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MET4300/5355
Lecture2PropertiesoftheAtmosphere
(textbookCH1)
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AtmosphericProperties
• Temperature(oCoroF)• Pressure(mborhPa)• Moisture
– Variables(%orhPaordimensionless)– Clouds(Typeanddimensions)– Hydrometeors(Rain,drizzle,hail,graupel…)
• Wind(ms-1)– Relatedtopressureindifferentwaysfordifferentscales
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Temperature• Temperatureisameasureoftheaveragespeedofmoleculesmoveinasubstance.
• Solid,Liquid,Gas• Units:Celsius(oC),Fahrenheit(oF),Kelvin(K)• Absolutezero:nomoreenergycanbeextracted(-273.15 oC)
• IntheUS,Fahrenheitisusedforsurfacetemperatures,Celsiusforupperatmospherictemperatures,andKelvinforscientificapplications.
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TemperatureScales
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• Troposphere:0-12kmabovesurface,Tdecreases
• Stratosphere:20-50km,Tincreases(Ozoneeffect)
• Mesosphere:50-85km,Tdecreases
• Thermosphere:>85km,Tincreasesagain
LayersofAtmosphere
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Tropopause• Definition:Theboundarybetweentroposphereandstratosphere.ItcanbethoughtofasalidonEarth’sweather.
• Tropopauseslopesdownfromthetropicstothepoles:– Tropicalregions:16-18km– Mid-latitude:11-13km– Polarlatitudes:8km
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TropopauseHeightasaFunctionofLatitude
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Tropopausefolds• Jetstreams:riversoffast-movingairintheuppertropopause(criticaltoweatherdevelopment)– Subtropicaljetstream:circletheglobeatlatitudesofabout25°.
– Polarjetstream:circletheglobeatlatitudesofabout50°.
• Tropopausefolds:justnorthofeachofthesejetstreams,airfromstratosphereoftendescendsinanarrowzone,leadingtothefolds:– Itisonewaytomixstratophereairwithtroposphereair.
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WorldwideTemperaturesinJanuary
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WorldwideTemperaturesinJuly
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HowtheEarth’sOrbitAffectsTemperature
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HighandLowLatitudeTemperatureVariations
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Pressure
• Pressureistheforceappliedbyalloftheairmoleculesthatstrikeoveraunitarea
• Pressureisequivalenttotheweightofacolumnofairaboveaunitarea(lb/in^2).
• Standardunit:mborhPa;Pascal=newton/m^2
• Otherunit:inchesofmercury
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IntheLargeScaleAtmosphere,PressureistheWeightofAirAbove
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PresssureasaFunctionofHeight
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HowaMercuryBarometerWorks• AirpressesonHginthe
dish• WeightofHgincolumnis
thesameastheweightoftheair
• Vacuumatthetopofthetube
• SinceTorricelli(1644)observershaveidentifiedlowpressurewithbadweather
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MeanSeaLevelPressure(MSLP)
• Meteorologistshavetoconvertstationpressuretoacommonaltitude,whichischosenasmeansealevel.
• Ifnoconversionlikethisismade,amapofstationpressurewilllookmuchlikeamapofthetopography
• Oncetheconversionismade,wecanseehowpressurevariesoveraregion.
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InClassActivity
• ReadChapter1&2
ForNextTime
• Ex.1.2-1.3
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Moisturevariables(1)• Vaporpressure:theforceappliedbyonlythewatervapormoleculesstrikingaunitarea.Itisameasureoftheabsoluteamountofmoistureintheair(mb).
• Theatmospherecannotholdunlimitedamountofmoisture.Ithasacapacityformoisture.
• Saturation:Whentheatmospherecannotcontainanymorewatervaporwithoutcondensingintoclouddroplets,wesaytheatmosphereissaturated.
• Saturationvaporpressure:Thevaporpressureatwhichtheatmospherebecomessaturated.Itisameasureoftheatmosphere’scapacityforwatervapor,whichdependsontemperatureonly(WHY??)!
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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
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Moisturevariables(2)• Relativehumidity=(vaporpressure/saturationvaporpressure)*100%,whichisameasureofthemoisturecontentoftheatmosphererelativetoitscapacityforwatervapor.Humansaresensitivetorelativehumidity,nottheabsoluteamountofmoisture.
• Dewpoint temperature(Td)isthelowesttemperaturetowhichaircanbecooledatconstantpressurebeforesaturationoccurs.Itisameasureofabsolutemoisturecontent,justlikevaporpressure.Tdiseasytomeasure.
• WecanqualitativelyestimatetherelativehumiditybycomparingTdandT(howtoestimatequantitatively???—useFig.1.9,thefigureinpreviousslide).
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DailyVariationofRelativeHumiditywithWithTemperaturewhenthemoisturecontent(DewPoint)doesnotvary
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Bothvaporpressureanddewpoint temperaturearemeasuresofabsoluteamountthemoisturecontent
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Clouds
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WaterPhaseChangesandCloud
• Solidà Liquidà GasMelting,evaporation,sublimation:absorblatentheatenergyFreezing,condensation,deposition:releaselatentheat
• Latentheat:theenergythatisrequiredforaphasechange.Forexample,acceleratingthemoleculestohighspeedsofvaporandbreakingthestrongbondsinliquid&ice.
• Supercooledliquidwaterandicenuclei(-15~-10degC)
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HydrometeorsandPrecipitation• Hydrometeor:Anywater
condensedfromtheatmosphere
• Precipitation:Waterthatfallsfromcloudstotheground
• Liquid:Rainordrizzle(<0.5mm)
• Ice:Hailorgraupel (<5mm)• Icecrystals:Snow• Depthisinmmofliquid
water,i.e.meltsnoworhail
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FourCloudTypes
• Cumulus: withverticaldevelopment,towering,withcauliflower-likelobes
• Stratus: layeredandwidespread• Cirrus: high,wispy,fibrous• Nimbus:raining/precipitatingclouds• Thesetypescanbecombinedtonameclouds:Highclouds(>6km):cirro-??;middleclouds(2-6km):alto-??
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Wind
• Windissimplythemovementofair• Windisavector:winddirection&windspeed(measuredbyanemometer).
• Themeteorologicalwinddirection,byconvection,isthedirectionfromwhichthewindisblowing.
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CodingofWinds(Review)
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AnExample
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Summary• Temperature
– ControlledbySunandSeason– 6.5oCkm-1 lapserateinTroposphere
• Pressure:– Weightofairabove– Decreasesupward
• Moisture&Humidity– Vaporpressure,dewpoint,saturationvaporpressureand
relativehumidity– Phasechangesofwater– Latentheat
• Wind:Codedonmapsusingwindbarbs