1. Clouds, Stability, Adiabatic Processes, Precipitation

2. What is a cloud? Dense mass
suspended water droplets, supercooled water droplets and/or ice particles
on condensation / ice nuclei of solid matter (sea salt, soil particles, organic matter, pollutants, aerosols)

3. Clouds form
When a parcel of air rises and cools to the dew point temperature.
Then the water vapor in the air parcel condenses into water droplets or deposits into ice crystals

4. Why would a parcel of air rise?

5. If surrounding air is more dense / colder than the parcel, parcel will rise.
warm
Colder / more dense
surrounding air
THAT IS
CALLED
UNSTABLE

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

7. If surrounding air is less dense / warmer than the parcel, parcel will sink; or will not rise.
Warmer / less dense air
STABLE;
NO CLOUDS
cold

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

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

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

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

12. QUIZ 5 1. What is this? Fog B. a cloud C. Frost D. dew

13. 3. What is this?
Fog B. a cloud
C. Frost D. dew
4. What is this?
Fog B. a cloud
C. Frost D. dew

14. 5. For the previous 4 questions, which of the
following was required?
A legal ID B. saturation
C. Low relative humidity D. extreme heat
6. What do you call a dense mass of water
droplets, supercooled water droplets and/or ice
crystals in the troposphere?
A cloud B. the frozen three
C. Dunno D. aerosols

15. Yesterday’s heavy fog
On radar

16. On surface analysis

17. On visible satellite image

18. On thermal IR

19. Water vapor

21. Long range forecast
A vigorous upper-level trough moving from the Rockies to the central U.S. on Wed is expected to quickly deepen and spin up a significant low pressure system. The surface low is expected to move from the Central High Plains Wed morning to the Midwest on Thu and the Great Lakes by Fri, before quickly moving northeast into Quebec by Fri night/Sat. Confidence in the forecast for this system has improved significantly over the past hours.

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

23. 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.

25. Lifting condensation level

28. General cloud forms: Stratiform: layer Cumuliform : globular
layer of air forced to rise over denser layers
Cumuliform : globular
parcels of air forced to rise above denser surrounding air

31. Altitude classes: High Cloud (cirro-): ice crystal clouds
> 7000 m (23,000 ft.)
CIRRUS, CIRROSTRATUS, CIRROCUMULUS
Middle Cloud (alto-)
2000 – 7000 m ( ,000 ft.)
ALTOSTRATUS, ALTOCUMULUS
Low Cloud (stratus)
<2000 m (6500 ft)
STRATUS, NIMBOSTRATUS, STRATOCUMULUS
Vertically developed clouds (CUMULONIMBUS)

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

34. GOES

41. Cold front

42. Warm front

43. 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

44. Why do cloud droplets need to grow?

46. 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

48. Bergeron Findeisen process

49. 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

51. Precipitation Types

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

53. virga

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

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

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

57. Sleet hurts! and is loud.

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

59. Very destructive;
Causes tree limbs and power lines to come down