Atmospheric Stability

1. Atmospheric Stability • Adiabatic Processes • The concept of a parcel • Parcel and environmental lapse rates • Atmospheric dry stability • Determining stability

2. Air parcels • A parcel is a “blob” of air • Small enough to have only one value of T, p, ρ, etc. • Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?)

3. Lapse Rates • Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude • Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases

4. The Adiabatic Lapse Rate • An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings • Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment • Examples: • rising air in a thunderstorm • Air rising over a topographic barrier (like a mountain)

5. Adiabatic Processes (Chalkboard)

6. The Adiabatic Lapse Rate • The adiabatic lapse rate for DRY air on Earth is • Γd = g/cp • Γd = 9.81 m s-2 / 1004 J kg-1 K-1 • Γd = 0.00977 K m-1 • Γd = 9.77 K km-1

7. The Adiabatic Lapse Rate • This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 oC per km of ascent(descent) unless: • It exhanges significant mass or heat with the environment • It becomes saturated with respect to water vapor • It rises(sinks) so slowly that radiation heat transfer is possible

8. The Adiabatic Lapse Rate • What is the dry adiabatic lapse rate (Γd = g/cp) in these atmospheres?

9. The Adiabatic Lapse Rate • We have thus far only discussed the • DRY ADIABATIC LAPSE RATE • Water vapor condensation releases 2.5 MJ of energy for each kg of water condensed – this latent heat changes the adiabatic lapse rate for condensing air parcels to the • MOIST ADIABATIC LAPSE RATE

10. Atmospheric Stability

11. Atmospheric Stability • stable and unstable equilibria • air parcels • adiabatic process • adiabatic lapse rates • Stability does not control whether air will rise or sink.Rather, it controls whether rising air will continue to riseor whether sinking air will continue to sink.

12. Determining Stability (Chalkboard)

13. A Stable Atmosphere • environmental lapse rate • absolute stability • stabilizing processes • Stable air provides excellent conditions for high pollution levels.

14. An Unstable Atmosphere • absolute instability • warming of surface air • destabilizing processes • superadiabatic lapse rates • Unstable air tends to be well-mixed.

15. Conditionally Unstable Air • conditional instability • dry and moist adiabatic lapse rates are different • Environmental lapse rate is between the two

16. Atmospheric Moisture Twice now, we’ve mentioned moist adiabatic lapse rates. Maybe we should talk about atmospheric moisture before we go down that road any further…

17. Humidity, Condensation and Clouds • Circulation of water in the atmosphere • Evaporation, condensation and saturation • Humidity • Dew and frost • Fog • Clouds

18. Circulation of Water in the Atmosphere

19. Circulation of Water in the Atmosphere • evaporation • condensation • precipitation • hydrologic cycle • The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

20. Fig. 4-1, p. 80

21. Stepped Art Fig. 4-1, p. 80

22. Evaporation, Condensation and Saturation

23. Evaporation, Condensation and Saturation • saturation • condensation nuclei • In very clean air, about 10,000 condensation nucleiare typically found in one cubic centimeter of air,a volume approximately the size of your fingertip.

24. Humidity

25. Mixing Ratio • The ratio of the mass of water vapor in air to the mass of dry air: • w = mv / md • Usually expressed in g kg-1 • Some typical values: • Tropical marine boundary layer air: w ≈ 18 g kg-1 • Polar air: w ≈ 1 g kg-1 • Stratospheric air: w ≈ 0.1 g kg-1

26. Specific Humidity The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor): SH = mv / (md + mv) w = SH / (1 – SH) SH = w / (1 + w)

27. Vapor Pressure • actual vapor pressure • saturation vapor pressure • “Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

28. Vapor Pressure Actual vs. Saturation (or equilibrium) vapor pressure… (Chalkboard)

29. Vapor Pressure Saturation vapor pressure depends only on temperature… Formula: Saturation vapor pressure Saturation vapor pressure at 273 K = 6.11 mb Latent heat of vaporization = 2.5x106 J kg-1 Gas constant for water vapor = 461 J kg-1 K-1 273 K Temperature

30. Vapor Pressure Saturation vapor pressure depends only on temperature… Formula:

31. Vapor Pressure Saturation vapor pressure depends only on temperature… Graph:

32. Relative Humidity • definition of relative humidity • saturation and supersaturation • condensation • relative humidity and temperature • When the general public uses the term “humidity”, they mean “relative humidity.”

33. Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure. rh = e / es Since es depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content.

34. Fig. 4-5, p. 83

35. Fig. 4-7, p. 85

36. Relative Humidity and Dew Point • dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity • dew point depression and relative humidity • The dew point temperature is useful for forecasting heatindex, precipitation probabilities, and the chance of frost.

37. Measuring Humidity • psychrometers • hygrometers

38. Dew and Frost

39. Dew and Frost • dew • frost • frost point and deposition • Frost is one of the few examples of deposition in nature.

40. Fog

41. Fog • radiation fog • advection fog • upslope fog • evaporation (mixing) fog • Fog is an extreme hazard to aircraft.

42. Clouds

43. Classification of Clouds • major cloud types • cloud appearance • cloud base • It’s easy to identify clouds, but it takes practice.The ability to identify clouds allows you to forecastmany aspects of the weather using nothing but youreyes.

44. Table 4-2, p. 98

45. Cloud Identification • high clouds • middle clouds • low clouds • clouds with vertical development

46. High Clouds • cirrus • cirrocumulus • cirrostratus • Cirrostratus clouds can sometimes be quite thick.

47. Middle Clouds • altocumulus • altostratus • Altocumulus clouds are very pretty, especially just after sunrise or just before sunset.

48. Low Clouds • nimbostratus • stratocumulus • stratus • Marine stratocumulus is the most common cloud typein the world.

49. Clouds with Vertical Development • cumulus • cumulus congestus • cumulonimbus • Not all cumulus clouds grow to be thunderstorms, butall thunderstorms start out as cumulus clouds.

50. Some Unusual Clouds • lenticular clouds • pileus • mammatus clouds • contrails • Several alleged ‘flying saucer’ reports have turned out to be lenticular clouds.