ReviewClouds form in RISING AIRAs air rises, it reaches lower pressures and expandsAs air expands, it coolsTemperature drops but moisture content remains the same, so RH increasesParcel eventually saturates, condensation occurs, and a cloud formsSinking air à CLEAR SKIESIn general, the faster air rises, the more interesting the weather will be!Rising AirHow fast can air rise?Forced ascent:Non-precipitqting layered cloud or drizzle (stratus clouds)? 1cm/s or lessSteady rain or snow? Few cm/sBuoyant ascentCumulus cloud? Few m/s (5 mph)Hurricane? 5-10 m/s (15 mph)Ordinary thunderstorm? 10 m/s (22 mph)Severe thunderstorm? Up to 50 m/s (110 mph)Stability and Thunderstorm DevelopmentTo forecast thunderstorm development, we must be able to assess if the atmosphere will support buoyant ascentRising air parcels will be buoyant and rise on their own if they are warmer than air around themRecall from way back when that warm air is less dense (lighter) than cold airAn air parcel that is warmer than the air around it will rise (like a hot air balloon)An air parcel that is colder than the air around it will sinkReview: SoundingsRight line is always temperatureLeft line is always dewpointRecall that T ≥ TdWhat is a pressure level?A layer is bounded by two pressure levelsExample: 850-500 mbEnvironmental Lapse RateRate of decrease of environmental temperature with heightRelated to slope of temperature lineELR says nothing about how temperature of rising or sinking air parcel changes!Dry Adiabatic Lapse RateThe rate at which a rising unsaturated air parcel cools is called the dry adiabatic lapse rateIt is constant, equal to 10°C/kmSinking unsaturated air also warms at this rateUnsaturated air means temperature and dewpoint are NOT equal!!Moist Adiabatic Lapse RateRising saturated air also cools, but at a different rate, called the moist adiabatic lapse rateVariable, but will take to be 6°C/kmSinking saturated air also warms at this rateSaturated: Temperature = DewpointLapse RatesWhy are moist and dry adiabatic lapse rates different?As saturated air rises, it coolsWater vapor must condense out of air parcel to keep RH at 100%Condensation releases latent heat, offsetting some cooling due to ascentSaturated parcel cools at slower rateSimilar to driving with your windows down and AC on during a hot dayCar still cools, but not as quicklyLapse Rates and SoundingsCan plot temperature of rising air parcel on a sounding!Moist, dry adiabatic lapse ratesWhen rising parcel temperature (black) is to RIGHT of temperature line (red), parcel is WARMER than environment and will continue to RISEUNSTABLEWhen rising parcel temperature (black) is to LEFT of temperature line (red), rising parcel is COOLER than environment and will begin to SINKSTABLELifting Condensation Level (LCL)Lifting Condensation Level (LCL) – Level at which a rising surface air parcel becomes saturatedChange from dry to moist adiabatic lapse rateLook for inflection in parcel temperature curveLevel of Free Convection (LFC)Level of Free Convection (LFC) – Level at which a rising surface air parcel becomes positively buoyant (warmer than its environment)Parcel temp crosses to right of environmental tempEquilibrium Level (EL)Equilibrium Level (EL) – Level at which a rising surface air parcel is no longer positively buoyant (typically found near tropopause)Parcel temp crosses back to left of environmental tempCAPE and CINCAPE – Convective Available Potential Energy. Area between parcel temp and environmental temp when parcel is warmer than environmentBetween LFC and ELBig CAPE = Big Storm (if storm forms)Rising air parcel warmer than environmentCIN – Convective INhibition. Area between parcel temp and environmental temp when parcel cooler than environmentBelow LFCWant small, or zero if possible (for storms to form)Rising air parcel cooler than environmentLifted IndexTo calculate Lifted Index, lift parcel from surface to 500 mbHow? Follow parcel temperature line to 500 mbSubtract parcel temperature at 500 mb from environmental temperature at 500 mbLI = Tenv – TparcelWill calculate this in lab this weekNegative LI means rising air parcel warmer than environment and thunderstorms possiblePositive LI means thunderstorms not likelyAtmospheric StabilityAtmosphere unstable whenWarm and moist near surfaceCold aloftTemperature cools rapidly with heightRising parcel more likely to be warmer than environment, especially if parcel is saturatedAtmosphere stable whenCool or dry near surfaceWarm aloftTemperature cools slowly with heightInversions are VERY STABLE!!!Make the Atmosphere More StableWarm the air aloftWarm air advectionCool the surfaceNocturnal coolingCold air advectionAtmosphere typically most stable around sunriseMake the Atmosphere More UnstableCool the air aloftCold air advectionWarm the surfaceSolar heatingWarm air advectionAtmosphere typically most unstable in late afternoonRapid decrease in temperature with height = InstabilityStability and ThunderstormsThunderstorms most likely whenAtmosphere is unstableCAPE largeCIN small or zeroLI negativeMost likely in late afternoon/early evening hoursThunderstorms not likely whenAtmosphere is stableCAPE small or zeroCIN largeLI positiveLeast likely during morning hoursAtmospheric Stability and Thunderstorm Development 11/28/2012ReviewClouds form in RISING AIR-As air rises, it reaches lower pressures and expands-As air expands, it cools-Temperature drops but moisture content remains the same, so RH increases-Parcel eventually saturates, condensation occurs, and a cloud formsSinking air à CLEAR SKIESIn general, the faster air rises, the more interesting the weather will be!Rising AirHow fast can air rise?-Forced ascent:oNon-precipitqting layered cloud or drizzle (stratus clouds)? 1cm/s orlessoSteady rain or snow? Few cm/s-Buoyant ascentoCumulus cloud? Few m/s (5 mph)oHurricane? 5-10 m/s (15 mph)oOrdinary thunderstorm? 10 m/s (22 mph)oSevere thunderstorm? Up to 50 m/s (110 mph)Stability and Thunderstorm Development-To forecast thunderstorm development, we must be able to assess if the atmosphere will support buoyant ascentoRising air parcels will be buoyant and rise on their own if they are warmer than air around them-Recall from way back when that warm air is less dense (lighter) than cold air-An air parcel that is warmer than the air around it will rise (like a hot air balloon)-An air parcel that is colder than the air around it will