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Chapter 7
Clouds, Precipitation, and Weather
Radar
Clouds
• A cloud is the visible product of
condensation or deposition of water vapor
in the atmosphere
• Need more than just saturation to form
clouds
• When the relative humidity of the air is
>100% we say the air is supersaturated:
and a cloud forms; vapor in excess of 100%
Nuclei
• Recall that that the atmosphere is composed of
gases and aerosols
• Nuclei – naturally occurring particles that promote
condensation or deposition in the atmosphere
• Nuclei have a radius greater than 1.0 μm - droplets
grow at RH near 101%, which does occur in the
atmosphere
• Sources: volcanoes, forest fires, pollution, soil
erosion, and sea spray
Nuclei
• Cloud Condensation Nuclei (CCN)
– Condensation of water vapor at temperatures
above and below the freezing point of water
• Ice Forming Nuclei (IN)
– Formation of ice crystals at temperatures well
below freezing
• Freezing nuclei – water vapor condenses and freezes
• Deposition nuclei – water vapor deposits directly as ice
Supercooled Water
• Water that cools below freezing, but does not
freeze (as cold as –38.2oF)
• Homogeneous Nucleation
– Supercooled water drops collect on a tiny ice crystal
spontaneously at a temperature less than –38.2oF
• Heterogeneous Nucleation
– Supercooled water drops collect on a foreign particle at a
temperature less than freezing, but warmer than –38.2oF
Classification of Clouds
• General Appearance
• Altitude of Cloud Base
– Stratiform: high, middle, or low level
– Cumuliform: clouds with vertical development
• Temperature
– Warm cloud > 0oC
– Cold cloud at or below 0oC
• Composition
– Ice crystals, supercooled droplets, or water droplets
International Visual Cloud
Classification• Cumulus (literally, heap or pile)
• Stratus (literally, flattened out or
covered with a layer)
• Cirrus (literally, a lock of hair or
a tuft of horsehair)
• Nimbus (precipitating cloud)
• Altum (height)
High Clouds
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Altitude: above 5000m - stratiform
Temperature: -25oC
Composition: almost entirely ice crystals
Appearance:
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Thin and wispy
Transparent to allow sunlight through
Rarely cover the entire sky
No precipitation
Cirrus (Ci)
Cirrostratus (Cs)
Cirrocumulus (Cc)
Middle Clouds
• Altitude: 2000-7000m
• Temperature: Between 0oC and -25oC
• Composition: ice crystals, water droplets, or a
combination of both
• Appearance:
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Thicker and larger than cirrus clouds
Sun is dimly visible
Completely or partially cover the sky
Rarely produce precipitation that reaches the ground
Altostratus (As)
Altocumulus (Ac)
Low Clouds
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•
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•
Altitude: ground to 2000m
Temperature: temperatures above -5oC
Composition: mostly water droplets
Appearance:
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Low lying thick gray clouds
Sun is obscured
Completely cover the sky
Generally light, but steady precipitation
Stratocumulus (Sc)
Stratus (St)
Nimbostratus (Ns)
Clouds With Vertical Development
• Altitude: height of Convective Condensation Level
(CCL)
– generally about 1000-2000m
– Clouds tops can be as high as 20,000m (stratosphere)
• Composition: water drops, supercooled water drops,
and ice crystals
• Appearance:
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–
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White puffy clouds
Cotton, Cauliflower
No precipitation with “fair weather” cumulus
Significant storms with cumulonimbus
Cumulus (Cu)
Cumulus Congestus (CuCon)
Cumulonimbus (Cb)
Mountain Wave Cloud
Cap Cloud
Lee-Wave Clouds
Lenticular
Mammatus
Nacreous Clouds
Noctilucent Cloud
Fog
• Fog
– a cloud (stratus) in contact with the ground
– Restricts visibility to 1000m or less
– If this visibility restriction is not met, then it is
called mist
• The air needs to be saturated for fog to
develop
Radiation Fog
• Air becomes saturated due to radiational cooling
• Conditions for development:
– Clear night sky
– Light winds (calm winds would favor dew)
– Humid air at the ground with dry air aloft
• Generally occurs over land where rain or snowmelt has
occurred the day before
• Is often burned off by the sun a few hours after sunrise
Advection Fog
• Air becomes saturated due to advective
cooling
• Conditions for development:
– Warm humid air advecting over a cold surface
– The cold surface chills the air to its saturation
point at the lowest layers
• Warm air flowing over snow covered ground
or a cold water surface (Great Lakes)
Steam Fog
• Air becomes saturated due to the addition of
water vapor
• Conditions for development:
– In the winter when cold dry air flows over an unfrozen
lake
– The lower layer warms and becomes more humid due
to evaporation  this mixes with the cold dry air aloft
to form fog
• Fog resembles smoke coming out of a smokestack
Upslope Fog
• Air becomes saturated due to expansional
cooling
• Conditions for development
– As humid air ascends up a mountain it expands
and cools, thus reaching saturation
• Sometimes the fog reaches the top of the
hill and spreads as a stratus cloud over a
valley – this is called high fog
Precipitation Processes
• Most clouds do not bring any rain or snow
• For clouds to precipitate the cloud particles must
be large enough for their terminal velocity to be
greater than the updraft in the cloud
• For this to happen drops need to be about 2mm
in diameter, but cloud drops are only 10-20μm in
diameter – so how does it rain (or snow)?
Warm Air Clouds
• Collision-Coalescence Process
– Droplets that grow by colliding and then
coalescing (merging) with one another
• Droplets with larger diameters have a larger
terminal velocity, so as they move through
the cloud, they “pick up” smaller droplets
• As droplets become large enough they fall
out of the cloud as precipitation
Cold Air Clouds
• Bergeron-Findeisn Process
– The growth of ice crystals in a cloud at the
expense of supercooled water droplets
• Same idea as warm air clouds  as the
frozen particles grow they overtake more
droplets and fall out of the cloud
Virga
• Once a large droplet leaves the base of the
cloud there is no guarantee that it will reach
the surface
• Often the drop will evaporate
• Virga
– Water or ice particles that vaporize before they
reach the earth’s surface
Types of Precipitation
• Rain
– Diameters between 0.5 and 6mm
• Drops break apart if diameter gets too large
• Drizzle
– Diameters between 0.2 and 0.5mm
– Generally occurs in stratus clouds
– Occur with fog and contribute to low visibility
Types of Precipitation
• Snow
– An agglomeration of ice crystals in the form of flakes
– Crystals come in 4 types
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Needles
Dendrites
Plates
Columns
– Snow flakes vary in size, but can be as big as 5-10 centimeters
in diameter
– Snow Pellets: supercooled droplets that collide and freeze on
an ice crystal
– Snow Grains: like drizzle, except they freeze before reaching
the ground
Types of Precipitation
• Ice Pellets
– Also called sleet
– Snowflakes that partially or completely melt and then refreeze
before hitting the ground
• Freezing Rain (or drizzle)
– Liquid drops that supercool and partially freeze on contact on
cold surfaces at the ground
– This forms a coat of ice on road, trees, and stuff
• Hail
– Chunks of ice
– Forms in thunderstorms with strong updrafts that cause ice
rock to grow
– Mostly melt before hitting the surface
Weather Radar
• A remote sensing tool for determining the location,
movement, and intensity of areas of precipitation
• National Weather Service uses a WSR-88D
– WSR – weather surveillance radar
• Reflectivity Mode
– Location, movement, and intensity of areas of precipitation
– Maximum range of 285 miles
• Velocity (Doppler) Mode
– Air motions directly toward or away from the radar associated
with the circulation of the weather system
– Maximum range of 143 miles
Doppler Effect
• A shift in the frequency of sound waves
emanating from a moving source
– How the sound of a train or ambulance changes as it
moves towards and then away from you
• Doppler radar monitors the motion of
precipitation toward or away from the radar
– Meteorologists can detect circulations and rotations
(tornados) and thus give advanced warnings
Reflectivity Mode
Doppler Effect
Radar Stuff
• Clear Air Mode
– Very sensitive radar setting
– Radar can detect dust particles or swarms of
bugs that collect along boundaries of air masses
– These boundaries are potential sites for
thunderstorms development
• Ground Clutter
– Nearby objects (buildings, trees) that reflect
back to the radar