Atmospheric Stability

Stability of Atmosphere is affected by Temperature of the surrounding environment Temperature of the air parcel

Environmental Lapse Rate (ELR) Altitude 6oC/1,000m Temperature

Isothermal Lapse Rate Temperature Altitude

Temperature Inversion Altitude (km) 2 1 10 20

Conditions for Radiation Inversion At night Clear sky Calm condition Relatively dry air Snow cover Radiation loss Ground

Air drainage inversion In a valley At night Cold air of higher density Warm air

Frontal Inversion Warm air mass Cold air mass INVERSION Ground

Advection of warm air over a cold surface

Adiabatic lapse rate Adiabatic cooling Decrease in temperature without heat exchange with surrounding air When an air parcel is forced to rise, It expands due to the lower air pressure of surrounding air Increase of volume leads to decrease in temperature

Dry adiabatic lapse rate DALR Temperature change of dry air mass 1oC/100 m

Saturated adiabatic lapse rate SALR - Temperature change of air mass when condensation occurs 6oC/1,000 m in usual cases SALR depends on Moisture content & temperature Higher moisture content leads to smaller lapse rate SALR is smaller when temperature is lower

Absolutely Unstable Air Temperature Altitude DALR SALR ELR ELR% > DALR > SALR

Absolutely Unstable Air Temperature Altitude SALR ELR% > DALR > SALR ELR Condensation level DALR

Absolutely Unstable Air ELR% > DALR > SALR Therefore, the rising air parcel is always hotter than the surrounding air The density is lower Further upwards displacement is resulted For a sinking air parcel, further downwards displacement is resulted

Conditionally Stable / Unstable Air Temperature Altitude ELR SALR DALR> ELR > SALR DALR

Conditionally Stable / Unstable Air DALR > ELR Means the air parcel is cooler than surrounding air It will not rise if the external force discontinues However, when condensation takes place Latent heat is released The air parcel cools more slowly : ELR>SALR The air parcel will become warmer than surrounding air - Unstable

Absolutely Stable Air ELR% < SALR < DALR Altitude ELR SALR Temperature Altitude ELR SALR Air parcel tends to return to its original position DALR

Stability and Cloud formation Instability leads to continuous rise of air parcel Thick clouds such as cumulus and cumulonimbus will be formed Stability will make rising air parcel go back to original position Only thin clouds such as stratus

Atmospheric stability and cloud development Temperature Altitude ELR Cloud top SALR Air parcel tends to rise because of higher temperature Condensation level DALR

Reference Website www.piercecollege.com/offices/weather/stability.html

Lapse Rate Exercise

Describe the change of ELR Below 250m, temperature increases with height causing a temperature inversion From 250m upward, temperature generally decreased with increasing altitude at a rate about 12.4℃/1,000m The 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.

Dew point=5℃ DALR=10℃/1km SALR=5℃ Condensation level

Dew 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 2,100m Freezing process ocurs with formation of ice crystals if freezing temperature is reached.

Dew point The coalescence of water droplets and ice crystals may induce precipitation

Stability of air Below 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.

Stability of air As 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

Stability of air As 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.

Natural mechanism lead to uplift Air movement encountering mountian barrier Meeting of air masses with different properties along frontal surface Convection uplift due to local heating Convergence of air masses at a lower pressure zone

Uplift in a highly urbanized environment Urban heat island promotes convective turbulence and the upward movement of air Surface roughness due to the presence of high buildings enhances vertical air motion

Urban Heat Island 26℃ 24℃ 22℃ 28℃

Urban heat island encourages condensation and cloud formation However, high pressure in surrounding rural areas leads to closed circulation Pollutants in urban can hardly diffused Pollutants together with fog and cloud leads to the formation of smog Pollution induced temperature inversion create stable condition which hinders diffusion of pollutants