1. Meteo 3: Chapter 8 Stability and Cloud Types Read Chapter 8

2. Parcel Theory  Parcel: volume of air assumed to behave separately from the air surrounding it  Environment: everything outside the air parcel

3. Hydrostatic Balance means small vertical motions

4. Hydrostatic Balance can be violated on small space and time scales…requires instability (buoyancy)

5. Role of Stability in Cloud Formation  We already have relationships to determine if net condensation is occurring (e.g. LCL, vapor pressure vs. saturation vapor pressure)  If we can determine how air moves vertically, we can determine whether clouds are likely to form and how deep they might be!

6. Concept of Buoyancy

7. Lifting a Parcel of Air

8. Unstable equilibrium: T parcel > T environment

10. Stable equilibrium: T parcel < T environment

11. Neutral equilibrium  A parcel is given a push upward and comes to a stop after a short rise (T parcel = T environment ) (T parcel = T environment )  The key to determining stability is comparing a parcel’s temperature to the atmospheric temperature at the altitude of interest

12. Dry Adiabatic Lapse Rate (DALR)

13. More on DALR  An unsaturated sinking air parcel warms at 10°C/km  These temperature changes result from molecules in the parcel doing work for parcel expansion when rising and gaining kinetic energy via compression when sinking  Adiabatic: no exchange of heat energy between a parcel and its surroundings

14. Even more  Environmental lapse rate is what is measured by a weather balloon –Determined locally by height above ground and temperature advection, among other things  On average, is ~6.5°C/km  If temperature decreases with height, lapse rates are positive  If lapse rate is negative (temperature increases with height), an inversion is present

15. Moist Adiabatic Lapse Rate (MALR)

16. Importance of MALR  Since rising, saturated air parcels cool at a slower rate, the parcel has a better chance of staying warmer than its environment longer, thus has a better chance of being positively buoyant  Not constant

17. Conditional Instability

18. Simple Model of Stability  Instability results when an air parcel is warmer than its environment (warm below cool ideal) –Solar heating & low-level warm advection increase instability…sometimes lapse rate near ground is greater than DALR = superadiabatic –Cooling ground increases stability (radiational cooling/evaporational cooling) –Upper-level cooling (warming) increases (decreases) instability  Warm, moist low levels and chilly mid-levels are an ideal setup for an unstable atmosphere (thunderstorms)

19. Warm below, cold above

20. Cloud Classification

21. High Clouds  Composed of ice crystals  Cirrostratus can lead to formation of halo around sun or moon

24. Middle clouds  If cirrostratus lower and thicken into altostratus, that’s a good sign precipitation may be on the way  Alto-, strato- & cirro- cumulus form from destabilization of cloudy region in a generally stable environment

28. Low clouds  Stratiform clouds to north of warm front where overrunning is occurring  “Nimbus” means rain

32. Cumuliform clouds  Form via convection  “Clouds of vertical development”  Can last for minutes-few hours  Cumulonimbus produce thunderstorms, tornadoes, hail  Updrafts range from tens of cm/s (cumulus) to tens of m/s (cumulonimbus)

39. Wave Clouds  Indicate a stable atmosphere

41. Smoke as a stability clue

42. Summertime afternoons and wind

43. How does precipitation form?  Liquid water can become supercooled => drops of water exist as liquid with temperatures below 0°C  At temperatures >-40°C, tiny particles are needed to start ice crystal formation (ice nuclei)  Ice, liquid, and vapor coexist in clouds  Equilibrium vapor pressure over ice is less than the equilibrium vapor pressure over water at the same sub-freezing temperature

44. Bergeron Process  Saturation is reached over ice but not over water, thus ice crystals grow at the expense of liquid drops….eventually become large and fall

45. Collision-Coalescence Process  Occurs in “warm” clouds  Falling ice crystals melt into rain  Bigger raindrops fall faster than smaller ones  Larger drops collide with smaller drops  Grow as all or part of smaller drop’s water coalesces with larger drop

46. Virga