1. Cloud Formation
Formation of clouds requires a mechanism that results in the uplift of air.

2. First, recall

3. Adiabatic process - process in which temperature changes but heat energy is not added or removed.

4. For adiabatic systems gases warm as they contract.
p . ∆V = - Cv . ∆T
∆ H - Change in heat
p - pressure
∆V - Volume Change
Cv - Specific heat
∆ T - Temperature Change
Adiabatic process - process in which temperature changes but heat energy is not added or removed.
For adiabatic systems gases warm as they contract.
For adiabatic systems gases cool as they expand.

5. For adiabatic systems gases warm as they contract.
p . ∆V = - Cv . ∆T
∆ H - Change in heat
p - pressure
∆V - Volume Change
Cv - Specific heat
∆ T - Temperature Change
Adiabatic process - process in which temperature changes but heat energy is not added or removed.
For adiabatic systems gases warm as they contract.
For adiabatic systems gases cool as they expand.

6. p . ∆V = - Cv . ∆T
For adiabatic systems gases warm as they contract.
For adiabatic systems gases cool as they expand.

7. dry adiabatic lapse rate (DALR) –
rate at which a rising parcel of dry (unsaturated) air cools.
about 1oC/100m (5.5oF/1000 feet)
Lifting Condensation level (LCL) –
the height at which a rising parcel of air undergoes condensation or deposition.
Saturated adiabatic lapse rate (SALR) –
rate at which a rising parcel of wet (saturated) air cools.
about 0.5oC/100m (3.3oF/1000 feet)
Environmental Lapse Rate (ELR) - rate at temperature in the troposphere decreases with height.

8. Mechanisms That Lift Air
1) Orographic Lifting
2) Frontal Lifting
3) Convergence
4) Convection

9. Mechanisms That Lift Air
Orographic Lifting - the forcing of air above a mountain barrier.
-air flowing toward a topographic high (hill or mountain) will be deflected over the barrier and displaced upward.
-downwind of the mountain, air descends the slope and warms by compression
Rain Shadow - an area on the downwind side of a mountain barrier having relatively low precipitation.

10. Mechanisms That Lift Air
Rain Shadow Deserts
Nevada
Gobi Desert, China

11. Mechanisms That Lift Air
2) Frontal lifting - displacement of one air mass over another.
Front - A transition zone between two dissimilar air masses with differing temperatures and/or moisture. Results in clouds in two ways.

12. Mechanisms That Lift Air
2) Frontal lifting - displacement of one air mass over another.
Front - A transition zone between two dissimilar air masses with differing temperatures and/or moisture. Results in clouds in two ways.
a) Cold Front - cold air mass advancing toward a warmer air mass.
The denser cold air displaces the lighter warm air ahead of it.
Results on cumulonimbus clouds.

13. Mechanisms That Lift Air
2) Frontal lifting - displacement of one air mass over another.
b) Warm Front - warm air mass advancing toward a cooler air mass.
The warm air rides over the wedge of cold air as if it is rising over a mountain barrier.
Overrunning - warm air flows upward over the cold air.
Results in nimbostratus clouds.

14. Mechanisms That Lift Air
Pressure differences in the atmosphere set air in motion causing winds.
When there is a center of low pressure, winds tend to converge toward the center from all directions
3) Convergence - horizontal movement of air toward a center of low pressure.
4) Convection - heating air near the surface causes the air to become less dense and rise upward.

15. Stability Once given an initial boost air may continue to rise.
Other times it may resist rising
Stable air - resists rising.
Unstable air - air that will continue to rise once given it initial boost.
Stability is related to density
low density air tends to rise upward.
high density air tends to sink.
Warmer air is less dense and will tend to rise
Cooler air is more dense and will resist rising

16. Stability
If a parcel of rising air cools at a rate that makes it colder than its surrounding air, it will become relatively dense - this inhibits rising.
If the lifted air cools more slowly than its surrounding air, it will become warm and will continue to rise upward.
The cooling of air is governed by the DALR or the SALR

17. Stability
Absolutely unstable
Absolutely stable
Conditionally unstable

18. Absolutely Unstable Air
400
300
200
100
Absolutely Unstable Air
7oC
8.5oC
5.5oC
8oC
9oC
7oC
Height (m)
9oC
9.5oC
8.5oC
10oC
10oC
ELR (1.5) DALR (1.0) SALR (0.5)
oC/100m

19. Stability Absolutely Unstable Air Once lifted it continues rise upward
Rule 1: Whenever the ELR exceeds the DALR, the air is absolutely unstable and will continue to rise once lifted, regardless of whether it is saturated or not.

20. Absolutely Stable Air Height (m) oC/100m 400 300 200 100 9.1oC 8oC 9oC
Absolutely Stable Air
9.1oC
8oC
9oC
9.4oC
Height (m)
9oC
9.5oC
9.7oC
10oC
10oC
ELR (0.3) DALR (1.0) SALR (0.5)
oC/100m

21. Stability Absolutely Stable Air
Lifted air that has a negative buoyancy and sinks back down to its initial position.
Rule 2: Whenever the ELR is less than the SALR, the air is absolutely stable and will resist lifting, regardless whether it is saturated or not.

22. Conditionally Unstable Air
400
300
200
100
Conditionally Unstable Air
7oC
8.5oC
8.1oC
8oC
9oC
8.8oC
Height (m)
9oC
9.5oC
9.3oC
10oC
10oC
ELR (0.7) DALR (1.0) SALR (0.5)
oC/100m

23. Stability Conditionally Unstable Air
The tendency of the air to rise depends on whether it is saturated or not.
Rule 3: Whenever the ELR is between the DALR and the SALR, the air is conditionally unstable and whether it rise or sinks depends on whether it becomes saturated or not.

24. Air makes up the greatest
Composition of Clouds
Water droplets
Ice crystals
Air
Air makes up the greatest
mass of a cloud!

25. Cloud Types General Characteristics of Clouds
Assume a variety of shapes
Most occur within the troposphere
Contain a combination of liquid droplets and ice crystals

26. Cloud Types First widely accepted system for classification of clouds:
Luke Howard (1803) - English naturalist
(4 basic categories)
Cirrus (Latin for curl) - thin wispy clouds of ice.
Stratus (Latin for layer) - layered clouds.
Cumulus (Latin for heap) - clouds having vertical development
Nimbus (Latin for rain) - rain clouds
Today we use a modified version of Howard’s classification scheme.

28. Cirrus

29. Cirrus

30. Cirrostratus

31. Cirrocumulus

32. altocumulus

33. altocumulus

34. altostratus

35. Stratus

36. Nimbostratus

37. Fair Weather Cumulus

38. Fair Weather Cumulus

39. Cumulonimbus

40. Cumulonimbus

41. Mamatus Clouds

42. Lenticular Clouds

43. Nacreous Clouds

44. Cloud Types High Clouds > 6000m Cirrus Cirrostratus Cirrocumulus
Ten major cloud types grouped by their shape and height
High Clouds > 6000m
Cirrus
Cirrostratus
Cirrocumulus
Middle Clouds m
Altostratus
Altocumulus

45. Cloud Types C. Low Clouds <2000 m Stratus Stratocumulus
Ten major cloud types grouped by their shape and height
C. Low Clouds <2000 m
Stratus
Stratocumulus
Nimbostratus
D. Clouds with vertical development
Cumulus
Cumulonimbus

46. Cloud Types High Clouds Almost always composed of ice
above 6000m 1900 feet)
Almost always composed of ice
Temperatures <-35oC
Cirrus (Ci)
wispy aggregates of ice crystals
~1.5 km thick
Fall streaks - ice crystals in cirrus clouds fall and descend below the cloud.
Usually form in fair weather.

47. Cloud Types b. Cirrostratus (Cs) Composed entirely of ice
High Clouds
b. Cirrostratus (Cs)
Composed entirely of ice
Moon or sun can be viewed through these clouds but will have a whitish appearance
Sometimes light from the sun or moon will be bent by the ice and create a halo around the sun or moon.
Cirrocumulus (Cc)
Ice crystals arranged in rows of individual puffy clouds
Rows form from wind shear - wind changing speed or direction.
Mackeral sky

48. Cloud Types B. Middle Clouds 2000-6000 feet
composed of liquid droplets
alto - middle
a. altostratus (As)
- similar to cirrostratus but at middle level in the atmosphere.
- composed of liquid
- scatter back a large percentage of incoming solar radiation, reducing the sunlight reaching the surface.
- No shadows
- No halo as in cirrostratus

49. Cloud Types B. Middle Clouds b. altocumulus (Ac)
- layered clouds that form long bands of puffs.
- often gray in color
- mostly liquid
Usually form by convection in unstable air, as a cold front approaches.

50. Cloud Types cloud base below 2000m a. stratus (St) - layered clouds.
C. Low Clouds
cloud base below 2000m
a. stratus (St)
- layered clouds.
- form from extensive areas of unstable air
- can exceed several states in extent
b. nimbostratus (Ns)
- low, layered clouds that yield precipitation.
- Usually light steady precipitation

51. Cloud Types c. stratocumulus (Sc)
C. Low Clouds
c. stratocumulus (Sc)
- low layered clouds with some vertical development.
- thicker sections may appear dark.

52. Cloud Types a. Cumuloform (Cu)
D. Clouds with vertical development
a. Cumuloform (Cu)
- clouds that have substantial vertical displacement.
Several subtypes
- cumulus humilis - fair weather cumulus
- cumulus congestus - multiple towers of clouds

53. Cloud Types b. Cumulonimbus (Cb)
D. Clouds with vertical development
b. Cumulonimbus (Cb)
- clouds that have substantial vertical displacement that produce precipitation.
- can extent from a few hundred meters from the ground to the top of the troposphere.
- anvil shaped top composed of ice.
- hailstones - form in the anvel
- updrafts and downdrafts causes turbulence.

54. Unusual Cloud Types
Lenticular Clouds

55. Unusual Cloud Types
Nacreous Clouds

56. Unusual Cloud Types
Mamatus Clouds

57. Cloud Types Unusual Cloud types Lenticular clouds lens-shaped
form downwind of moutain barriers
can appear to stay stationary
Banner clouds
Similar to Lenticular but form over mountain tops.
Mamatus Clouds
cumulonimbus clouds with portions that hang down below the cloud base.
Nacreous clouds
soft white clouds that are present in the stratosphere (heights of ~30 km)

58. Cloud Types