1. L15 Atmospheric Stability
Stability and lapse rate
Stability on skew T – ln p charts
Stable, neutral, unstable, conditionally unstable
Stable atmosphere
Inversions – fog/air pollution
Unstable atmosphere
Convection
Thunderstorm development

2. Static stability of dry air
1st case: Real lapse rate, Γ is less than the DALR, Γd
What happens to air at ‘O’ that is displaced upwards by a disturbance?
And what happens if it is displaced downwards?
See Wallace & Hobbs section 3.6

3. Static stability of dry air
2nd case: Real lapse rate, Γ > Γd
What happens to air at ‘O’ that is displaced upwards by a disturbance?
And what happens if it is displaced downwards?
See Wallace & Hobbs section 3.6

4. Useful analogy for stability: ball on a surface
Stable
Unstable
Neutral
Conditional instability
Unstable profiles cannot persist, so tend to rapidly evolve to be ~neutral

5. Stability and air pollution
Match the plume dispersion pattern to the temperature profiles below
Atmospheric temperature profile is the solid line. Dry adiabat is dashed line.

6. Stability and air pollution
Neutral profile – smoke goes anywhere
Stable profile – smoke stays at emitted height
Stable above, neutral below – smoke mixes down but not up
Neutral above, stable below – smoke mixes up but not down
Atmospheric temperature profile is the solid line. Dry adiabat is dashed line.

7. Stability on skew T-ln p diagram
Conditionally
unstable
Stable
stable
Absolutely unstable

8. Absolutely unstable example
Tiny unstable layer near ground

9. Any form of strong heating of the ground can cause instability
Extreme example of instability: a forest fire
Any form of strong heating of the ground can cause instability

10. Inversion layer example
Very stable layer probably defining the top of the
boundary layer.
Very stable layers where temperature increases with height are called ‘inversions’.
Top of the BL often capped by an inversion – tends to trap pollution in BL

11. Edinburgh, c 1950
‘auld reekie’
London, 1952

12. Buncefield oil depot fire, 2005

15. Satellite image at 1045 GMT, ~5 hours after start of fire

16. Nottingham Skew T-log P on day of Buncefield Fire (12Z, 11 Dec 2005)
Strong
inversion near ground

17. Top of BL

18. Scott-Amundsen South Pole station
South Pole: Note it is cold!! Surface pressure is only ~700 hPa Very stable thin layer close to surface
Scott-Amundsen South Pole station

19. Kuching Indonesia

20. Conditionally unstable - tropics

21. (Image from Google earth)
Small scale cumulus clouds – typical of convection and an unstable tropical atmosphere. In the tropics these clouds typically build up during the day to cumulonimbus as surface heating increases the instability.
Douala, Cameroon
(Image from Google earth)

22. Summary Stability and lapse rate Stability on skew T – ln p charts
Stable, neutral, unstable, conditionally unstable
Stable atmosphere
Inversions – fog/air pollution
Unstable atmosphere
Convection
Thunderstorm development – more next time