Chapter 4: Humidity,
Condensation and
Clouds
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By the end of this chapter you should:
Know the hydrologic cycle
Understand the idea of saturation
Grasp the idea of humidity
Understand the different cloud
types, including fog
Fig. 4-1, p. 80
Circulation of Water in the
Atmosphere
Evaporation from oceans that take up 70% of the Earth’s
surface (solar radiation does the evaporation)
 Precipitation clouds produce rain on land (runs back to
the ocean) and over the ocean
 Condensation this evaporated water (water vapor) turns
back into liquid water in the form of clouds
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Evaporation, Condensation
and Saturation
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What is equilibrium?
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It is a state in a system where competing forces are
balanced
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For example, look at the two beakers below
Evaporation, Condensation
and Saturation
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In the first beaker is water with molecules bumping,
vibrating at different speeds (temperature)
Occasionally, one of the fast-moving molecules near
the top of the water will break away and evaporate
Evaporation, Condensation
and Saturation
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At the same time, some of the water vapor molecules
outside the water change phase from vapor to liquid
Now let’s look at the second beaker
Evaporation, Condensation
and Saturation
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The second beaker has a lid on it
Water molecules still evaporate into the air, and vapor still
condenses into liquid
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But, what will happen if we let this process go on for a
long time (think of a soda can in the sun)
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Evaporation, Condensation
and Saturation
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Eventually, the number of water molecules changing
phase into vapor will equal the number of vapor
molecules changing phase into water
This is called saturation. Basically, the air is in equilibrium
Evaporation, Condensation
and Saturation
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If we remove the lid and blow, water vapor molecules
would go away. The air isn’t saturated anymore. This
allows for more evaporation
Thus, wind enhances evaporation
Evaporation, Condensation
and Saturation
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In the air above the beakers is Nitrogen, Oxygen, etc.
But also dirt, smoke, salt, etc.
Condensation nuclei – particles that allow vapor to
condense into liquid
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Condensation is more likely when the air is cooled
Vapor Pressure
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In a parcel of air,
the pressure is
1000 millibars
Pressure is the sum
of the forces of
every molecule
pushing against the
side of the parcel
Vapor Pressure
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Thus, since air is
78% Nitrogen, the
Nitrogen air
pressure is 780 mb
(1000 * 78%)
Oxygen’s (21%)
pressure is 210 mb
Water vapor’s (1%)
pressure is 10 mb.
This is called actual
vapor pressure
Vapor Pressure
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So actual vapor
pressure is a small
portion of the total
pressure
Actual vapor
pressure is an
indication of how
much water vapor is
in the air
Vapor Pressure
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If the air inside the
balloon was
saturated, the water
molecules would
exert a pressure.
This is called
saturation vapor
pressure
Vapor Pressure
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So, if we increase the temperature in the second beaker,
more water molecules will escape to vapor
To maintain equilibrium, the air must be able to hold
more vapor
At higher temperatures, it takes more vapor to saturated
the air
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Fig. 4-5, p. 83
Relative Humidity
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Relative humidity – basically tells us how close the air is
to being saturated
 Definition: RH = water vapor content / water vapor
capacity OR RH = act vapor pressure / sat vapor
pressure
 So an RH of 50% tells us what? Or 100% How about
104%
• When the general public uses the term “humidity”,
they mean “relative humidity.”
Relative Humidity
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Supersaturation – air with a relative humidity greater
than 100%
Name two ways we can change the humidity
Change the water vapor content (put more vapor in or
take some out)
Changing the air temperature
Relative Humidity
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Higher temperatures – faster moving molecules and less
likely they are to collide and condense
Condensation occurs at saturation
So what happens to RH during the day typically?
Fig. 4-7, p. 85
Relative Humidity and Dew
Point
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Suppose the morning temperature is 10°C and the RH is
100%. From graph, saturation vapor pressure is 12 mb
at 10°C. What is the actual vapor pressure?
Suppose the afternoon temperature is 30°C with no
change is vapor content. What is the RH?
Answer 1: 12 mb
Answer 2: 29% (saturation vapor pressure is 412 mb at
30°C)
For the second scenario, what temperature do we have
to cool the temperature to to get saturation?
Answer: 10°C
For this scenario, 10°C is the dew point
Relative Humidity and Dew
Point
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Dew point – the temperature to which the air would have
to be cooled to obtain saturation (with no moisture
content change)
So, temp of 15°C and RH of 100%. What is the dew
point?
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High dew points represent a lot of moisture. Low dew
points, low moisture
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Relative Humidity and Dew
Point
Relative Humidity and Dew
Point
Relative Humidity and Human
Discomfort
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So why do humid days “feeling” hotter than drier days?
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First, why do we perspire?
 The amount of evaporation from the skin is directly
related to how saturated the air is
 Drier air will evaporate sweat more quickly than air
that is closer to saturation
 More evaporation, more cooling
• “It’s not the heat, it’s the humidity”
Relative Humidity and Human
Discomfort
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Heat index – combines temperature and relative
humidity to determine what the air “feels like”
Dew and Frost
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On calm, clear nights, surface cools rapidly by what
process?
Air near the ground cools to dew point quickly,
reaches saturation
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Dew - vapor that condenses on blades of grass
Dew and Frost
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If the air temperature cools to the freezing point, you get
frozen dew
Frost - If the dew point
is below the freezing
point, then vapor goes
directly to ice (what is
this called)?
Fog
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Fog – a cloud resting near the ground
Types of fog:
 Radiation fog
 Advection fog
 Upslope fog
 Evaporation fog
Fog
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Radiation fog – is produced as a result of radiational
cooling
Forms best at night with a shallow moist layer below a
drier layer
 Moist layer cools rapidly and reaches dew point
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 Usually “burns off” by afternoon from bottom up
Radiation Fog
Fog
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Advection fog – produced with warm, moist air flows over
cool surface
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Very common along West Coast
Cool water very near coast, warm water and moist air
farther out. Afternoon breeze pushes air to coast, and
cools it to dew point
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Moisture in fog important to redwood coast
Advection Fog
Advection Fog
Advection Fog
Fog
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Upslope fog – produced as moist air flows up an
elevated surface
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As the air rises, it cools and reaches its dew point
Common on the eastern slopes of the Rocky
Mountains
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Can last for days
Upslope Fog
Fog
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Evaporation fog – produced by mixing two unsaturated
air masses
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Breathing on a cold day produces a cloud
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Occurs above swimming pools
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cold air moves over a swimming pool, warm water
will evaporate into the air, raising the dew point
Foggy Weather
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What areas will be the foggiest? Coastal or Interior?
Where the water is!!!! The coast!
Table 4-2, p. 98
High Clouds
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Cirrus
High clouds made of ice
crystals
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Can look like a pulled apart cotton ball
High Clouds
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Cirrocumulus (mackerel sky)
Made of ice crystals
 Looks a little like the scales of a fish
 “Cumulus” means has a puffy look
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High Clouds
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Cirrostratus
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Made of ice crystals
“Status” means layered across entire sky
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Middle Clouds
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Altocumulus
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Mid-level clouds mostly made of liquid water
“Cumulus” means puffy
Can cause very pretty sunsets or sunrises
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Middle Clouds
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Altostratus
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Mid-level clouds mostly made of liquid water
“Stratus” means layered across sky
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Low Clouds
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Nimbostratus
Low cloud
“Nimbo” or “Nimbus” means rain
“Stratus” means
layered across
the entire sky
Low Clouds
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Stratocumulus
Most common cloud type in the world
“Cumulus” means puffy, “Strato” means layered
Clouds With Vertical
Development
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Cumulus
“Cumulus” means puffy
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Not all cumulus clouds grow into thunderstorms
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Clouds With Vertical
Development
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Cumulus congestus
“Cumulus” means puffy
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“Congestus” means bunched together
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Clouds With Vertical
Development
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Cumulonimbus
These are the big thunderstorms
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“Nimbus” means rain
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Some Unusual Clouds
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Lenticular clouds
Pileus
Mammatus clouds
Contrails
Nacreous
Noctilucent
• Several alleged ‘flying
saucer’ reports have turned
out to be lenticular clouds.
Mammatus Clouds
Lenticular Clouds
Contrails
Noctilucent Clouds
Nacreous Clouds