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Atmospheric Stability

Atmospheric Stability. Stability of Atmosphere is affected by. Temperature of the surrounding environment Temperature of the air parcel. Altitude. 0. 0. Temperature. Environmental Lapse Rate (ELR). 6 o C/1,000m. Altitude. 0. 0. Temperature. Isothermal Lapse Rate. Altitude (km). 0.

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Atmospheric Stability

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  1. Atmospheric Stability

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

  3. Altitude 0 0 Temperature Environmental Lapse Rate (ELR) 6oC/1,000m

  4. Altitude 0 0 Temperature Isothermal Lapse Rate

  5. Altitude (km) 0 0 Temperature Temperature Inversion 2 1 10 20

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

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

  8. Warm air mass INVERSION Cold air mass Frontal Inversion Ground

  9. Advection of warm air over a cold surface

  10. 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

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

  12. 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

  13. Altitude DALR SALR ELR 0 0 Temperature Absolutely Unstable Air ELR% > DALR > SALR

  14. Altitude ELR 0 0 Temperature Absolutely Unstable Air SALR ELR% > DALR > SALR Condensation level DALR

  15. 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

  16. Altitude SALR DALR 0 0 Temperature Conditionally Stable / Unstable Air ELR DALR> ELR > SALR

  17. 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

  18. Altitude SALR DALR 0 0 Temperature Absolutely Stable Air ELR% < SALR < DALR ELR Air parcel tends to return to its original position

  19. 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

  20. Altitude Cloud top SALR Condensation level DALR 0 0 Temperature Atmospheric stability and cloud development ELR Air parcel tends to rise because of higher temperature

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

  22. Lapse Rate Exercise

  23. 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.

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

  25. 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.

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

  27. 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.

  28. 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

  29. 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.

  30. 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

  31. 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

  32. 22℃ 24℃ 28℃ 26℃ Urban Heat Island

  33. 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

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