MET4300/5355

Lecture2PropertiesoftheAtmosphere

(textbookCH1)

AtmosphericProperties

• Temperature(oCoroF)• Pressure(mborhPa)• Moisture

– Variables(%orhPaordimensionless)– Clouds(Typeanddimensions)– Hydrometeors(Rain,drizzle,hail,graupel…)

• Wind(ms-1)– Relatedtopressureindifferentwaysfordifferentscales

Temperature• Temperatureisameasureoftheaveragespeedofmoleculesmoveinasubstance.

• Solid,Liquid,Gas• Units:Celsius(oC),Fahrenheit(oF),Kelvin(K)• Absolutezero:nomoreenergycanbeextracted(-273.15 oC)

• IntheUS,Fahrenheitisusedforsurfacetemperatures,Celsiusforupperatmospherictemperatures,andKelvinforscientificapplications.

TemperatureScales

• Troposphere:0-12kmabovesurface,Tdecreases

• Stratosphere:20-50km,Tincreases(Ozoneeffect)

• Mesosphere:50-85km,Tdecreases

• Thermosphere:>85km,Tincreasesagain

LayersofAtmosphere

Tropopause• Definition:Theboundarybetweentroposphereandstratosphere.ItcanbethoughtofasalidonEarth’sweather.

• Tropopauseslopesdownfromthetropicstothepoles:– Tropicalregions:16-18km– Mid-latitude:11-13km– Polarlatitudes:8km

TropopauseHeightasaFunctionofLatitude

Tropopausefolds• Jetstreams:riversoffast-movingairintheuppertropopause(criticaltoweatherdevelopment)– Subtropicaljetstream:circletheglobeatlatitudesofabout25°.

– Polarjetstream:circletheglobeatlatitudesofabout50°.

• Tropopausefolds:justnorthofeachofthesejetstreams,airfromstratosphereoftendescendsinanarrowzone,leadingtothefolds:– Itisonewaytomixstratophereairwithtroposphereair.

WorldwideTemperaturesinJanuary

WorldwideTemperaturesinJuly

HowtheEarth’sOrbitAffectsTemperature

HighandLowLatitudeTemperatureVariations

Pressure

• Pressureistheforceappliedbyalloftheairmoleculesthatstrikeoveraunitarea

• Pressureisequivalenttotheweightofacolumnofairaboveaunitarea(lb/in^2).

• Standardunit:mborhPa;Pascal=newton/m^2

• Otherunit:inchesofmercury

IntheLargeScaleAtmosphere,PressureistheWeightofAirAbove

PresssureasaFunctionofHeight

HowaMercuryBarometerWorks• AirpressesonHginthe

dish• WeightofHgincolumnis

thesameastheweightoftheair

• Vacuumatthetopofthetube

• SinceTorricelli(1644)observershaveidentifiedlowpressurewithbadweather

MeanSeaLevelPressure(MSLP)

• Meteorologistshavetoconvertstationpressuretoacommonaltitude,whichischosenasmeansealevel.

• Ifnoconversionlikethisismade,amapofstationpressurewilllookmuchlikeamapofthetopography

• Oncetheconversionismade,wecanseehowpressurevariesoveraregion.

InClassActivity

• ReadChapter1&2

ForNextTime

• Ex.1.2-1.3

Moisturevariables(1)• Vaporpressure:theforceappliedbyonlythewatervapormoleculesstrikingaunitarea.Itisameasureoftheabsoluteamountofmoistureintheair(mb).

• Theatmospherecannotholdunlimitedamountofmoisture.Ithasacapacityformoisture.

• Saturation:Whentheatmospherecannotcontainanymorewatervaporwithoutcondensingintoclouddroplets,wesaytheatmosphereissaturated.

• Saturationvaporpressure:Thevaporpressureatwhichtheatmospherebecomessaturated.Itisameasureoftheatmosphere’scapacityforwatervapor,whichdependsontemperatureonly(WHY??)!

SaturationVaporPressurevs.Temperature

• The atmosphere has little capacity to hold water vapor when the temperatures are very cold.

• You can use the same figure for Vapor pressure vs. Dew point temperature

Moisturevariables(2)• Relativehumidity=(vaporpressure/saturationvaporpressure)*100%,whichisameasureofthemoisturecontentoftheatmosphererelativetoitscapacityforwatervapor.Humansaresensitivetorelativehumidity,nottheabsoluteamountofmoisture.

• Dewpoint temperature(Td)isthelowesttemperaturetowhichaircanbecooledatconstantpressurebeforesaturationoccurs.Itisameasureofabsolutemoisturecontent,justlikevaporpressure.Tdiseasytomeasure.

• WecanqualitativelyestimatetherelativehumiditybycomparingTdandT(howtoestimatequantitatively???—useFig.1.9,thefigureinpreviousslide).

DailyVariationofRelativeHumiditywithWithTemperaturewhenthemoisturecontent(DewPoint)doesnotvary

Bothvaporpressureanddewpoint temperaturearemeasuresofabsoluteamountthemoisturecontent

Clouds

WaterPhaseChangesandCloud

• Solidà Liquidà GasMelting,evaporation,sublimation:absorblatentheatenergyFreezing,condensation,deposition:releaselatentheat

• Latentheat:theenergythatisrequiredforaphasechange.Forexample,acceleratingthemoleculestohighspeedsofvaporandbreakingthestrongbondsinliquid&ice.

• Supercooledliquidwaterandicenuclei(-15~-10degC)

HydrometeorsandPrecipitation• Hydrometeor:Anywater

condensedfromtheatmosphere

• Precipitation:Waterthatfallsfromcloudstotheground

• Liquid:Rainordrizzle(<0.5mm)

• Ice:Hailorgraupel (<5mm)• Icecrystals:Snow• Depthisinmmofliquid

water,i.e.meltsnoworhail

FourCloudTypes

• Cumulus: withverticaldevelopment,towering,withcauliflower-likelobes

• Stratus: layeredandwidespread• Cirrus: high,wispy,fibrous• Nimbus:raining/precipitatingclouds• Thesetypescanbecombinedtonameclouds:Highclouds(>6km):cirro-??;middleclouds(2-6km):alto-??

Wind

• Windissimplythemovementofair• Windisavector:winddirection&windspeed(measuredbyanemometer).

• Themeteorologicalwinddirection,byconvection,isthedirectionfromwhichthewindisblowing.

CodingofWinds(Review)

AnExample

Summary• Temperature

– ControlledbySunandSeason– 6.5oCkm-1 lapserateinTroposphere

• Pressure:– Weightofairabove– Decreasesupward

• Moisture&Humidity– Vaporpressure,dewpoint,saturationvaporpressureand

relativehumidity– Phasechangesofwater– Latentheat

• Wind:Codedonmapsusingwindbarbs