1. Introduction to Atmospheric Sciences Plymouth State University
CLOUDS
Dr. Sam Miller
Introduction to Atmospheric Sciences
Plymouth State University 1

2. Formation Processes

3. What do you need to form a cloud?
Condensation of water vapor in the air into liquid cloud droplets
Occurs when T = Td (RH = 100%)
A way to reduce the temperature to the dew point
The best way to cool air in the atmosphere is LIFT IT
Cooling is adiabatic

4. Rising air parcels expand and cool
Sinking parcels compress and warm

5. Cloud bases Form at the LCL
Lifting Condensation Level
Altitude at which temperature within a rising parcel drops down to the dew point
Altitude at which lifted parcel reaches saturation
Visible cloud droplets appear
LIFTING CONDENSATION LEVEL

6. Type of cloud that forms
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

7. Type of cloud that forms
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

8. Convection
Surface heating causes hot “bubbles” of air (thermals) to break away from the surface and rise
Warm air rises, since it is less dense than cold air
Can produce “popcorn” thunderstorms during warm, humid afternoons

9. Topography Air is forced over mountain by strong, perpendicular wind
Rising air expands and cools on windward side of mountain
Often produces showery precipitation
Air descending leeward side of mountain is compressed and heated
Produces a rain shadow on the leeward side of the mountain

10. Low-Level Convergence
Air near the surface moves toward a low pressure system
Away from a high pressure
“Fronts” are areas of low pressure
When air converges at surface, it is forced to rise
Often assisted by divergence aloft
Upper-level divergence sometimes occurs without low-level convergence

11. Fronts Warm air meets cold air and is forced to rise
Type of clouds depends on the type of front
Cold fronts can produce lines of thunderstorms called squall lines
Warm fronts usually produce widespread, flat clouds via overrunning

12. Type of cloud that forms
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

13. Type of cloud that forms
Long-term, widespread lifting mechanisms will create the largest clouds
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

14. Type of cloud that forms
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

15. Type of cloud that forms
Very moist airmasses (high dew point) can create larger clouds with lower bases
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

16. Type of cloud that forms
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

17. Type of cloud that forms
Stable airmasses will produce little, if any, cloud cover – What does form will be “stratiform”
Neutral airmasses are more likely to produce stratiform clouds and continuous precipitation
Unstable airmasses will produce puffy, “cumuliform” clouds and showery precipitation.
Type of lifting mechanism
Convection
Topography
Low-level convergence and fronts
Scale and duration of lifting mechanism
Widespread or localized
Short or long term
Amount of water vapor available
Stability

18. Cloud Classification

19. Sources
International Cloud Atlas, published in 1956 by the World Meteorological Organization.
My webpage: vortex.plymouth.edu/~stmiller/. See “Tutorials,” then “Cloud Classification.” This is based on the Federal Meteorological Handbook and the International Cloud Atlas.
Various meteorology textbooks.

20. Cloud nomenclature originally proposed in 1802
Sources
International Cloud Atlas, published in 1956 by the World Meteorological Organization.
My webpage: vortex.plymouth.edu/~stmiller/. See “Tutorials,” then “Cloud Classification.” This is based on the Federal Meteorological Handbook and the International Cloud Atlas.
Textbook: Essentials of Meteorology (Ahrens, 2005).
Based on work of Luke Howard, 19th Century British pharmacist, chemist and amateur meteorologist
Cloud nomenclature originally proposed in 1802

21. Cloud classification is based on:
Height of the base (determines “etage”)
Physical appearance (determines “form” and “genera”)

22. Cloud Etages (Mid-Latitudes)
Low etage
Bases below 2,000 m (6,500 ft)
Usually composed of water droplets
May build upward into the mid- and high etages
Middle etage
Bases between 2,000 and 7,000 m (6,500 and 23,000 ft)
Some overlap with high clouds
Composed of water droplets
(mixed with ice crystals when temp is low enough)
High etage
Bases generally above 5,000 m (16,000 ft)
Usually composed of ice crystals

23. Cloud Forms and Genera FORMS
Cumuliform: “Heaped up” – significant vertical development – Unstable
Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable
Cirriform: Feather-like, generally composed of ice crystals
LOW ETAGE MIDDLE ETAGE HIGH ETAGE
(St) Stratus (AS) Altostratus (Ci) Cirrus
(NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus
(SC) Stratocumulus (CC) Cirrocumulus
Low etage clouds with significant vertical development
(Cu) Cumulus
(Cb) Cumulonimbus
Note: This is not exhaustive.

24. Cloud Forms and Genera FORMS
Cumuliform: “Heaped up” – significant vertical development – Unstable
Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable
Cirriform: Feather-like, generally composed of ice crystals
LOW ETAGE MIDDLE ETAGE HIGH ETAGE
(St) Stratus (AS) Altostratus (Ci) Cirrus
(NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus
(SC) Stratocumulus (CC) Cirrocumulus
Low etage clouds with significant vertical development
(Cu) Cumulus
(Cb) Cumulonimbus
Note: This is not exhaustive.

25. Cloud Forms and Genera FORMS
Cumuliform: “Heaped up” – significant vertical development – Unstable
Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable
Cirriform: Feather-like, generally composed of ice crystals
LOW ETAGE MIDDLE ETAGE HIGH ETAGE
(St) Stratus (AS) Altostratus (Ci) Cirrus
(NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus
(SC) Stratocumulus (CC) Cirrocumulus
Low etage clouds with significant vertical development
(Cu) Cumulus
(Cb) Cumulonimbus
Note: This is not exhaustive.

26. Cloud Forms and Genera FORMS
Cumuliform: “Heaped up” – significant vertical development – Unstable
Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable
Cirriform: Feather-like, generally composed of ice crystals
LOW ETAGE MIDDLE ETAGE HIGH ETAGE
(St) Stratus (AS) Altostratus (Ci) Cirrus
(NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus
(SC) Stratocumulus (CC) Cirrocumulus
Low etage clouds with significant vertical development
(Cu) Cumulus
(Cb) Cumulonimbus
Note: This is not exhaustive.

27. 1. Low Clouds (St) Stratus (NS) Nimbostratus (SC) Stratocumulus
base below 2,000 m (6,500 ft)
composed of water droplets
(St) Stratus
(NS) Nimbostratus
Considered a mid-cloud in the International Cloud Atlas and Federal Meteorological Handbook
(SC) Stratocumulus

28. Stratus

29. Stratus Lifting mechanism(s): None, or weak upslope
Scale and duration: Local – regional; hours
Amount of water vapor: Varied
Stability: Stable

30. Stratus Fractus

31. Stratus Fractus Lifting mechanism(s): None
Scale and duration: Local; minutes
Amount of water vapor: Varied
Stability: Stable

32. Stratus Abbreviated St May cover the entire sky
Stratus – Latin for layer or veil
Stratus Fractus – Ragged bits of stratus
Resembles fog above the ground
No precipitation (or only very light drizzle)
Usually cannot see Sun through it

33. Stratocumulus

34. Stratocumulus

35. Stratocumulus Lifting mechanism(s): All
Scale and duration: Local – regional; hours
Amount of water vapor: small - moderate
Stability: Neutral - stable

36. Stratocumulus Abbreviated SC Marks a half-way point between Cu and St
Lumpy cloud layer
Blue sky visible in between
Individual “lumps”
Size of fist when arm extended

37. Nimbostratus

38. Nimbostratus and Stratus Fractus

39. Nimbostratus Lifting mechanism(s): Warm frontal
Scale and duration: Regional; hours - days
Amount of water vapor: High
Stability: Neutral - stable

40. Nimbostratus Abbreviated NS Steady precipitation
Nimbo – rain
Stratus – layer or veil
Often accompanied by Stratus Fractus
Associated with widespread weather systems, esp. in winter months

41. 2. Low clouds with significant vertical development
(Cu) Cumulus
(CB) Cumulonimbus

42. Cumulus

43. Cumulus

44. Cumulus
Lifting mechanism(s): Surface heating (convection); weak upslope; or cold frontal
Scale and duration: Local (elements); minutes - hours
Amount of water vapor: Small
Stability: Slightly unstable

45. Cu congestus (Towering Cumulus)

46. Cu congestus (Towering Cumulus)
Lifting mechanism(s): Surface heating (convection), upslope, or cold frontal
Scale and duration: Local (elements); minutes - hours
Amount of water vapor: Small - moderate
Stability: Unstable
Cu congestus (Towering Cumulus)

47. Cumulus Abbreviated Cu White or gray puffy clouds
Generally fair weather
Composition: water droplets
Cumulus Congestus
Also known as Towering cumulus (TCu)
May produce light showers

48. Cumulonimbus

49. Cumulonimbus

50. Cumulonimbus

51. Cumulonimbus

52. Cumulonimbus
Lifting mechanism(s): Surface heating (convection), upslope, or cold frontal
Scale and duration: Local (elements); hours
Amount of water vapor: Moderate - high
Stability: Very unstable

53. Cumulonimbus Thunderstorm cloud Abbreviated CB Dark base, anvil shaped
Nimbus – Latin for rain
Abbreviated CB
Dark base, anvil shaped
Extends to tropopause (base of stratosphere)
Composition varies with height
Water droplets near surface
Ice crystals higher up
May produce tornadoes/funnel clouds
Heavy precipitation
Showery
Rain, hail, snow

54. Cumulonimbus Often have other cloud structures associated with them
Some of these are:
Shelf clouds
Mammatus
Overshooting domes
Wall clouds

56. Shelf cloud

58. Mammatus

59. Mammatus

60. Mammatus

61. Mammatus

62. Mammatus

64. Overshooting top

65. Overshooting top

66. Overshooting top

67. Overshooting top

68. Overshooting top

70. Wall Cloud

71. Wall Cloud

72. Wall Cloud

73. 3. Middle Clouds (AS) Altostratus (AC) Altocumulus 2,000 m to 7,000 m
6,500 ft to 23,000 ft
Some overlap with high clouds
composed of water droplets
(mixed with ice crystals when temp is low enough)
(AS) Altostratus
(AC) Altocumulus

74. Altostratus

75. Altostratus

76. Altostratus Lifting mechanism(s): Usually warm frontal
Scale and duration: Regional; hours - days
Amount of water vapor: Moderate - high
Stability: Stable – neutral

77. Altostratus Abbreviated AS Covers entire sky Sun dimly seen though it
No shadows cast

78. Altocumulus

79. Altocumulus

80. Altocumulus

81. Altocumulus Lifting mechanism(s): Upper-level divergence
Scale and duration: Regional; hours - days
Amount of water vapor: Low - moderate
Stability: Stable – neutral

82. Altocumulus Abbreviated AC Resembles SC elevated to middle etage
Grey puffy masses
Individual “lumps”
Size of thumbnail when arm extended

83. 4. High Clouds (Ci) Cirrus (CS) Cirrostratus (CC) Cirrocumulus
base generally above 5,000 m (16,000 ft)
composed of ice crystals
(Ci) Cirrus
(CS) Cirrostratus
(CC) Cirrocumulus

84. Cirrus

85. Cirrus

86. Cirrus

87. Cirrus Lifting mechanism(s): None
Scale and duration: Regional; hours - days
Amount of water vapor: Low
Stability: Stable – neutral

88. Cirrus Abbreviated Ci Thin, wispy, feathery clouds
Cirro – Latin for feather
Associated with fair weather
Composed primarily of ice crystals

89. Cirrostratus

90. Cirrostratus with halo

91. Cirrostratus with everything!

92. (Photo taken in Finland, October 2010. Source: www.spaceweather.com)

93. Cirrostratus with halo
Lifting mechanism(s): None
Scale and duration: Regional; hours - days
Amount of water vapor: Low
Stability: Stable – neutral

94. Cirrostratus Abbreviated CS
Thin, sheet-like clouds that often cover the whole sky
Sun or Moon easily visible
Can be accompanied by a halo and other optical phenomena

95. Cirrocumulus

96. Cirrocumulus

97. Cirrocumulus

98. Cirrocumulus Lifting mechanism(s): Upper-level divergence
Scale and duration: Regional; hours - days
Amount of water vapor: Low
Stability: Neutral

99. Cirrocumulus Abbreviated CC Small rounded white puffs
Size of pencil eraser held at arm’s length
Resembles scales of a fish
“Mackerel sky”
Less common than Ci and CS
More likely to occur in tropics than in mid-latitudes

100. Differentiating Similar Clouds

101. Distinguishing between SC, AC and CC
fist

102. Distinguishing between SC, AC and CC
thumb nail

103. Distinguishing between SC, AC and CC
fish scales

104. Distinguishing between St, AS and CS
no sun no shadows

105. Distinguishing between St, AS and CS
dim sun no shadows

106. Distinguishing between St, AS and CS
brighter sun shadows and halos

107. Review

108. Type of cloud that forms depends on four things:
Type of lifting mechanism
Scale and duration of lifting mechanism
The amount of water vapor available
Stability
Clouds are classified by form and the height of the bases
Three forms
Three etages

109. Precipitation-producing clouds
Types of front associated with each
Stability conditions associated with each
Distinguish between St, AS and CS
Distinguish between SC, AC and CC

110. Additional Graphics Sources