lapse rates & stability

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Lapse Rates & Stability. Learning outcomes: - What are condensation nuclei? - What is the difference between DALR & SALR? - What are the different states of air stability?. Condensation nuclei. Large numbers of these in atmosphere Hygroscopic nuclei (attract water) Volcanic dust - PowerPoint PPT Presentation


  • Lapse Rates & StabilityLearning outcomes:- What are condensation nuclei?- What is the difference between DALR & SALR?- What are the different states of air stability?

  • Condensation nuclei Large numbers of these in atmosphereHygroscopic nuclei (attract water)Volcanic dustDust from windblown soilSmoke urb & indSulphuric acid urb & indSalt from sea sprayOver cities 1 mill nuclei per cubic cmOver oceans 10 nuclei per cubic cm* If large quantities of nuclei, condensation occurs at low RH e.g. 75% RH = smog in Los Angeles

  • If air is pure:

    No particles or condensation nucleiAir can be cooled below its dew pointAir can have more than 100% RHLab tests have cooled air to -40 before condensation / sublimation

    Pure air is very rare

  • Environmental Lapse Rate (ELR)ELR varies with local air conditions:HeightTime of yearType of surface that the air is over (land or sea)

    Decrease intemperature withheight6.5 oC / 1000m

  • Adiabatic Lapse Rate (ALR)

    Decrease in temp

    Increase in volume

    Decrease in pressure

    Parcel of air risesSinking air

    Increasing pressure

    Decreasing volume

    Air warms up

  • There are 2 ALRs1. Dry Adiabatic Lapse Rateenergy (heat) used up by expansion causes fall in tempno condensation, air not saturated

    9.8oC / 1000mAver. 1oC / 100m

  • Adiabatic Lapse RatesSaturated Adiabatic Lapse Rate (SALR)Happens when enough uplift to reach Dew Point Temp Cooling causes saturationCondensation occursLatent heat is releasesThis partly compensates for loss of temp with heightSALR varies with air temp 4-9oC / 1000mAverage 5.4oC / 1000m, about 0.5oC / 100m

  • Freezing Adiabatic Lapse RateHappens when temp below zeroSame rate as DALRVery little moisture at low temps, as no evaporation


    Ground level temp 20oC1000m up parcel of aircools to 10.2oCSurrounding air Cools at ELR6.5oC / 1000mSurrounding air 14oC at 1000mWill the parcel of air continue to rise above 1000m? Why?What will happen to the parcel?

  • StabilityParcel will sink because..

    No clouds formed because..

    Creates stable conditions because.

    Weather conditions associated with.

  • InstabilityCommon on hot daysSurrounding air heated by conductionELR increasesParcel of rising air remains warmer and lighterRises above DPTLatent heat released so it continues to riseCumulonimbus form, thunderstorms likely

  • Conditional Instability- most common in Britain- instability is conditional upon the air being forced to rise in the first place, until condensation triggers the SALR and further unforced uplift- gives cloudy and showery weatherOver mountain or cold air mass

  • SummaryLearning outcomes:- What are condensation nuclei?- What is the difference between DALR & SALR?- What are the different states of air stability?


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