Understanding Atmospheric Stability: Processes and Conditions

1. Chapter 7Water and Atmospheric Moisture Geosystems 6e An Introduction to Physical Geography Robert W. Christopherson Charles E. Thomsen

2. Water and Atmospheric Moisture • TOPICS: • Adiabatic Processes • Atmospheric Stability  • Clouds  • Fog  

3. Adiabatic Processes   • Adiabatic processes • Begins with a parcel of air • Bouyancy caused initially by differences in (near) surface temperature • Less dense, warmer air rises, more dense, colder air sinks, after which… • Ascending or descending air will undergo changes in temperature with no exchange of heat. This is an adiabatic process.

4. Buoyancy Figure 7.15

5. Adiabatic Processes   • Adiabatic processes • Normal lapse rate: the average decrease in temperature with increasing altitude • Environmental lapse rate: the actual lapse rate at a particular place and time • Ascending or descending air will undergo changes in temperature with no exchange of heat. This is an adiabatic process. • Heat WAS exchanged = diabatic process.

6. Adiabatic Processes Figure 7.17

7. Adiabatic Processes   • Dry adiabatic rate (DAR) • Also called the Dry Adiabatic Lapse Rate (DALR) • 10 C°/ 1000 m • 5.5 F°/ 1000 ft • Lifting Condensation Level (LCL) is reached, then… • Moist adiabatic rate (MAR) • Also called the Wet Adiabatic Lapse Rate (WALR) • 6 C°/ 1000 m • 3.3 F°/ 1000 ft

10. AtmosphericStability Figure 7.20

11. Atmospheric Stability   • Stable and unstable atmospheric conditions • Involves a parcel of air and its surrounding environment in the atmosphere • Stable atmosphere: • A parcel of air is discouraged from rising • Kind of weather normally associated? • Unstable atmosphere: • A parcel of air is encouraged to rise • Kind of weather normally associated?

12.  Examples of Stability Unstable AtmosphereParcel of air is encouraged to rise Figure 7.20

13.  Examples of Stability Stable AtmosphereParcel of air is discouraged from rising Figure 7.20

14. Atmospheric Stability   • To determine atmospheric stability: • Compare the ELR with the DAR (or DALR) and MAR (or WALR) • If ELR < MAR < DAR = STABLE • If ELR > DAR > MAR = UNSTABLE • If MAR < ELR < DAR = CONDITIONALLY UNSTABLE

15. Atmospheric Stability   • For example: • We measure and find the ELR to be 12 Cº/ 1000 m • We know the DAR is 10 Cº/ 1000 m. • We know the MAR is 6 Cº/ 1000 m. • If ELR (12) > DAR (10) > MAR (6) then? • If ELR > DAR > MAR = UNSTABLE

16. Atmospheric Stability   • If ELR is large (shallow slope), it will be to the left of both the DAR and MAR lines • This describes unstable atmospheric conditions! • If ELR is small (steep slope), it will be to the right of both the DAR and MAR lines. • This describes stable atmospheric conditions!

17. ELR ELR Atmospheric Temperatures and Stability Figure 7.19

18. Moisture Droplets Figure 7.20

19. Bergeron process Raindrop and Snowflake Formation Collision-coalescence process Figure 7.22

20. 3 1 2 2 10 5 4 6 9 7 8 Cloud Types and Identification   Figure 7.23

21. Cirrus Figure 7.23

22. Cirrostratus Figure 7.23

23. Cirrocumulus Figure 7.23

24. Altocumulus

25. Altostratus

26. Stratus Figure 7.23

27. Nimbostratus Figure 7.23

28. Stratocumulus Figure 7.23

29. Cumulus Figure 7.23

30. Cumulonimbus Figure 7.23

31. Cumulonimbus Development Figure 7.24

32. Fog   • Fog – a cloud layer at or very close to the surface formed when surface air temperatures and dewpt temperature are nearly identical • Advection fog – warm, moist air layers moves over a cold surface • Evaporation fog – dole air moves over warmer water body • Radiation fog – loss of longwave radiation over moist surface

33. When warm, moist air moves over cooler body of water, what happens? Advection Fog Figure 7.25

34. Cold air lies over warmer body of water, and evaporation from water surface causes saturation and fog. Also, sea smoke = hazard. Evaporation Fog Figure 7.26

35. Loss of longwave radiation at night over moist surface causes saturation. Radiation Fog Figure 7.28