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Lab 6: Saturation & Atmospheric Stability

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  • Lab 6: Saturation & Atmospheric Stability

  • Review Lab 5 Atm. Saturation

    Relative humidity?Mixing ratio / saturation mixing ratio?Function of temp.. Clausius-Clapeyron curveSling psychrometer what does this give us?Dew point?Looking at RH equation above, when temp is reduced, all else being equal, what happens to the RH of a sample of air? Does RH go up or down?Air is saturated when RH=100%

  • Lab 6Lab 6: Saturation and Atmospheric Stabilityprocesses that influence atmospheric saturation i.e., cause cooling and/or increase in water vapor contentatmospheric processes that change either the temp and/or water vapor content of an air sample In this lab, well focus on atmospheric mixing and adiabatic cooling and some processes that drive these conditions

  • Saturation & Atmospheric StabilityTwo main ways for air to reach saturation:Cooling to its dew point temperature (most common)Increasing water vapor contentRememberCondensation produces:FogDewClouds*ALL require saturated air to form!

  • Atmospheric MixingWhen two air masses of different temps and water vapor content mix When they mix, the new air mass will change in temp and water vapor resulting in new mixing and saturation mixing ratios Changes relative humidity

  • Mixing Ratio = SMR * RH Unsaturated air 0 99.9% RHAssuming the two mixing air masses are the same size and you know the temps and RH find: The new temp of mixed air massThe new mixing ratio of the mixed air massFrom the above, you can find the new RH (due to change in temp and water vapor)

  • Adiabatic CoolingAdiabatic temperature changes:Temperature changes in which heat was neither added nor subtracted (closed system)Average internal energy decreases with expansion changes in average kinetic energyCompressed air = warm airExpanded air = cooler airNOTE: If a parcel moves , it passes through regions of successively lower pressure:Ascending air: EXPANDSDescending air: COMPRESSES

  • Saturation & Atmospheric StabilityDRY adiabatic rate: unsaturatedcools at a constant rate of 10C/1km of ascentwarms at constant rate of 10C/km of descent

    WET adiabatic rate: saturated(has RH 100%)Slower rate of cooling caused by the release of latent heatRates vary between 5C & 9C/1kmAmount of LH released depends on quantity of moisture in the airDew Point rate:2C/1km to the LCLAt the WALR after the LCL

    LCL = altitude at which a parcel reaches saturation & cloud formation begins

  • Saturation & Atmospheric StabilityDALR = 10C/1kmWALR = 5 9C/1kmLCL

    Air decreases by 5C2318131.510.5Air decreases by 2.5C

    Parcel A Temperature (C)Height (km)Parcel B Temperature (C)5.04.54.03.53.02.52.0

    1.00.528surface10

  • DALR: 10C/kmLCLWALR: dependentAir is cooling by 528 5 = 23C23 C18 C13 C10.5 C8 C5.5 C3 C0.5 C 2C 4.5CAir is cooling by 523 5 = 18CAir is cooling by 518 5 = 13CAir is cooling by 2.513 2.5 = 10.5CAir is cooling by 2.510.5 2.5 = 8CAir is cooling by 2.58 2.5 = 5.5CAir is cooling by 2.55.5 2.5 = 3CAir is cooling by 2.53 2.5 = 0.5CAir is cooling by 2.50.5 2.5 = -2CAir is cooling by 2.5-2 2.5 = -4.5C

    Parcel A Temperature (C)Height (km)Parcel B Temperature (C)

    5.04.54.03.53.02.52.01.51.00.528Csurface10C

  • Lifting Condensation Level (LCL):Reached when ascending air cools to its dew point (saturation = 100% RH) clouds formIf it continues to rise:Cools at the wet adiabatic lapse rate (between 5& 9C)Calculated based on:Surface temperature & dew point temperature

  • Part II

  • ReviewWhat is adiabatic cooling?Wet versus dryThinking about atmospheric saturation, how does this influence cloud formation (hint: think about dew point temperature, etc.)What is environmental lapse rate?

  • Atmospheric lifting forces:Surface heating (air expansion, less dense, rise, etc..)Two surface air masses colliding (convergence)Contact of dissimilar air masses along warm & cold fronts (convergence)Topographic barriers (e.g. orographic lift)Upper air divergence

    Rising air doesnt mix substantially with the surrounding atmosphere. Once the initial lifting force stops, the continued rising of an air parcel depends on atmospheric stability (the state of the atmosphere surrounding the parcel).

  • Orographic LiftingAir ascends: adiabatic cooling often generates clouds & lots of precipitationAir descends: warms adiabatically, making condensation & precipitation less likely

  • LCL T(C) Td(C)/8 25C 13C = 12C/8 = 1.5 kmLCLTd (dew point) cools at: 2C/km below the LCLThe WALR above the LCLIncorporating Dew Point

  • Absolute stability:Environmental lapse rate is less than the wet adiabatic rate (surrounding air cools slower with height)Stable air resists vertical movement, and doesnt want to move. If it gets forced above LCL it would remain cooler and return to surfaceNote: air parcel cools faster than ELRLAYERED CLOUDS not much vertical development

  • Absolute instability:Environmental lapse rate is greater than the dry adiabatic rate (surrounding air cools faster w/ height)Unstable air rises because of its buoyancyParcel of air cools slower than ELRVERTICAL CLOUDS potential for thunderstorms

  • Conditional stability:Moist air has an environmental lapse rate between the dry & wet adiabatic rates

    Relative humidity measures how close an air sample is to the saturation point

    Mixing ratio: ratio of water vapor mass to the mass of dry air (g/kg)

    Saturation ratio: maximum water vapor a sample of air will hold at a given temperature.

    Is the temp at which air must be cooled in order to reach saturationRelative humidity measures how close an air sample is to the saturation point

    Mixing ratio: ratio of water vapor mass to the mass of dry air (g/kg)

    Saturation ratio: maximum water vapor a sample of air will hold at a given temperature.

    Is the temp at which air must be cooled in order to reach saturation*Condensation: occurs when water vapor is cooled enough to change to a liquid.Cloud formation: typically occurs during the warmest part of the day.Dew: ground loses heat in the evening, dew may condense on the grass (fog may form in the air near the surface); Surface cooling that occurs after sunset accounts for some condensation.Need to find mixing ratio

    **Dry adiabatic rate: only applies to unsaturated air. Ascending air is expanded, making it cooler; descending air is compressed, making it warmer.Wet adiabatic rate: once the parcel reaches the LCL, the latent heat that was absorbed by the water vapor when it evaporated is liberated. Although the parcel will continue to cool adiabatically, the release of this latent heat slows the rate of cooling. When a parcel of air ascends above the lifting condensation level, the rate of cooling is reduced because the release of latent heat partially offsets the cooling due to expansion. Because the amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 & 4%), the WAR varies from 5C per 1,000 meters for air with a high moisture content to 9C per 1,000 meters for air with a low moisture content*Dry adiabatic rate: only applies to unsaturated air. Ascending air is expanded, making it cooler; descending air is compressed, making it warmer.Wet adiabatic rate: once the parcel reaches the LCL, the latent heat that was absorbed by the water vapor when it evaporated is liberated. Although the parcel will continue to cool adiabatically, the release of this latent heat slows the rate of cooling. When a parcel of air ascends above the lifting condensation level, the rate of cooling is reduced because the release of latent heat partially offsets the cooling due to expansion. Because the amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 & 4%), the WAR varies from 5C per 1,000 meters for air with a high moisture content to 9C per 1,000 meters for air with a low moisture content***Rising air doesnt mix substantially with the surrounding atmosphere. Once the initial lifting force stops, the continued rising of an air parcel depends on atmospheric stability (the state of the atmosphere surrounding the parcel).*Illustration: prevailing winds force warm, moist air over a 3,000-meter-high mountain range. As the unsaturated air ascends the windward side of the range, it cools at the rate of 10C per 1,000 meters (dry adiabatic rate) until it reaches the dew-point temperature of 20C. Because the dew-point temperature is reached at 1,000 meters, we can say that this represents the LCL & the height of the cloud base. From the cloud base to the top of the mountain, water vapor within the rising air is condensing to form more & more cloud droplets. As a result, the windward side of the mountain range experiences abundant precipitation. We will assume that the air that was forced to the top of the mountain is cooler than the surround air & hence begins to flow down the leeward slop of the mountain. As air descends, it is compressed & heated at the dry adiabatic rate. Upon reaching the base of the mountain range, the temperature of the descending air has risen to 40C, or 10C warmer than the temperature at the base of the mountain on the windward side. The higher temperature on the leeward side is the result of the latent heat that was released during condensation as the air ascended the windward slope of the mountain range.*if temperature of air , relative humidity *Even if this stable air were forced above the LCL, it would remain cooler than its environment & would have a tendency to return to the surface.**