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Theatmosphericpressureistheratiobetweentheforceoftheatmospheric
aircolumnacting(pressing)onasurfaceunitofEarth(oranygivenplanet),
andthesurfaceonwhichitpresses.Thevalueoftheatmosphericpressure
isdeterminedbyaltitude,latitudeandtemperature.Theincreaseinaltitude
resultsinthedecreaseintheatmosphericpressure.
Atmosphericdensityistheratiobetweenairmassandvolume,or
alternatively,themassof1m3ofair.Theatmosphericairdensityisnotaconstant
value:itdropswithincreasingaltitudeduetochangesintemperatureand
pressure.
Oneoftherudimentarymeteorologicalparameters,airtemperature
describesthethermalstateoftheambientair(theatmosphere).Temperature
isavariablephysicalquantitywhich,withrespecttotheEarth)satmosphere,
dependsonsuchparametersas:latitude(determiningtherelativepositionofthe
Sun),height,thesurfacestructure(waterorland,owingtothedifferentenergy
transferratesinthermodynamicsystems),thespatialdistributionoflarge
bodiesofwater,seacurrents,landforms,thelarge-scalesystemofaircurrents
abovetheEarth)ssurface(atmosphericcirculation),cloudcoverandvegetation
(dispersionofelectromagneticsolarwaves).Thechangesintemperature
dependingonagivenfactorarereferredtoasthetemperaturepattern,and
thedifferencebetweenthetemperatureextremesoveragivenperiodisthe
amplitude,whichistypicallyconsideredindailyorannualperspective,and
thereforediurnalandannualtemperatureamplitudesaredetermined.
Humidityisanotherparameterrefectingthestateoftheatmosphere.
Itdescribestheconcentrationofwatervapourintheair.Threemainformal
typesofhumidityaredistinguished:
-absolute-describingthemassofwatervapour,expressedingrams,in1m3;
-relative-thequotientofthepartialpressure3ofwatervapourinthe
airtothesaturationvapourpressure4aboveafatliquidsurfaceatthesame
temperature;
-specific-i.e.theratiobetweenthemassofwatervapouringramsto1kg
ofairmassincludingthemassofwatervapour.
Itshouldbenotedthatairhumidityislargelydependentontemperature:
thegrowthintemperatureleadstotheincreaseintheamountofwatervapour
intheatmosphere.Bothhumidityandtemperatureconvergeintheconcept
ofthedewpoint,i.e.isthetemperatureatwhichthewatervapourcontained
intheaircondenses,thusformingliquidwater.Thedewpointtemperatureis
3
Partialpressure-thepressurethatagivencomponentofgaswouldexertonthewalls
ofatankifitweretooccupyitsentirevolumeatagiventemperature.
4
Vapourpressure-vapourpressureovertheliquiddeterminedatagiventemperature.
18
