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Likeafishintheocean,manisconfinedtoaveryshallowlayerofatmosphere.ThegaseousenvelopeoftheEarthisphysicallyinhomogeneousinboththeverticalandhorizontaldirections,althoughthehorizontalinhomogeneityismuchlessmarkedthantheverticalinhomogeneity.

Variouscriteriahavebeendevisedfordividingtheatmosphereintolayers.Thisdivisioncanbebasedonthenatureoftheverticaltemperatureprofile,onthegaseouscompositionoftheairatdifferentaltitudes,andtheeffectoftheatmosphereonaircraftatdifferentaltitudes,etc.Thedivisionbasedonthevariationoftheairtemperaturewithaltitudeisusedmostcommonlyinthemeteorologicalliterature.

AccordingtoapublicationoftheagrologicalcommissionoftheWorldMeteorologicalOrganization(WMO)in1961,theEarth’satmosphere,isdividedintofivemainlayers:thetroposphere,thestratosphere,themesosphere,thethermosphereandtheexosphere.Theselayersareboundedbyfourthintransitionregions:thetropospause,thestratospause,themesospause,thethermospause.

ThetroposphereisthelowerlayeroftheatmospherebetweentheEarth’ssurfaceandthetropopause.Thetemperaturedropswithincreasingheightinthetroposphere,atameanrateof6.5℃perkilometer(lapserate).Theupperboundaryofthetroposphereliesataheightofapproximately8to12kminthepolarandtropospherecontainsabout75%ofthetotalmassofatmosphericair,whileinthetropicsitcontainsabout90%.Thetropoauseisanintermediatelayerinwhicheitheratemperatureinversionoranisothermaltemperaturedistributionisobserved.

Thestratosphereistheatmosphericlayerabovethetroposphere.Inthestratospherethetemperatureeitherincreaseswithheightorremainsnearlyconstant.Inthelowerpartofthestratosphere(uptoapproximately20kmabovetheEarth’ssurface)thetemperatureispracticallyconstant(about－56℃).Whilefurtherupthetemperatureincreaseswithaltitudeatarateofabout1℃/kmatheightsof20to30kmandabout2.8℃/kmataltitudesfrom32to47km.Underthestandardconditionsthetemperatureatthe47kmlevelisnormally-2.5℃.

ThisincreaseintemperaturewithheightisduetotheabsorptionofUVsolarradiationbyozonemolecules.Itshouldbenotedthatabout99%ofthetotalmassofatmos