Chapter 4: Humidity,
Condensation and
Clouds
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Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Foggy weather
Clouds
Circulation of Water
in the Atmosphere
Q: do we have more water vapor in the hot, `dry’ air
the Sahara desert than in the cold, `damp’ polar air?
(A: yes and no, depending on the type of humidity
we mean)
Circulation of Water in the
Atmosphere
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evaporation
condensation
Precipitation
Runoff
hydrologic cycle
• The total amount of water vapor stored in the
atmosphere amounts to only one week’s supply of
precipitation for the planet.
Fig. 4-1, p. 80
Evaporation, Condensation
and Saturation
•Saturation
saturation is more likely
to occur in cool air
•Evaporation is increased
by
stronger wind;
Higher T
Evaporation, Condensation and
Saturation
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condensation nuclei
• In very clean air, about 10,000 condensation nuclei
are typically found in one cubic centimeter of air,
a volume approximately the size of your fingertip.
• Condensation occurs
primarily when the air
is cooled
Humidity
•Water vapor density (absolute
humidity (kg/m3)
•Specific humidity (kg/kg)
•Mixing ratio (kg/kg)
•Vapor pressure (mb)
•Relative humidity
Vapor Pressure
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actual vapor pressure
saturation vapor pressure
• “Saturation” describes a condition of equilibrium:
liquid water is evaporating at exactly the same rate that
water vapor is condensing.
The actual values are
Also provided in Table
B.1 on p. 440.
Fig. 4-5, p. 83
Relative Humidity
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definition of relative
humidity:
vapor pressure divided by
saturation vapor pressure
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saturation and
supersaturation
relative humidity and
temperature
Fig. 4-7, p. 85
Relative Humidity and Dew
Point
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dew point temperature
• The It is the T to which air would have to be cooled (with
no change in air pressure and moisture content) for
saturation to occur
• Higher Td indicates higher water vapor content
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dew point depression (T – Td) and relative
humidity
Higher (T – Td) indicates lower RH
Ta = -2C
Td = -2C
Ta – Td = 0C
RH = 100%
Ta = 35C
Td = 10C
Ta – Td = 25C
RH = 21%
Fig. 4-9a, p. 87
Relative Humidity and Human
Discomfort
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wet bulb temperature
Tw: attained by evaporating water into the air;
a good measure of how cool the skin can become
Td: reached by cooling the air to saturation;
a good measure of actual vapor content
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Heat index
• “It’s not the heat, it’s the humidity” - both temperature and
relative humidity contribute to warm-weather discomfort.
Fig. 4-10, p. 89
Measuring Humidity
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Psychrometers
Wet-bulb T (Tw);
Dry-bulb T (Ta);
wet-bulb depression (Ta-Tw);
Find Td and RH (based on
Table D on p. 443-446) for
Ta = 20C, Ta-Tw = 5C; or
Ta = 90F, Ta-Tw = 10F
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Hygrometers
hair hygrometer and electrical hygrometer: RH
infrared hygrometer and dew cell: moisture content;
dew-point hygrometer (for ASOS)
Dew and Frost
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dew
frost
frost point and deposition
• Frost is one of the few examples of deposition in nature.
Fog
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radiation fog:
cooling from ground
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advection fog:
warm, moist air over cold surface
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upslope fog: cooling
evaporation (mixing) fog
when moist air from your mouth or nose
meets the cold air and mixes with it
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haze: hygroscopic (`water
seeking’) condensation nuclei
allows water vapor to condense
when RH is below 100%
Foggy Weather
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coastal vs. interior areas
hazard to aircraft
• Some airports use fog-dispersal equipment.
Clouds
The cloud chart is provided at the end
of the textbook
Classification of Clouds
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major cloud types
low, middle, high, vertical
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cloud appearance
sheetlike, puffy, wispy, rain cloud
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cloud base
0-2 Km, 2-6 km, 6-10km
• It’s easy to identify clouds, but it takes practice.
The ability to identify clouds allows you to forecast
many aspects of the weather using nothing but your
eyes.
Table 4-2, p. 98
High Clouds
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Cirrus: thin, wispy
Cirrocumulus: small, white puffs
Cirrostratus: usually thin, often producing a halo
• Cirrostratus clouds can sometimes be quite thick.
Middle Clouds
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Altocumulus: gray, puffy (larger, darker than Cc)
Altostratus: gray layer cloud with `watery sun’
(difference from Cs: darker, dimly visible, no ground
shallows)
Low Clouds
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Nimbostratus
dark gray with light rain
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Stratocumulus:
larger cloud elements with
lower cloud base than Ac
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Stratus
uniform grayish cloud;
has a more uniform base than
Ns;
has a lower base and dark gray
than As;
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Clouds with Vertical
Development
Cumulus: puffy, floating `cotton’ with flat base
cumulus congestus: line of towering Cu
Cumulonimbus: thunderstorm with anvils
• Not all cumulus clouds grow to be thunderstorms, but
all thunderstorms start out as cumulus clouds.
Some Unusual Clouds
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lenticular clouds:
lens-like; UFO
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Pileus: `cap’
mammatus clouds:
baglike
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Contrails: condensation
trail from engine exhaust