1. GEOG 100:
Physical Geography
Clouds and Precipitation:
The Transfer of Latent Heat

2. The Global Water Budget
• Earth has a global water budget—if water
is lost in one place or in one form, it is
moved to another place or another form
• The total amount of water (in whatever
form) varies from place to place, but stays
constant over the planet as a whole

3. Where is all of Earth’s water found?
• Oceans = 97.2%
• Glaciers = 2.0%
• Underground sources
(aquifers, underground pools & groundwater) = 0.5%
• Lakes (half saline, half fresh) = 0.2%
• Pore spaces in soil (“soil water”) = 0.04%
• Atmospheric water, streams, living things = 0.01%

4. Residence Time
• The amount of time a given amount of
water may remain in a particular segment
of the hydrologic cycle is its residence
time.
• Residence time can vary from hours
(evaporation followed by a
thundershower), to millions of years
(trapped in deep aquifers)

5. Residence Time
• As water changes its “residence,” it may
also change state.
• When water changes state it moves
around latent heat. The evaporation and
condensation phase changes are
especially significant...

6. Residence Time
• As water changes its “residence,” it may
also change state.
• When water changes state it moves
around latent heat. The evaporation and
condensation phase changes are
especially significant...
How about a diagram???

7. Latent Heat Transfer
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water vapor (gas) water (liquid)
evaporation—latent heat absorbed
condensation—latent heat released

8. Saturation

9. Saturation
 The saturation point is the point at which a given
parcel of air is holding the maximum amount of water
vapor that it can possibly hold at a given temperature
and pressure.

10. Saturation
 The saturation point is the point at which a given
parcel of air is holding the maximum amount of water
vapor that it can possibly hold at a given temperature
and pressure.
– Temperature is the key!

11. Saturation
 The saturation point is the point at which a given
parcel of air is holding the maximum amount of water
vapor that it can possibly hold at a given temperature
and pressure.
– Temperature is the key!
 If the air is not saturated, evaporation can continue,
as long as there is moisture available to be
evaporated

12. Three Factors Influencing
the Rate of Evaporation

13. Three Factors Influencing
the Rate of Evaporation
1. Temperature of the water

14. Three Factors Influencing
the Rate of Evaporation
1. Temperature of the water
– The warmer the water, the faster the
molecules are moving and the more likely
they will be able to escape the surface
(evaporate)

15. Three Factors Influencing
the Rate of Evaporation

16. Three Factors Influencing
the Rate of Evaporation
2. Temperature of the air

17. Three Factors Influencing
the Rate of Evaporation
2. Temperature of the air
– Warm air can hold more water vapor
suspended in it

18. Three Factors Influencing
the Rate of Evaporation
2. Temperature of the air
– Warm air can hold more water vapor
suspended in it
– Warm air transfers heat to the water and
speeds up water molecules to the point
where they can evaporate

19. Three Factors Influencing
the Rate of Evaporation
2. Temperature of the air
– Warm air can hold more water vapor
suspended in it
– Warm air transfers heat to the water and
speeds up water molecules to the point
where they can evaporate
– Cold air can hold less water as a vapor
and reaches its saturation point more
quickly

20. Three Factors Influencing
the Rate of Evaporation

21. Three Factors Influencing
the Rate of Evaporation
3. Degree of windiness

22. Three Factors Influencing
the Rate of Evaporation
3. Degree of windiness
– Saturation is reached quickly right above
the water

23. Three Factors Influencing
the Rate of Evaporation
3. Degree of windiness
– Saturation is reached quickly right above
the water
– Wind blowing over a wet surface will
reduce saturation above that surface by
moving water vapor molecules away from
the surface. This leaves room for more
molecules to evaporate

24. Vapor Pressure
 Vapor pressure--the portion of total air
pressure made up of water vapor molecules
 Saturation vapor pressure--the pressure
exerted by the maximum amount of water
vapor a parcel of air can hold at a given
temperature.
11

26. Relative Humidity

27. Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water vapor which
could be in the air if it were saturated

28. Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water vapor which
could be in the air if it were saturated
RH = actual/maximum x 100 = ___ %

29. Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water vapor which
could be in the air if it were saturated
RH = actual/maximum x 100 = ___ %
 RH = relative humidity

30. Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water vapor which
could be in the air if it were saturated
RH = actual/maximum x 100 = ___ %
 RH = relative humidity
 Actual = the actual amount of water vapor in the air right now

31. Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water vapor which
could be in the air if it were saturated
RH = actual/maximum x 100 = ___ %
 RH = relative humidity
 Actual = the actual amount of water vapor in the air right now
 Maximum = the maximum amount of water vapor the air can hold
at the given temperature and pressure (in other words, saturation
point)

32. RH Example:

33. RH Example:
If the room you’re sitting in has 5 grams of
water vapor actually suspended in it, but the
maximum amount of water vapor that the air
could possibly hold is 10 grams, then:

34. RH Example:
If the room you’re sitting in has 5 grams of
water vapor actually suspended in it, but the
maximum amount of water vapor that the air
could possibly hold is 10 grams, then:
RH = actual/maximum x 100 = ___ %

35. RH Example:
If the room you’re sitting in has 5 grams of
water vapor actually suspended in it, but the
maximum amount of water vapor that the air
could possibly hold is 10 grams, then:
RH = actual/maximum x 100 = ___ %

36. RH Example:
If the room you’re sitting in has 5 grams of
water vapor actually suspended in it, but the
maximum amount of water vapor that the air
could possibly hold is 10 grams, then:
RH = actual/maximum x 100 = ___ %
RH = 5g / 10g x 100 = 50%

37. Relative Humidity

38. Relative Humidity
 What happens when relative humidity
reaches 100%?

39. Relative Humidity
 What happens when relative humidity
reaches 100%?
– Saturation

40. Relative Humidity
 What happens when relative humidity
reaches 100%?
– Saturation
– Condensation

41. Relative Humidity
 What happens when relative humidity
reaches 100%?
– Saturation
– Condensation
 Clouds or fog (if cooling continues)

42. Two Ways to Change
Relative Humidity

43. Two Ways to Change
Relative Humidity
 Change the temperature of the air

44. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up

45. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up

46. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up

47. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up

48. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up
 Add or subtract water vapor

49. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up
 Add or subtract water vapor
– In the atmosphere, water is added through
evaporation, or lost through precipitation (rain, snow,
etc.)

50. The Dew Point
 The dew point is the temperature at which
saturation is reached.

51. The Dew Point
 The dew point is the temperature at which
saturation is reached.
Note: The temperature
at which the dew point
is reached depends on
variables such as
absolute humidity. For
example...

52. The Dew Point
 The dew point is the temperature at which
saturation is reached.
Note: The temperature
at which the dew point
is reached depends on
variables such as
absolute humidity. For
example...
At 10g water vapor/m3,

53. The Dew Point
 The dew point is the temperature at which
saturation is reached.
Note: The temperature
at which the dew point
is reached depends on
variables such as
absolute humidity. For
example...
At 10g water vapor/m3,
dew pt. = 50℉

54. The Dew Point
 The dew point is the temperature at which
saturation is reached.
Note: The temperature
at which the dew point
is reached depends on
variables such as
absolute humidity. For
example...
At 10g water vapor/m3,
dew pt. = 50℉
At 20g/m3,

55. The Dew Point
 The dew point is the temperature at which
saturation is reached.
Note: The temperature
at which the dew point
is reached depends on
variables such as
absolute humidity. For
example...
At 10g water vapor/m3,
dew pt. = 50℉
At 20g/m3,
dew pt. = 68℉

56. The Adiabatic Process
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57. The Adiabatic Process

58. The Adiabatic Process
 The process by which rising air cools (as it
expands) and sinking air warms (as it is
compressed) in the atmosphere

59. The Adiabatic Process
 The process by which rising air cools (as it
expands) and sinking air warms (as it is
compressed) in the atmosphere
 The physical principle involved:

60. The Adiabatic Process
 The process by which rising air cools (as it
expands) and sinking air warms (as it is
compressed) in the atmosphere
 The physical principle involved:
– When a gas expands, it cools

61. The Adiabatic Process
 The process by which rising air cools (as it
expands) and sinking air warms (as it is
compressed) in the atmosphere
 The physical principle involved:
– When a gas expands, it cools
– When a gas is compressed, it warms

62. The Adiabatic Process

63. The Adiabatic Process
 As an air mass rises through the atmosphere, it
moves into an area of lower density, allowing the
molecules the freedom to expand.

64. The Adiabatic Process
 As an air mass rises through the atmosphere, it
moves into an area of lower density, allowing the
molecules the freedom to expand.
 As air expands, there are fewer collisions between
molecules and the air begins to cool.

65. The Adiabatic Process
 As an air mass rises through the atmosphere, it
moves into an area of lower density, allowing the
molecules the freedom to expand.
 As air expands, there are fewer collisions between
molecules and the air begins to cool.
 So rising air expands and cools down. If the air mass
cools enough to reach the dew point temperature,
condensation will occur and a cloud will form.

66. The Adiabatic Process

67. The Adiabatic Process
 On the other hand, a sinking air mass will move
down through the atmosphere into a region of
increasingly more molecules of air.

68. The Adiabatic Process
 On the other hand, a sinking air mass will move
down through the atmosphere into a region of
increasingly more molecules of air.
 The pressure of all of these molecules will compress
the air mass, forcing the molecules closer to one
another.

69. The Adiabatic Process
 On the other hand, a sinking air mass will move
down through the atmosphere into a region of
increasingly more molecules of air.
 The pressure of all of these molecules will compress
the air mass, forcing the molecules closer to one
another.
 This increases the number of molecular collisions,
speeding up the molecules, which translates into an
increase in temperature.

70. The Adiabatic Process
 On the other hand, a sinking air mass will move
down through the atmosphere into a region of
increasingly more molecules of air.
 The pressure of all of these molecules will compress
the air mass, forcing the molecules closer to one
another.
 This increases the number of molecular collisions,
speeding up the molecules, which translates into an
increase in temperature.
 So sinking air is compressed and warms up.

71. The DAR

72. The DAR
 The rate at which unsaturated air will cool as
it rises is called the Dry Adiabatic lapse
Rate, or DAR (the air is not actually “dry”, it’s
just not saturated).

73. The DAR
 The rate at which unsaturated air will cool as
it rises is called the Dry Adiabatic lapse
Rate, or DAR (the air is not actually “dry”, it’s
just not saturated).
 Although this rate can vary based on several
atmospheric variables, a commonly-used
average value is:

74. The DAR
 The rate at which unsaturated air will cool as
it rises is called the Dry Adiabatic lapse
Rate, or DAR (the air is not actually “dry”, it’s
just not saturated).
 Although this rate can vary based on several
atmospheric variables, a commonly-used
average value is:
10ºC/1000m (5.5ºF/1000ft)

75. The LCL

76. The LCL
 The lifting condensation level (LCL) is the
elevation at which condensation occurs.

77. The LCL
 The lifting condensation level (LCL) is the
elevation at which condensation occurs.
 As it rises, expands, and cools, the air’s
relative humidity increases (getting closer to
100%) until eventually the air parcel reaches
its dew point temperature.

78. The LCL
 The lifting condensation level (LCL) is the
elevation at which condensation occurs.
 As it rises, expands, and cools, the air’s
relative humidity increases (getting closer to
100%) until eventually the air parcel reaches
its dew point temperature.
 At that point, saturation has been reached
and a cloud begins to form.

79. The LCL
 The lifting condensation level (LCL) is the
elevation at which condensation occurs.
 As it rises, expands, and cools, the air’s
relative humidity increases (getting closer to
100%) until eventually the air parcel reaches
its dew point temperature.
 At that point, saturation has been reached
and a cloud begins to form.
 The elevation where this happens is the LCL.

80. You can “see” the LCL:

81. You can “see” the LCL:

82. You can “see” the LCL:
Look at the flat bottom of the cloud

83. A Quick Reminder!

84. A Quick Reminder!
The following five conditions all occur at
the same time:

85. A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation

86. A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation
 Condensation

87. A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation
 Condensation
 RH=100%

88. A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation
 Condensation
 RH=100%
 Dew point temperature

89. A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation
 Condensation
 RH=100%
 Dew point temperature
 LCL (lifting condensation level)

90. The Latent Heat of Condensation

91. The Latent Heat of Condensation
 Once condensation occurs, the water
molecules begin to give off the latent heat of
condensation. This heat becomes sensible
heat that can be measured.

92. The Latent Heat of Condensation
 Once condensation occurs, the water
molecules begin to give off the latent heat of
condensation. This heat becomes sensible
heat that can be measured.
 This heat interferes with the adiabatic cooling
that is going on, slowing down the cooling
process. So the air continues to get colder as
it rises, but it cools at a slower rate.

93. The SAR (or MAR)

94. The SAR (or MAR)
 The rate at which a saturated parcel of air will
cool as it rises is called the Saturated
Adiabatic lapse Rate, or SAR (also called
the MAR, or moist adiabatic lapse rate)
 Again, the rate varies, but we’ll use an
average value of:

95. The SAR (or MAR)
 The rate at which a saturated parcel of air will
cool as it rises is called the Saturated
Adiabatic lapse Rate, or SAR (also called
the MAR, or moist adiabatic lapse rate)
 Again, the rate varies, but we’ll use an
average value of:
5ºC/1000m (3.3ºF/1000ft)

96. The SAR (or MAR)
 The rate at which a saturated parcel of air will
cool as it rises is called the Saturated
Adiabatic lapse Rate, or SAR (also called
the MAR, or moist adiabatic lapse rate)
 Again, the rate varies, but we’ll use an
average value of:
5ºC/1000m (3.3ºF/1000ft)
 As the air parcel continues to rise, it
continues to cool, though more slowly.

97. Stability vs. Buoyancy

98. Stability vs. Buoyancy
 Buoyancy
– The tendency of a substance to rise, especially in a fluid

99. Stability vs. Buoyancy
 Buoyancy
– The tendency of a substance to rise, especially in a fluid
 An air-filled balloon (or you!) in water

100. Stability vs. Buoyancy
 Buoyancy
– The tendency of a substance to rise, especially in a fluid
 An air-filled balloon (or you!) in water
 A helium balloon in air
– Density is the key
 Equilibrium level

101. Stability vs. Buoyancy
 Buoyancy
– The tendency of a substance to rise, especially in a fluid
 An air-filled balloon (or you!) in water
 A helium balloon in air
– Density is the key
 Equilibrium level
– Where both the rising and the still air are the same density
 The opposite of buoyancy is stability

102. Stability vs. Buoyancy
 Buoyancy
– The tendency of a substance to rise, especially in a fluid
 An air-filled balloon (or you!) in water
 A helium balloon in air
– Density is the key
 Equilibrium level
– Where both the rising and the still air are the same density
 The opposite of buoyancy is stability
– The substance does NOT want to rise

103. Stable air

104. Unstable air

105. Conditionally unstable air

107. Four Causes of Uplift:
1. Convective Uplift

108. 2. Frontal lifting

109. 3. Convergent lifting

110. 4. Orographic Lifting

111. 4. Orographic Lifting
Waaaait…something else is going on here…the air is sinking on the other side!

112. So what happens to temperature if the air
sinks on the other side of the mountain?

113. So what happens to temperature if the air
sinks on the other side of the mountain?
 You can not have greater than 100% relative humidity on the
way up (except in rare cases like a lack of condensation
nuclei).

114. So what happens to temperature if the air
sinks on the other side of the mountain?
 You can not have greater than 100% relative humidity on the
way up (except in rare cases like a lack of condensation
nuclei).
 Anything over 100% will condense into liquid water droplets,
forming a cloud. Right?

115. So what happens to temperature if the air
sinks on the other side of the mountain?
 You can not have greater than 100% relative humidity on the
way up (except in rare cases like a lack of condensation
nuclei).
 Anything over 100% will condense into liquid water droplets,
forming a cloud. Right?
 So, as air sinks, it is compressed by the weight of more air
above it and it begins to warm up adiabatically. It moves away
from 100% RH (99%, 98%, 97%...and so on).

116. So what happens to temperature if the air
sinks on the other side of the mountain?
 You can not have greater than 100% relative humidity on the
way up (except in rare cases like a lack of condensation
nuclei).
 Anything over 100% will condense into liquid water droplets,
forming a cloud. Right?
 So, as air sinks, it is compressed by the weight of more air
above it and it begins to warm up adiabatically. It moves away
from 100% RH (99%, 98%, 97%...and so on).
 Because RH becomes < 100% as soon as the air begins to
sink, we can’t use the Saturated Adiabatic Lapse Rate any
more. (It is only used when RH = 100%.)

117. So what happens to temperature if the air
sinks on the other side of the mountain?
 You can not have greater than 100% relative humidity on the
way up (except in rare cases like a lack of condensation
nuclei).
 Anything over 100% will condense into liquid water droplets,
forming a cloud. Right?
 So, as air sinks, it is compressed by the weight of more air
above it and it begins to warm up adiabatically. It moves away
from 100% RH (99%, 98%, 97%...and so on).
 Because RH becomes < 100% as soon as the air begins to
sink, we can’t use the Saturated Adiabatic Lapse Rate any
more. (It is only used when RH = 100%.)
 It therefore warms as it sinks at the Dry Adiabatic Lapse Rate,
the whole way back down.

118. This is why the back side of a mountain is
hotter and drier than the side facing the wind
windward side leeward side

119. Condensation nuclei and cloud droplets

120. Classifying Clouds

121. Classifying Clouds
Are you paying attention?

123. Extra Credit Section!!!

125. Cloud types…

126. Fog: A cloud on the ground

127. The Four Common Types of Fog

128. Dew: Condensation on Earth’s surface

130. Formation of Precipitation:
The Bergeron Process

131. The Collision-coalescence process

132. Some Different Forms of
Precipitation
• Rain
– Drizzle vs. showers
• Snow
• Sleet
• Glaze (ice storm)
• Hail

133. The Formation of Hail

134. The Formation of Hail

135. Hail

136. Hail

137. Hail

138. Hail

139. Hail

140. Global Precipitation

141. Global Precipitation:
The ITCZ Connection

142. Precipitation in the U.S.

143. Acidity

144. Acid Precipitation in the U.S.