1. Stability, Adiabatic Processes, Precipitation

2. Stability of a parcel of air is tendency to resist displacement (stay put!)

3. 997
998
999
1000
surface
Air moves according to pressure gradient force (from High to Low )
So, why don’t air molecules escape into space?
Pressure gradient balanced by gravity.

4. “hydrostatic equilibrium” :
Upward pressure gradient is balanced by downward force of gravity
Atmosphere (in general) is in hydrostatic equilibrium.

5. But…
In any given place at any given time, there are large bodies of air (parcels) that are not in hydrostatic equilibrium with their surroundings

6. If surrounding air is more dense / colder than the parcel, parcel will rise.
warm
Colder / more dense
surrounding air
UNSTABLE

7. A parcel will rise until it gets to an altitude at which surrounding air is at same density / temperature

8. If surrounding air is less dense / warmer than the parcel, parcel will sink.
Warmer / less dense air
cold
STABLE

9. Therefore,
stability is determined by temperature difference between parcel and surrounding air.

10. Temperature of surroundings decreases with altitude at
AVERAGE environmental lapse rate:
(3.5°F / 1000 ft; 6.4°C / 1000 m)

11. What about parcel?
As parcel rises, it expands due to lower atmospheric pressure of surrounding air; as parcel expands, temperature drops.

12. As a parcel of air descends, it contracts due to greater atmospheric pressure of surrounding air; as parcel contracts, temperature rises. .

13. These changes in temperature:
NOT due to heat exchange with surroundings,
due to expanding and contracting.
called ADIABATIC
(change in temperature with no gain or loss of heat)

14. Dry Adiabatic Lapse Rate
Rate at which temperature drops in rising body of unsaturated air:
5.5°F / 1000 ft OR °C / 1000 m.

15. Wet Adiabatic Lapse Rate
Rate at which temperature drops in saturated rising air (temperature at dew point):
3.3°F / 1000 ft. OR 6°C / 1000 m.
NOTE: this rate is lower than dry rate
WHY?

16. Mechanisms causing air parcels to rise:
Spontaneous:
Convergence
Convection
Forced :
Orographic Uplift
Frontal Lifting

18. GOES

25. Cold front

26. Warm front

28. Lifting condensation level

31. Top & bottom elevations of clouds:
Bottom: lifting condensation level: temperature of rising air reaches dew point
Top: Cloud rises until cloud temperature reaches temperature of surroundings (becomes stable).
Cloud thickness depends on thickness of unstable layer.

32. Back to stability….
To test stability, push parcel up and see if it continues to rise.
Several possible results….

33. Example #1:
ELR = 11 degrees / 1000m
Parcel is saturated so use Wet AR: (What is it?)
6 degrees / 1000 m
Therefore, surroundings cool faster than parcel. Parcel’s tendency will be to RISE.
UNSTABLE
(ELR>WAR)

34. Example #2:
ELR = 11 degrees / 1000m
Parcel is unsaturated so use Dry AR: (What is it?)
10 degrees / 1000 m
Therefore, surroundings cool faster than parcel. Parcel’s tendency will be to
RISE.
UNSTABLE
(ELR>DAR)

35. Example #3:
ELR = 4 degrees / 1000 m
Parcel is unsaturated so use Dry AR: (What is it?)
10 degrees / 1000 m
Therefore, parcel cools faster than surroundings. Parcel’s tendency will be to
STAY PUT.
STABLE
(ELR<DAR)

36. Example #4:
ELR = 4 degrees / 1000m
Parcel is saturated so use Wet AR: (What is it?)
6 degrees / 1000 m
Therefore, parcel cool faster than surroundings. Parcel’s tendency will be to STAY PUT.
STABLE
(ELR<WAR)

37. If air is saturated, use Wet AR. Stable or unstable? unstable
Example #5:
ELR = 8 degrees / 1000 m
If air is saturated, use Wet AR. Stable or unstable? unstable
If air is unsaturated, use Dry AR. Stable or unstable? stable

38. summary If ELR > WAR , DAR : UNSTABLE If ELR < WAR, DAR : STABLE
If WAR < ELR < DAR: CONDITIONALLY STABLE (depends on saturation)
(Atmosphere is usually conditionally stable).

39. Sinking air warms at dry adiabatic rate.

40. PRECIPITATION (rain , snow, sleet, freezing rain, hail)
Large parcel of air rises and cools
Adiabatic cooling
Cools to dew point
Cloud droplets grow large enough to fall

41. Why do cloud droplets need to grow?

43. Processes by which cloud droplets/crystals grow :
A. Bergeron Findeisen Process
Mid- and High latitudes: “Cold Clouds”
(tops colder than 0°C)
Saturation vapor pressure is greater over water than ice
It takes more water vapor to saturate air over water than over ice
ice crystals grow at the expense of water droplets

45. Bergeron Findeisen process

46. Collision/Coalescence process (tops warmer than -15°C)
Low latitudes: “Warm Clouds”
(tops warmer than -15°C)
Collision and coalescence: some big drops form; they fall faster than small drops and collect small drops

48. Precipitation Types

49. rain Precipitation as liquid water Height Melting level 0°C
Temperature

50. virga

51. snow
Snowflake is several aggregated crystals.
Height
0°C
Temperature

52. graupel
“riming”: shell forms around crystal as it passes through tiny suspended water drops: forms “graupel” (ice pellets)

53. Sleet Frozen raindrops; raindrops that freeze as they descend. Height
Melting level
0°C
Temperature

55. Freezing rain
Supercooled droplets hit a surface which is below freezing
Height
Melting level
0°C
Temperature