Class #3: Humidity,
condensation, and clouds
Chapters 4 and 5
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Chapter 4
Atmospheric Humidity
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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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Stepped Art
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Fig. 4-1, p. 90
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Fig. 4-2, p. 91
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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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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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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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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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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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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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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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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Fig. 4-13, p. 98
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Fig. 4-13, p. 98
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Fig. 4-13, p. 98
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Fig. 4-13, p. 98
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Fig. 4-15a, p. 100
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Fig. 4-15b, p. 100
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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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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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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Humidity
• Measuring humidity
– Sling psychrometer
– Hygrometer
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Chapter 5
Condensation: DEW, Fog, &
clouds
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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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Condensation Nuclei
• Particles suspended in the air that around
which water condenses or freezes.
– Hydrophobic/hygroscopic
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Table 5-1, p. 113
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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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Fig. 5-4, p. 114
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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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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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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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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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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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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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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Fig. 5-26, p. 130
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Fig. 5-27, p. 130
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Fig. 5-28, p. 130
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Fig. 5-29, p. 130
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Fig. 5-30, p. 131
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Fig. 5-31, p. 131
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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Fig. 5, p. 132
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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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