Cloud Formation
Matt Rogers (rogers@atmos.colostate.edu)
Department of Atmospheric Science
Colorado State University
AIM Workshop
25 July 2006
Anchorage, Alaska
Water Cycle Revisited
Sources of water for
clouds come from
evaporation and
transpiration
Sinks of water for clouds
come in the form of
precipitation and
evaporation
What are the mechanisms that happen in between?
Thirty Seconds of
Thermodynamics
Temperature is a measure of molecular energy - if a
collection of molecules has a lot of energy, odds are, it’ll
have a higher temperature as well (and vice versa.)
Gases behave (well, ideally, at least) independently of
each other
If matter in the form of a liquid gains enough energy
(which we call the latent heat of evaporation) it may
undergo a phase change and become a gas.
If matter in the form of a gas loses enough energy
(which we call the latent heat of condensation) it may
turn back into a liquid.
Water Saturation
Thought experiment - which beaker likely has
The temperature
of a gas (say, Why?
water vapor) is closely
a warmer temperature?
related to its energy
The amount of water vapor and liquid water will depend on
the total energy of the system - obeying the laws of
thermodynamics and staying in equilibrium
Describing Saturation
In terms of a saturation vapor pressure, which is simply the
water vapor pressure (for a given temperature and
atmospheric pressure) that maintains equilibrium.
In terms of the Relative Humidity, which is the fraction of the
current vapor pressure (whatever it might be) to the saturation
vapor pressure it would have if it were in equilibrium.
Or, in terms of a dewpoint temperature, which is the
temperature that the vapor (at its current vapor pressure)
would have to be cooled to to reach its saturation vapor
pressure.
Nothing here depends on the dry air!
Dew
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Surfaces cool strongly at
night
 Strongest on clear, calm
nights
If a surface cools below the
dew point, water condenses
on the surface and dew
drops are formed
Frost
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If the temperature is below
freezing, the dew point is
called the frost point
If the surface temperature
falls below the frost point
water vapor is deposited
directly as ice crystals
The resulting crystals are
known as frost, hoarfrost, or
white frost
Cloud droplet formation
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If the air temperature cools below the dew point (RH >
100%), water vapor will tend to condense and form
cloud/fog drops
As with dew and frost, cloud drop formation prefers to
condense on a surface of some sort - we call these
particles cloud condensation nuclei (CCN)
CCN surfaces offer a locally lower saturation vapor
pressure that facilitates condensation
The most effective CCN are water soluble.
Without these particles clouds would not form in the
atmosphere
 RH of several hundred percent required for pure
water drop formation
Aerosol Sources
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Terrestrial Sources
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Oceanic Sources
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Dust/sand/dirt particles
Smoke - volcanic, fires, and pollution (sulfates)
Pollens and spores
Sea Salts
Chemical sources
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Photodissociation
Heterogeneous/Homogeneous chemistry
Typical sizes
Cloud Formation Mechanisms
…can be anything that causes water vapor cool and condense
Direct cooling…
…lifting of an airmass…
Cloud Formation Mechanisms
…can be anything that causes water vapor cool and condense
…buoyant lifting…
…or mechanical forcing
Cloud classification
Clouds are classified by height, appearance, precipitation,
and category of vertical development
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High Clouds - generally above 16,000 ft at middle latitudes
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Middle Clouds – 7,000-16,000 feet
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Main types – Stratus, stratocumulus, nimbostratus
Clouds of Vertical Development
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Main types – Altostratus, Altocumulus
Low Clouds - below 7,000 ft
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Main types - Cirrus, Cirrostratus, Cirrocumulus
Main types – Cumulus, Cumulonimbus
Nimbo- and -nimbus prefix/suffix
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Denotes precipitation
Low Clouds
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Stratus
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Stratocumulus
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Uniform, gray
Resembles fog that does not
reach the ground
Usually no precipitation, but light
mist/drizzle possible
Low lumpy clouds
Breaks (usually) between cloud
elements
Lower base and larger elements
than altostratus
Nimbostratus
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Dark gray
Continuous light to moderate rain
or snow
Evaporating rain below can form
stratus fractus
Stratiform cloud layers
Stratocumulus cloud streets
Stratus undulatus
Looking down on an eastern
Atlantic stratus deck
Middle Clouds
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Altocumulus
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<1 km thick
mostly water drops
Gray, puffy
Differences from
cirrocumulus
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Larger puffs
More dark/light contrast
Altostratus
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Gray, blue-gray
Often covers entire sky
Sun or moon may show
through dimly
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Usually no shadows
Altostratus
Altocumulus
High Clouds
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High clouds
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White in day;
red/orange/yellow at sunrise
and sunset
Made of ice crystals
Cirrus
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Thin and wispy
Move west to east
Indicate fair weather
Cirrocumulus
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Less common than cirrus
Small, rounded white puffs
individually or in long rows
(fish scales; mackerel sky)
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Cirrostratus
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Thin and sheetlike
Sun and moon clearly visible
through them
Halo common
Often precede precipitation
Cirrus
Cirrocumulus
Cirrostratus
Cirrostratus with Halo
Vertically developed
clouds
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Cumulus
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Puffy “cotton”
Flat base, rounded top
More space between cloud
elements than stratocumulus
Cumulonimbus
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Thunderstorm cloud
Very tall, often reaching
tropopause
Individual or grouped
Large energy release from
water vapor condensation
Cumulonimbus with Pileaus caps
Mechanically Forced Clouds
Lenticularis
Wave clouds
Clouds - Why We Care
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Clouds transport energy from one area to
another
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Evaporation takes latent heat out of warm
surface waters
Condensation releases same latent heat into
atmosphere in a different location
Heat has been ‘carried’ by the water inside a
cloud
Latent Heat Release
An average thunderstorm
contains several thousand
metric tons of water
Condensing 1 kg of water
releases ~ 2.26x106 J of
latent heat energy
An average thunderstorm containing around 1500 tons of
water will release 3.45 billion Joules of energy
Clouds - Why We Care
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Clouds affect the radiation budget by
reflecting visible light from the sun (thus
cooling the planet) and by trapping
infrared radiation from the surface (thus
warming the planet)
Reflection/trapping behavior depends on
type of cloud…
Cloud Radiative Effects
Salient Tidbits
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In the infrared
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Clouds absorb radiation from below, re-emit at the
temperature of the cloud
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Cirrus - very cold, emit little radiation
Stratus - very warm, basically emit like the surface
In the visible
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Clouds reflect radiation based on the amount of cloud
droplets
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Cirrus - thin, not as reflective as thicker water clouds
Stratus - thicker, many water droplets, highly reflective
High Clouds
Thin, cold ice clouds
reflect less sunlight
Extremely cold, emits
infrared at colder
temperatures, prevents
warmer surface infrared
from escaping to space
NET EFFECT: Warming
Low Clouds
Very thick water clouds reflect
large amounts of sunlight
Very near the surface,
temperature of the cloud
effectively the same as surface.
Infrared radiation is therefore
about the same - almost like the
cloud wasn’t there!
NET EFFECT: Cooling
Challenges
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Climate modelling with clouds - need to get both
type and amount correct, which is difficult
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Understanding cloud feedbacks
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Overestimating high clouds - too much warming
Overestimation low clouds - not enough
What does CO2 warming do to cloud populations?
How does increased aerosol pollution affect cloud
types and amounts?
Observations
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Layered cloud structures not seen from satellites