoftenresultsfromtemperaturedifferencesfromoneplacetoanother.Onlargescales,temperaturedifferencesoccurbecauseareasclosertotheequatorreceivemoreenergyperunitareafromtheSunthandoregionsclosertothepoles.Onlocalscales,temperaturedifferencescanoccurbecausedifferentsurfaces(suchasoceans,forests,icesheets,orman-madeobjects)havedifferingphysicalcharacteristicssuchasreflectivity,roughness,ormoisturecontent.
Surfacetemperaturedifferencesinturncausepressuredifferences.Ahotsurfaceheatstheairaboveitandtheairexpands,loweringtheairpressure.Theresultinghorizontalpressuregradientacceleratestheairfromhightolowpressure,creatingwind,andEarth'srotationthencausescurvatureoftheflowviatheCorioliseffect.Thesimplesystemsthusformedcanthendisplayemergentbehaviourtoproducemorecomplexsystemsandthusotherweatherphenomena.LargescaleexamplesincludetheHadleycellwhileasmallerscaleexamplewouldbecoastalbreezes.
Thestrongtemperaturecontrastbetweenpolarandtropicalairgivesrisetothejetstream.Mostweathersystemsinthemid-latitudesarecausedbyinstabilitiesofthejetstreamflow(seebaroclinity).Weathersystemsinthetropicsarecausedbydifferentprocesses,suchasmonsoonsororganizedthunderstormsystems.
BecausetheEarth'saxisistiltedrelativetoitsorbitalplane,sunlightisincidentatdifferentanglesatdifferenttimesoftheyear.InJunetheNorthernHemisphereistiltedtowardsthesun,soatanygivenNorthernHemispherelatitudesunlightfallsmoredirectlyonthatspotthaninDecember(seeEffectofsunangleonclimate).Thiseffectcausesseasons.Overthousandstohundredsofthousandsofyears,changesinEarth'sorbitalparametersaffecttheamountanddistributionofsolarenergyreceivedbytheEarthandinfluencelong-termclimate(seeMilankovitchcycles).
OnEarth,commonweatherphenomenaincludesuchthingsaswind,cloud,rain,snow,fogandduststorms.Lesscommoneventsincludenaturaldisasterssuchastornadoes,hurricanesandicestorms.Almostallfamiliarweatherphenomenaoccurinthetroposphere(thelowerpartoftheatmosphere).Weatherdoesoccurinthestratosphereandcanaffectweatherlowerdowninthetroposphere,buttheexactmechanismsarepoorlyunderstood.[1]
Theatmosphereisachaoticsystem,sosmallchangestoonepartofthesystemcangrowtohavelargeeffectsonthesystemasawhole.Thismakesitdifficulttoaccuratelypredictweathermorethanafewdaysinadvance,thoughweatherforecastersarecontinuallyworkingtoextendthislimitthroughthescientificstudyofweather,meteorology.Itistheoreticallyimpossibletomakeusefulday-to-daypredictionsmorethanabouttwoweeksahead,imposinganupperlimittopotentialforimprovedpredictionskill.[1]Chaostheorysaysthattheslightestvariationinthemotionofthegroundcangrowwithtime.Thisideaissometimescalledthebutterflyeffect,fromtheideathatthemotionscausedbytheflappingwingsofabutterflyeventuallycouldproducemarkedchangesinthestateoftheatmosphere.Becauseofthissensitivitytosmallchangesitwillneverbepossibletomakeperfectforecasts,althoughtherestillismuchpotentialforimprovement.
Thesunandoceanscanalsoaffecttheweatherofland.Ifthesunheatsupoceanwatersforaperiodoftime,watercanevaporate.Onceevaporatedintotheair,themoisturecanspreadthroughoutnearbyland,thusmakingitcooler.
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