关于天气英语的作文.doc

关于天气英语的作文 Theweatherisasetofallthephenomenainagivenatmosphereatagiventime.Italsoincludesinteractionswiththehydrosphere.Thetermusuallyreferstotheactivityofthesephenomenaovershortperiods(hoursordays),asopposedtothetermclimate,whichreferstotheaverageatmosphericconditionsoverlongerperiodsoftime.Whenusedwithoutqualification,weatherisunderstoodtobetheweatherofEarth. Weathermostoftenresultsfromtemperaturedifferencesfromoneplacetoanother.Onlargescales,temperaturedifferencesoccurbecauseareasclosertotheequatorreceivemoreenergyperunitareafromtheSunthandoregionsclosertothepoles.Onlocalscales,temperaturedifferencescanoccurbecausedifferentsurfaces(suchasoceans,forests,icesheets,ormanmadeobjects)havedifferingphysicalcharacteristicssuchasreflectivity,roughness,ormoisturecontent. Surfacetemperaturedifferencesinturncausepressuredifferences.Ahotsurfaceheatstheairaboveitandtheairexpands,loweringtheairpressure.Theresultinghorizontalpressuregradientacceleratestheairfromhightolowpressure,creatingwind,andEarthsrotationthencausescurvatureoftheflowviatheCorioliseffect.Thesimplesystemsthusformedcanthendisplayemergentbehaviourtoproducemoreplexsystemsandthusotherweatherphenomena.LargescaleexamplesincludetheHadleycellwhileasmallerscaleexamplewouldbecoastalbreezes. Thestrongtemperaturecontrastbetweenpolarandtropicalairgivesrisetothejetstream.Mostweathersystemsinthemidlatitudesarecausedbyinstabilitiesofthejetstreamflow(seebaroclinity).Weathersystemsinthetropicsarecausedbydifferentprocesses,suchasmonsoonsororganizedthunderstormsystems. BecausetheEarthsaxisistiltedrelativetoitsorbitalplane,sunlightisincidentatdifferentanglesatdifferenttimesoftheyear.InJunetheNorthernHemisphereistiltedtowardsthesun,soatanygivenNorthernHemispherelatitudesunlightfallsmoredirectlyonthatspotthaninDecember(seeEffectofsunangleonclimate).Thiseffectcausesseasons.Overthousandstohundredsofthousandsofyears,changesinEarthsorbitalparametersaffecttheamountanddistributionofsolarenergyreceivedbytheEarthandinfluencelongtermclimate(seeMilankovitchcycles). OnEarth,monweatherphenomenaincludesuchthingsaswind,cloud,rain,snow,fogandduststorms.Lessmoneventsincludenaturaldisasterssuchastornadoes,hurricanesandicestorms.Almostallfamiliarweatherphenomenaoccurinthetroposphere(thelowerpartoftheatmosphere).Weatherdoesoccurinthestratosphereandcanaffectweatherlowerdowninthetroposphere,buttheexactmechanismsarepoorlyunderstood.1 Theatmosphereisachaoticsystem,sosmallchangestoonepartofthesystemcangrowtohavelargeeffectsonthesystemasawhole.Thismakesitdifficulttoaccuratelypredictweathermorethanafewdaysinadvance,thoughweatherforecastersarecontinuallyworkingtoextendthislimitthroughthescientificstudyofweather,meteorology.Itistheoreticallyimpossibletomakeusefuldaytodaypredictionsmorethanabouttwoweeksahead,imposinganupperlimittopotentialforimprovedpredictionskill.1Chaostheorysaysthattheslightestvariationinthemotionofthegroundcangrowwithtime.Thisideaissometimescalledthebutterflyeffect,fromtheideathatthemotionscausedbytheflappingwingsofabutterflyeventuallycouldproducemarkedchangesinthestateoftheatmosphere.Becauseofthissensitivitytosmallchangesitwillneverbepossibletomakeperfectforecasts,althoughtherestillismuchpotentialforimprovement. Thesunandoceanscanalsoa