10Adiabatic lapse rate Adiabatic cooling Decrease in temperature without heat exchange with surrounding airWhen an air parcel is forced to rise,It expands due to the lower air pressure of surrounding airIncrease of volume leads to decrease in temperature
11Dry adiabatic lapse rate DALRTemperature change of dry air mass1oC/100 m
12Saturated adiabatic lapse rate SALR - Temperature change of air mass when condensation occurs6oC/1,000 m in usual casesSALR depends onMoisture content & temperatureHigher moisture content leads to smaller lapse rateSALR is smaller when temperature is lower
13Absolutely Unstable Air TemperatureAltitudeDALRSALRELRELR% > DALR > SALR
14Absolutely Unstable Air TemperatureAltitudeSALRELR% > DALR > SALRELRCondensation levelDALR
15Absolutely Unstable Air ELR% > DALR > SALRTherefore, the rising air parcel is always hotter than the surrounding airThe density is lowerFurther upwards displacement is resultedFor a sinking air parcel, further downwards displacement is resulted
16Conditionally Stable / Unstable Air TemperatureAltitudeELRSALRDALR> ELR > SALRDALR
17Conditionally Stable / Unstable Air DALR > ELRMeans the air parcel is cooler than surrounding airIt will not rise if the external force discontinuesHowever, when condensation takes placeLatent heat is releasedThe air parcel cools more slowly : ELR>SALRThe air parcel will become warmer than surrounding air - Unstable
18Absolutely Stable Air ELR% < SALR < DALR Altitude ELR SALR TemperatureAltitudeELRSALRAir parcel tends to return to its original positionDALR
19Stability and Cloud formation Instability leads to continuous rise of air parcelThick clouds such as cumulus and cumulonimbus will be formedStability will make rising air parcel go back to original positionOnly thin clouds such as stratus
20Atmospheric stability and cloud development TemperatureAltitudeELRCloud topSALRAir parcel tends to rise because of higher temperatureCondensation levelDALR
23Describe the change of ELR Below 250m, temperature increases with height causing a temperature inversionFrom 250m upward, temperature generally decreased with increasing altitude at a rate about 12.4℃/1,000mThe ELR below 1,750m is greater than above, i.e. the temperature decreases more rapidly from 250m to 1,750m and less rapidly above 1,750m.
25Dew point Dew-point is reached at 1,500m Condensation process takes place with the changing of water vapour into water droplets.The development of rain clouds with a thickness of 2,100mFreezing process ocurs with formation of ice crystals if freezing temperature is reached.
26Dew pointThe coalescence of water droplets and ice crystals may induce precipitation
27Stability of airBelow 1,700m, the rising air parcel is colder, and hence denser, than its surrounding air. There is a tendency for the rising air to sink back to ground level. The air parcel is said to be stable.The temperature inversion reinforces the stable condition.
28Stability of airAs condensation process occurs at 1,500m with the release of latent heat, the rising air parcel cools less rapidly at the SALR.From 1,700m upwards, the air parcel becomes warmer, and hence lighter, than its surrounding environment. It will become unstable and will continue to rise its own buoyancy
29Stability of airAs a general, the rising air parcel is said to be conditionally unstable since it is stable when it is unsaturated but becomes unstable when saturated.
30Natural mechanism lead to uplift Air movement encountering mountian barrierMeeting of air masses with different properties along frontal surfaceConvection uplift due to local heatingConvergence of air masses at a lower pressure zone
31Uplift in a highly urbanized environment Urban heat island promotes convective turbulence and the upward movement of airSurface roughness due to the presence of high buildings enhances vertical air motion
33Urban heat island encourages condensation and cloud formation However, high pressure in surrounding rural areas leads to closed circulationPollutants in urban can hardly diffusedPollutants together with fog and cloud leads to the formation of smogPollution induced temperature inversion create stable condition which hinders diffusion of pollutants