1. Atmospheric Stability

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

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

4. Isothermal Lapse Rate
Temperature
Altitude

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

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

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

8. Frontal Inversion
Warm air mass
Cold air mass
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. Absolutely Unstable Air
Temperature
Altitude
DALR
SALR
ELR
ELR% > DALR > SALR

14. Absolutely Unstable Air
Temperature
Altitude
SALR
ELR% > DALR > SALR
ELR
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. Conditionally Stable / Unstable Air
Temperature
Altitude
ELR
SALR
DALR> ELR > SALR
DALR

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

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

21. Reference Website

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. Urban Heat Island
26℃
24℃
22℃
28℃

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