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Chapter 5: Cloud
Development and
Precipitation
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Atmospheric Stability
Determining stability
Cloud development and stability
Precipitation processes
Precipitation types
Measuring precipitation
Atmospheric Stability
Q1: Why does the air rise on some occasions and
not on others?
Q2: Why does the size and shape of clouds vary
so much when the air does rise?
Atmospheric Stability
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stable and unstable equilibria
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air parcels: rising air
parcel expands and cools
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adiabatic process
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dry adiabatic lapse rate
(9.8 C/km)
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moist adiabatic lapse rate (less than 9.8 C/km)
• Stability does not control whether air will rise or sink.
Rather, it controls whether rising air will continue to rise
or whether sinking air will continue to sink.
Determining Stability
Stable condition: If a rising parcel’s T is colder than its
environment, it is denser and would sink back.
Unstable condition: If the rising parcel’s T is warmer,
it is less dense and will continue to rise
A Stable Atmosphere
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environmental lapse
rate
absolute stability
stabilizing processes
surface cooling;
air aloft warming (e.g.,
subsidence inversions)
• Stable air provides excellent
conditions for high pollution levels.
An Unstable Atmosphere
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absolute instability
destabilizin processes
warming of surface air
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superadiabatic lapse
rates (> 9.8 C/km)
• Unstable air tends to be
well-mixed.
Conditionally Unstable Air
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dry and moist
adiabatic lapse rates
Conditional instability:
environmental lapse rate
between dry and moist
lapse rates
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Condensation level
cloud base
Cloud Development
and Stability
Layered clouds tend to form in a stable atmosphere;
whereas cumuliform clouds tend to form in a
Conditionally unstable atmosphere
Cloud Development and
Stability
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surface heating and
free convection
uplift along
topography
widespread ascent
lifting along weather
fronts
Convection and Clouds
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thermals
fair weather cumulus
• Fair weather cumulus
provide a visual marker of
thermals.
• Bases of fair-weather
cumulus clouds marks the
lifting condensation level,
the level at which rising air
first becomes saturated.
Topography and Clouds
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orographic uplift
rain shadow
• The rain shadow works for snow too. Due to frequent
westerly winds, the western slope of the Rocky Mountains
receives much more precipitation than the eastern slope.
Precipitation
Processes
Collision and Coalescence
Process
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Warm clouds (above freezing T)
terminal velocity
large drops fall faster than
small drops
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coalescence:
the merging of cloud
droplets by collision
Ice Crystal Process
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cold clouds (ice crystals and
liquid drop coexist)
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supercooled water
droplets
water drops below freezing T
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saturation vapor
pressures over liquid
water is higher than over
ice
Accretion
ice crystals grow by colliding with
supercooled water droplets to
form graupel (or snow pellets)
Fig. 5-22, p. 124
Precipitation in Clouds
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accretion
ice crystal process
Cloud Seeding and
Precipitation
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cloud seeding
inject (or seed) a cloud with small particles that will act as
nuclei so that the cloud particles will grow large enough to
fall to the surface as precipitation
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silver iodide: as cloud-seeding agent because it
has a crystalline structure similar to an ice crystal
• It is very difficult to determine whether a cloud seeding
attempt is successful. How would you know whether
the cloud would have resulted in precipitation if it hadn’t
been seeded?
Precipitation Types
Rain
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Rain: drop diameter of 0.5 ~
6mm
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Drizzle: drop diameter <
0.5mm
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Virga: rainfall not reaching
surface
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shower
small to
medium
large
droplet
Snow
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Snow
Fallstreaks: ice crystals and
snowflakes from high cirrus clouds
that usually do not reach surface
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Dendrite: Snowflake shape
depends on both temperature and
relative humidity
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Blizzard: low T and strong wind
bearing large amounts of snow,
reducing visibility to 1 few meters
Sleet and Freezing Rain
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Sleet: Sleet makes a ‘tap tap’
sound when falling on glass
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freezing rain
rime
Snow Grains and Snow
Pellets
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snow grains: snow equivalent of drizzle
snow pellets: larger and bounce on surface; formed
as ice crystals collide with supercooled water droplets
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Graupel:
when snow pellets accumulate a heavy coating of rime,
they are called graupel
Hail
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updraft cycles
accretion
• A hailstone can be sliced open to reveal accretion
rings, one for each updraft cycle.
Measuring
Precipitation
Instruments
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standard rain gauge: 0.01 inch interval ; trace
tipping bucket rain gauge: used in ASOS
• It is difficult to capture rain in a bucket when the
wind is blowing strongly.
Doppler Radar and
Precipitation
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Radar
radio detection
and ranging
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Doppler radar
provide precip.
area and intensity
as well as
horizontal speed of
falling rain
Fig. 5-40a,b, p. 136