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Atmospheric Moisture
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Atmospheric Moisture

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  • 1. Water and Moisture in theWater and Moisture in the AtmosphereAtmosphere
  • 2. OverviewOverview  WaterWater  DistributionDistribution  PropertiesProperties  HumidityHumidity  Relative HumidityRelative Humidity  Specific HumiditySpecific Humidity  Atmospheric StabilityAtmospheric Stability  Normal and Environmental Lapse RatesNormal and Environmental Lapse Rates  Dry and Moist Adiabatic RatesDry and Moist Adiabatic Rates  Stability and Instability in the AtmosphereStability and Instability in the Atmosphere  Clouds and FogClouds and Fog
  • 3. Global Distribution of WaterGlobal Distribution of Water  SourcesSources  OutgassingOutgassing  TranspirationTranspiration  Combustion and related processes (respiration,Combustion and related processes (respiration, decomposition)decomposition)  DistributionDistribution  Ocean and Land HemispheresOcean and Land Hemispheres  EustasyEustasy  PercentagesPercentages  Oceans: 97% (Pacific – 50%, Atlantic – 26%, Indian – 20%,Oceans: 97% (Pacific – 50%, Atlantic – 26%, Indian – 20%, Arctic – 4%)Arctic – 4%)  Freshwater: 3%Freshwater: 3%  Surface water: 2% (1.99% as ice and glaciers)Surface water: 2% (1.99% as ice and glaciers)  Groundwater: 0.6%Groundwater: 0.6%  Soil Moisture: < 0.5%Soil Moisture: < 0.5%
  • 4. Properties of WaterProperties of Water  Polar StructurePolar Structure  Chemical structure gives HChemical structure gives H22O molecules anO molecules an electrostatic polarityelectrostatic polarity  Unequal sharing of electronsUnequal sharing of electrons  Oxygen has a higher electronegativity thanOxygen has a higher electronegativity than HydrogenHydrogen  Covalent bondsCovalent bonds  Asymmetrical (bent) shapeAsymmetrical (bent) shape  Positive charge associated H, Negative with OPositive charge associated H, Negative with O  Hydrogen bondingHydrogen bonding  Cohesive force, surface tensionCohesive force, surface tension  Powerful solventPowerful solvent  Affects soil water availability, nutrientAffects soil water availability, nutrient transporttransport
  • 5. OxygenOxygen HH HH δ+ δ-
  • 6.  Heat PropertiesHeat Properties  Heat exchanges with water accounts for 30% of energyHeat exchanges with water accounts for 30% of energy driving atmospheric circulationdriving atmospheric circulation  PhasesPhases  Solid – Ice (also snow, sleet, hail, frost)Solid – Ice (also snow, sleet, hail, frost)  Maximum density at 4CMaximum density at 4Coo , Density of ice is 0.9 that of water, Density of ice is 0.9 that of water  Hydrogen bonding creates hexagonal crystal structureHydrogen bonding creates hexagonal crystal structure  Liquid – water (rain, dew)Liquid – water (rain, dew)  Hydrogen bonds are fewer, create surface tension, cohesionHydrogen bonds are fewer, create surface tension, cohesion  Gas – water vapor (steam, clouds, humidity)Gas – water vapor (steam, clouds, humidity)  Phase changesPhase changes  Latent Heat – amount of heat released/absorbed in a phaseLatent Heat – amount of heat released/absorbed in a phase changechange  Latent Heat of Freezing/MeltingLatent Heat of Freezing/Melting  Latent Heat of Evaporation/CondensationLatent Heat of Evaporation/Condensation  Freezing-meltingFreezing-melting  Condensation-evaporation/vaporization (precipitation, dew)Condensation-evaporation/vaporization (precipitation, dew)  The primary means of heat exchange with the environmentThe primary means of heat exchange with the environment  Deposition-sublimation (frost)Deposition-sublimation (frost)
  • 7. TempTemp. HeatHeat SpecificSpecific HeatHeat LatentLatent HeatHeat LatentLatent HeatHeat SpecificSpecific HeatHeat SpecificSpecific HeatHeat IceIce Water VaporVapor 80 Cal80 Cal 100 Cal100 Cal 540 Cal540 Cal20 Cal20 Cal
  • 8. HumidityHumidity  Humidity is water content of the airHumidity is water content of the air  Amount of water is directly proportional toAmount of water is directly proportional to temperaturetemperature  Relative Humidity is the ratio of waterRelative Humidity is the ratio of water content in the air to its capacity, expressedcontent in the air to its capacity, expressed as a percentageas a percentage  Capacity varies with temperature over the dailyCapacity varies with temperature over the daily range of temperaturesrange of temperatures  Content varies with the movement of airContent varies with the movement of air masses, annual pattern of pressure andmasses, annual pattern of pressure and oscillationsoscillations
  • 9.  SaturationSaturation  Occurs when relative humidity becomesOccurs when relative humidity becomes 100%100%  Most often occurs when capacity isMost often occurs when capacity is reduced due to a drop in temperaturereduced due to a drop in temperature  Further increases in content/reduction ofFurther increases in content/reduction of capacity results in condensationcapacity results in condensation  Clouds, fog, precipitationClouds, fog, precipitation  Dew-pointDew-point  The temperature at which air becomesThe temperature at which air becomes saturatedsaturated
  • 10.  Vapor PressureVapor Pressure  Water vapor is a gas, so it contributes toWater vapor is a gas, so it contributes to atmospheric pressureatmospheric pressure  Vapor pressure refers to the amount ofVapor pressure refers to the amount of pressure exerted by water vapor alonepressure exerted by water vapor alone  Expressed in mbExpressed in mb  Varies with temperatureVaries with temperature  Saturation occurs when water moleculesSaturation occurs when water molecules entering the air are in equilibrium withentering the air are in equilibrium with those leaving itthose leaving it  Saturation vapor pressureSaturation vapor pressure  The percentage ratio of VP to SVP givesThe percentage ratio of VP to SVP gives relative humidityrelative humidity
  • 11. At 30At 30oo C, if the vapor pressureC, if the vapor pressure is 15mb, then the relativeis 15mb, then the relative humidity is 33.3%humidity is 33.3% 15mb/45mb (SVP)15mb/45mb (SVP) ×× 100100
  • 12.  Specific HumiditySpecific Humidity  Humidity expressed as actual mass ofHumidity expressed as actual mass of water in the air per mass of air (g/kg)water in the air per mass of air (g/kg)  Maximum specific humidityMaximum specific humidity is the maximumis the maximum mass of water vapor that the air can hold atmass of water vapor that the air can hold at any given temperatureany given temperature  Relative humidity is the ratio of SH to MSHRelative humidity is the ratio of SH to MSH expressed as a percentageexpressed as a percentage
  • 13. At 20At 20oo C, if the specificC, if the specific humidity is 5g, then thehumidity is 5g, then the relative humidity is 33.3%relative humidity is 33.3% 5g/15g (MSH)5g/15g (MSH) ×× 100100
  • 14. Atmospheric StabilityAtmospheric Stability  Storms are caused by atmospheric instabilityStorms are caused by atmospheric instability  Instability is rising air that can produce cyclonic rotationInstability is rising air that can produce cyclonic rotation  Stability occurs when air masses are not risingStability occurs when air masses are not rising  The relative stability of the atmosphere is theThe relative stability of the atmosphere is the product of two cooling processesproduct of two cooling processes  Environmental Lapse RateEnvironmental Lapse Rate  The rate at which air cools with altitudeThe rate at which air cools with altitude  As measured at any given time.As measured at any given time.  The flow of heat energy through the atmosphere greatly affectsThe flow of heat energy through the atmosphere greatly affects this ratethis rate  Adiabatic RateAdiabatic Rate  The rate at which air cools as it becomes less denseThe rate at which air cools as it becomes less dense  Based on thermodynamic laws of gases relating temperature toBased on thermodynamic laws of gases relating temperature to density and pressure – it is the rate of cooling simply bydensity and pressure – it is the rate of cooling simply by expanding, without the flow of heat energy into or out if it.expanding, without the flow of heat energy into or out if it.
  • 15.  If the adiabatic rate exceeds the environmentalIf the adiabatic rate exceeds the environmental rate, then air will cool more quickly by rising thanrate, then air will cool more quickly by rising than the surrounding air, and will not risethe surrounding air, and will not rise  If the adiabatic rate is less than the environmentalIf the adiabatic rate is less than the environmental lapse rate, then rising air will have a higherlapse rate, then rising air will have a higher temperature than the surrounding air, and willtemperature than the surrounding air, and will continue to risecontinue to rise  Two adiabatic rates:Two adiabatic rates:  Dry adiabatic rate (DAR) if air is not saturatedDry adiabatic rate (DAR) if air is not saturated  1010oo C per 1000mC per 1000m  Moist adiabatic rate (MAR) if it is saturatedMoist adiabatic rate (MAR) if it is saturated  66oo C per 1000mC per 1000m  If a parcel of air rises far enough, it will cool, and itsIf a parcel of air rises far enough, it will cool, and its capacity for water vapor will decrease, and becomecapacity for water vapor will decrease, and become saturated. If so, the adiabatic rate for the parcelsaturated. If so, the adiabatic rate for the parcel changes from DAR to MARchanges from DAR to MAR
  • 16. Clouds and FogClouds and Fog  An expression of stability in the atmosphereAn expression of stability in the atmosphere  FormationFormation  Air at some point in the atmosphere becomesAir at some point in the atmosphere becomes saturatedsaturated  Water droplets condense around particles of dust,Water droplets condense around particles of dust, soot, pollution or sea-saltsoot, pollution or sea-salt  condensation nucleicondensation nuclei  ClassificationClassification  Based andBased and altitudealtitude andand shapeshape  FormsForms  Stratiform: flat and layeredStratiform: flat and layered  Cumuliform: puffy, globular and vertically developedCumuliform: puffy, globular and vertically developed  Cirroform: Wispy clouds (typicaly high altitude)Cirroform: Wispy clouds (typicaly high altitude)
  • 17.  Cloud TypesCloud Types  Low clouds (up to 2000 m)Low clouds (up to 2000 m)  Stratus: Dull, gray and featurelessStratus: Dull, gray and featureless  Stratocumulus: puffy stratiform cloudsStratocumulus: puffy stratiform clouds  Nimbostratus: Stratus clouds with drizzling precipationNimbostratus: Stratus clouds with drizzling precipation  Middle clouds (2000 – 6000 m)Middle clouds (2000 – 6000 m)  Altostratus: High, thin stratiform clouds. Sun just visibleAltostratus: High, thin stratiform clouds. Sun just visible  Altocumulus: Patches of “cotton balls” arranged in lines, groups,Altocumulus: Patches of “cotton balls” arranged in lines, groups, rippling wavesrippling waves  High clouds (6000 – 13,000 m)High clouds (6000 – 13,000 m)  Cirrus: wispy, feathery, hairlike, streaks or plumesCirrus: wispy, feathery, hairlike, streaks or plumes  Cirrostratus: Milky veil of ice crystals – sun and moon halosCirrostratus: Milky veil of ice crystals – sun and moon halos  Cirrocumulus: Tufts, dappled, in lines, groups or ripplesCirrocumulus: Tufts, dappled, in lines, groups or ripples  Vertically developed clouds (0 – 13,000 m)Vertically developed clouds (0 – 13,000 m)  Cumulus: Flat based puffy tops; sharply outlined; fair weatherCumulus: Flat based puffy tops; sharply outlined; fair weather  Cumulonimbus: Cirrus-topped plume blown by upper levelCumulonimbus: Cirrus-topped plume blown by upper level winds into an anvil-shape; explosive tops; extreme verticalwinds into an anvil-shape; explosive tops; extreme vertical development; thunderstorms, tornadoes.development; thunderstorms, tornadoes.
  • 18.  FogFog  Stratus clouds sitting on the groundStratus clouds sitting on the ground  Advection FogAdvection Fog  Warm air moving over a cool surface, becoming saturatedWarm air moving over a cool surface, becoming saturated  Cool air moving over a warm surface, saturating itCool air moving over a warm surface, saturating it  evaporation fogevaporation fog  Warm moist air being forced upslope, cooling andWarm moist air being forced upslope, cooling and becoming saturatedbecoming saturated  Upslope fogUpslope fog  Warm layer of air moving over trapped lower lying cool,Warm layer of air moving over trapped lower lying cool, saturated air, creating an inversion layersaturated air, creating an inversion layer  Valley FogValley Fog  Radiation FogRadiation Fog  Radiative cooling of a surface saturates the air above itRadiative cooling of a surface saturates the air above it

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