1. Class #3: Humidity, condensation, and clouds
Chapters 4 and 5
Class #3 July 9, 2010

2. Chapter 4
Atmospheric Humidity
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3. Circulation of Water in the Atmosphere
A general definition of humidity is the amount of water vapor in the air.
Remember, humidity is not constant through time or space, there is constant circulation of water through the hydrologic cycle.
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4. FIGURE 4.1 The hydrologic cycle.
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5. Stepped Art Figure 4.1: The hydrologic cycle. Class #3 July 9, 2010
Fig. 4-1, p. 90

6. Figure 4.2
The water molecule.
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Fig. 4-2, p. 91

7. The Many Phases of Water
Phase is related to molecular motion, an increase or decrease in motion creates a phase change.
Ice is the coolest/slowest phase
Water vapor is the warmest/fastest phase
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8. FIGURE 4.3 The three states of matter. Water as a gas, as a liquid,
and as a solid.
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9. Evaporation, Condensation, & Saturation
Evaporation is the change of liquid into a gas a requires heat.
Condensation is the change of a gas into a liquid and releases heat.
Condensation nuclei
Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses.
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10. FIGURE 4.4 (a) Water molecules at the surface of the water are
evaporating (changing from liquid into vapor) and condensing (changing
from vapor into liquid). Since more molecules are evaporating than
condensing, net evaporation is occurring. (b) When the number of water
molecules escaping from the liquid (evaporating) balances those returning
(condensing), the air above the liquid is saturated with water
vapor. (For clarity, only water molecules are illustrated.)
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11. FIGURE 4.5 Condensation is more likely to occur as the air cools.
(a) In the warm air, fast-moving H2O vapor molecules tend to bounce
away after colliding with nuclei. (b) In the cool air, slow-moving vapor
molecules are more likely to join together on nuclei. The condensing of
many billions of water molecules produces tiny liquid water droplets.
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12. Humidity
Any of a number of ways of specifying the amount of water vapor in the air.
Absolute humidity: mass of water vapor/volume of air
Water vapor density
Not commonly used due to frequent change of volume
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13. FIGURE 4.6 The water vapor content (humidity) inside
this air parcel can be expressed in a number of ways.
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14. FIGURE 4.7 With the same amount of water vapor in a
parcel of air, an increase in volume decreases absolute humidity,
whereas a decrease in volume increases absolute humidity.
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15. Humidity Specific Humidity: mass of water vapor/mass of air
Mixing ratio: mass of water vapor/mass of dry air
Neither measurement changes with volume, must add or subtract water vapor.
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16. FIGURE 4.8 The specific humidity does not change as air rises
and descends.
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17. FIGURE 4.9 The average specific humidity for each latitude. The
highest average values are observed in the tropics and the lowest values
in polar regions.
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18. Humidity
Vapor pressure: the pressure exerted by water vapor molecules in an air parcel (Dalton’s Law of Partial Pressure)
Fraction of total vapor pressure (1% or so)
More water molecules = high vapor pressure
Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature
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19. ACTIVE FIGURE 4.10 Saturation vapor pressure increases with
increasing temperature. At a temperature of 10oC, the saturation vapor
pressure is about 12 mb, whereas at 30oC it is about 42 mb. The insert
illustrates that the saturation vapor pressure over water is greater than
the saturation vapor pressure over ice.
Visit the Meteorology Resource Center to view this and other active
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20. Humidity Special Topic: Vapor Pressure & Boiling
Once water boils it requires more energy to increase temperature.
Water boils at a low temperature in the mountains and thus needs more energy and time to cook items as compared to sea level.
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21. Humidity
Relative Humidity: (actual water vapor/saturation water vapor)*100
RH can be changed two ways:
Change vapor content
Change saturation
Decrease temperature causes an increase in relative humidity (inverse relationship).
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22. FIGURE 4.11 (a) At the same air temperature, an increase in the
water vapor content of the air increases the relative humidity as the air
approaches saturation. (b) With the same water vapor content, an increase
in air temperature causes a decrease in relative humidity as the
air moves farther away from being saturated.
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23. Humidity Relative Humidity and Dew Point
Dew point is the temperature at which saturation occurs
Cool air parcel to dew point and liquid water condenses
A good measure of actual water vapor content
Relative humidity indicates how close to saturation, dew point indicates the amount of water vapor
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24. FIGURE 4.12 When the air is cool (morning), the relative humidity
is high. When the air is warm (afternoon), the relative humidity
is low. These conditions exist in clear weather when the air is calm or of
constant wind speed.
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25. Figure 4.13
On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.)
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Fig. 4-13, p. 98

26. Figure 4.13
On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.)
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Fig. 4-13, p. 98

27. Figure 4.13
On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.)
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Fig. 4-13, p. 98

28. Figure 4.13
On a calm, clear night, the lower the dew-point temperature, the lower the expected minimum temperature. With the same initial evening air temperature (80ºF) and with no change in weather conditions during the night, as the dew point lowers, the expected minimum temperature lowers. This situation occurs because a lower dew point means that there is less water vapor in the air to absorb and radiate infrared energy back to the surface. More infrared energy from the surface is able to escape into space, producing more rapid radiational cooling at the surface. (Dots in each diagram represent the amount of water vapor in the air. Red wavy arrows represent infrared (IR) radiation.)
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Fig. 4-13, p. 98

29. FIGURE 4.14 Average surface
dew-point temperatures (oF) for
(a) January and for (b) July.
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30. FIGURE 4.14 Average surface
dew-point temperatures (oF) for
(a) January and for (b) July.
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31. FIGURE 4.16 Relative humidity averaged for latitudes north and
south of the equator.
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32. FIGURE 4.17 Air from the Pacific Ocean is hot and dry over land, whereas air from the Gulf of Mexico is hot and
muggy over land. For each city, T represents the air temperature, Td the dew point, and RH the relative humidity. (All data
represent conditions during a July afternoon at 3 p.m. local time.)
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33. Figure 4.15
The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor.
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Fig. 4-15a, p. 100

34. Figure 4.15
The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor.
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Fig. 4-15b, p. 100

35. Humidity Relative Humidity in the Home
Due to an increase in temperature in a heated home there is a decrease in relative humidity, causing more evaporation from body, plants, etc
Humidifier, chapped lips
Swamp cooler
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36. FIGURE 4.18 When outside air with an air temperature and a
dew point of o15oC (5oF) is brought indoors and heated to a temperature
of 20oC (68oF) (without adding water vapor to the air), the relative
humidity drops to 8 percent, placing adverse stress on plants, animals,
and humans living inside. (T represents temperature; Td, dew point; and
RH, relative humidity.)
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37. Humidity Relative humidity & human comfort
“It’s not the heat, it’s the humidity.”
High relative humidity equates to less evaporative cooling.
Sweat cannot evaporate and cool the body
Wet bulb temperature
Heat Index
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38. FIGURE 4.19
Air temperature (oF) and
relative humidity are combined
to determine an apparent
temperature or heat index
(HI). An air temperature of
95oF with a relative humidity
of 55 percent produces an
apparent temperature (HI) of
110oF.
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39. Humidity Special Topic: Heavier humid air
Due to the molecular weight of water as compared to nitrogen, humid air is lighter than dry air.
Baseball announcers are incorrect.
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40. Humidity Measuring humidity Sling psychrometer Hygrometer
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41. FIGURE 4.20 The sling psychrometer.
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42. Condensation: DEW, Fog, & clouds
Chapter 5
Condensation: DEW, Fog, & clouds
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43. The Formation of Dew & Frost
Dew forms on objects near the ground surface when they cool below the dew point temperature.
More likely on clear nights due to increased radiative cooling
White frost forms when temperature cools below the dew point and the dew point is below 0°C
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44. FIGURE 5.1 Dew forms on clear nights when objects on the surface
cool to a temperature below the dew point. If these beads of water
should freeze, they would become frozen dew.
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45. FIGURE 5.2 These are the delicate ice-crystal patterns that frost
exhibits on a window during a cold winter morning.
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46. Condensation Nuclei
Particles suspended in the air that around which water condenses or freezes.
Hydrophobic/hygroscopic
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47. Class #3 July 9, 2010
Table 5-1, p. 113

48. FIGURE 5.1 Dew forms on clear nights when objects on the surface
cool to a temperature below the dew point. If these beads of water
should freeze, they would become frozen dew.
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49. FIGURE 5.3 Hygroscopic nuclei are “water-seeking,” and water
vapor rapidly condenses on their surfaces. Hydrophobic nuclei are “water-
repelling” and resist condensation.
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50. Haze
Dry condensation nuclei (above dew point) reflect and scatter sunlight creating blueish haze.
Wet condensation nuclei (75% relative humidity) reflect and scatter sunlight creating grayish or white haze.
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51. Figure 5.4
The high relative humidity of the cold air above the lake is causing a layer of haze to form on a still winter morning.
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Fig. 5-4, p. 114

52. Fog Saturation reached condensation forms a cloud near the ground
Radiation fog: ground cools through conduction and radiation; ground fog
Valley fog created by cold air drainage
High inversion fog
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53. FIGURE 5.5 Radiation fog nestled in a valley.
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54. FIGURE 5.6 Visible satellite image of dense radiation fog in the
southern half of California’s Central Valley on the morning of November
20, The white region to the east (right) of the fog is the snowcapped
Sierra Nevada range. During the late fall and winter, the fog,
nestled between two mountain ranges, can last for many days without
dissipating. The fog on this day was responsible for several auto accidents,
including a 14-car pileup near Fresno.
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55. Fog
Advection Fog: warm moist fog moves horizontally (advects) over a cool surface.
Summer fog on the Pacific coast
Observation: Headlands
Air converges and rises over headlands forming fog as compared to lower elevation beaches.
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56. FIGURE 5.7 Advection fog forms as the wind
moves moist air over a cooler surface. Here advection
fog, having formed over the cold, coastal water
of the Pacific Ocean, is rolling inland past the
Golden Gate Bridge in San Francisco. As fog moves
inland, the air warms and the fog lifts above the surface.
Eventually, the air becomes warm enough to totally
evaporate the fog.
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57. FIGURE 5.9 (a) Radiation fog tends
to form on clear, relatively calm nights when
cool, moist surface air is overlain by drier
air and rapid radiational cooling occurs.
(b) Advection fog forms when the wind
moves moist air over a cold surface and the
moist air cools to its dew point.
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58. Fog Upslope Fog: moist air flows up an orographic barrier
East side of the Rockies
Evaporation Fog:
Warm moist surface provides enough moisture to saturate a dry air parcel; short lived
Steam fog
Breath in winter
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59. FIGURE 5.10 Upslope fog forms as moist air slowly rises, cools,
and condenses over elevated terrain.
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60. FIGURE 5.11 Even in summer, warm air rising above thermal
pools in Yellowstone National Park condenses into a type of steam fog.
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61. Foggy Weather
In general fog not common for most location in the US. However several areas do exist with a high frequency of fog. Two causes:
Elevation
Ocean currents
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62. FIGURE 5.12
Average annual number of days with dense
fog (visibility less than 0.25 miles) through
the United States. (NOAA)
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63. Foggy Weather Environmental Issue: Fog dispersal
Mix air with air craft or fans
Introduce large particle into air to reduce total number of cloud droplets.
Use dry ice to lower temperature below freezing.
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64. Clouds
Classification of clouds: use Latin words to describe height and appearance.
Factors described
Height: low, mid, high, vertical
Appearance: shape, density, color
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65. TABLE 5.2 The Four Major Cloud Groups and Their Types
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66. TABLE 5.3 Approximate Height of Cloud Bases Above the Surface for Various Locations
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67. FIGURE 5.24 A generalized illustration of basic cloud types based on height above the surface and vertical development.
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68. FIGURE 5.13 Cirrus clouds.
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69. FIGURE 5.14 Cirrocumulus clouds.
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70. FIGURE 5.15 Cirrostratus clouds with a faint halo encircling the
sun. The sun is the bright white area in the center of the circle.
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71. FIGURE 5.16 Altocumulus clouds.
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72. FIGURE 5.17 Altostratus clouds. The appearance of a dimly visible
“watery sun” through a deck of gray clouds is usually a good indication
that the clouds are altostratus.
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73. FIGURE 5.18 The nimbostratus is the sheetlike cloud from
which light rain is falling. The ragged-appearing clouds beneath the
nimbostratus is stratus fractus, or scud.
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74. FIGURE 5.19 Stratocumulus clouds forming along the south
coast of Florida. Notice that the rounded masses are larger than those of
the altocumulus.
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75. FIGURE 5.20 A layer of low-lying stratus
clouds hides these mountains in Iceland.
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76. FIGURE 5.21 Cumulus clouds. Small cumulus
clouds such as these are sometimes called fair
weather cumulus, or cumulus humilis.
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77. FIGURE 5.22 Cumulus congestus.
This line of cumulus congestus clouds is
building along Maryland’s eastern shore.
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78. FIGURE 5.23A cumulonimbus cloud
(thunderstorm). Strong upper-level winds
blowing from right to left produce a welldefi
ned anvil. Sunlight scattered by falling ice
crystals produces the white (bright) area beneath
the anvil. Notice the heavy rain shower
falling from the base of the cloud.
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79. Some Unusual Clouds
Not all clouds can be placed into the ten basic cloud forms.
Unique atmospheric processes and environmental conditions create dramatic and exotic clouds.
Unusual clouds and weather balloons often cause of UFO reports.
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80. TABLE 5.4 Common Terms Used in Identifying Clouds
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81. FIGURE 5.25 Lenticular clouds forming on the leeward side of
the Sierra Nevada near Verdi, Nevada.
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82. Figure 5.26
The cloud forming over and downwind of Mt. Rainier is called a banner cloud.
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Fig. 5-26, p. 130

83. Figure 5.27
A pileus cloud forming above a developing cumulus cloud.
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Fig. 5-27, p. 130

84. Figure 5.28
Mammatus clouds forming beneath a thunderstorm.
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Fig. 5-28, p. 130

85. Figure 5.29
A contrail forming behind a jet aircraft.
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Fig. 5-29, p. 130

86. Figure 5.30
The clouds in this photograph are nacreous clouds. They form in the stratosphere and are most easily seen at high latitudes.
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Fig. 5-30, p. 131

87. Figure 5.31
The wavy clouds in this photograph are noctilucent clouds. They are usually observed at high latitudes, at altitudes between 75 and 90 km above the earth’s surface.
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Fig. 5-31, p. 131

88. Cloud Observations
Sky conditions: cloud coverage divided into eighths and each amount associated with term such as scattered clouds.
Observations: cloud ceilings
Ceilometer used at airports to determine height from clouds by light or laser striking clouds and then amount and speed of reflected light recorded.
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89. Figure 5
The laser-beam ceilometer sends pulses of infrared radiation up to the cloud. Part of this beam is reflected back to the ceilometer. The interval of time between pulse transmission and return is a measure of cloud height, as displayed on the indicator screen.
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Fig. 5, p. 132

90. TABLE 5.5 Description of Sky Conditions
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91. Cloud Observations Satellite Observations
Geostationary, polar orbiting
Visible light provides a black and white picture of clouds
Infrared approximates cloud temperature which infers height
Satellites measure many other variables: sea surface temperatures, ozone, upper level features, snow cover, land cover
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92. FIGURE 5.32 The geostationary satellite moves through space at
the same rate that the earth rotates, so it remains above a fixed spot on
the equator and monitors one area constantly.
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93. FIGURE 5.33 Polar-orbiting satellites scan from north to south,
and on each successive orbit the satellite scans an area farther to the
west.
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94. FIGURE 5.34 Generally, the lower the cloud, the warmer its top.
Warm objects emit more infrared energy than do cold objects. Thus, an
infrared satellite picture can distinguish warm, low (gray) clouds from
cold, high (white) clouds.
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