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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
Q: Why does the air rise on some occasions and not on others?
Q: Why do the size and shape of clouds vary so much when
the air does rise?
A: because of different atmospheric stability conditions
• Stable and unstable equilibrium
stability analysis: giving initial perturbations,
Stable:
parcel moves back
Unstable:
parcel moves away from
the original position
Atmospheric Stability
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adiabatic process: no heat exchange of the air parcel with the
environment so that rising air parcel expands and cools
dry adiabatic lapse rate for parcels (10 C/km)
moist adiabatic lapse rate for parcels (taken as 6 C/km)
environmental lapse rate for the atmosphere (~ 6.5 C/km)
Q1: if a rising parcel’s T decreases at 10 C/km in an adiabatic
process, does its T increases at 10 C/km as the parcel
descends?
a) yes, b) no
Q2: why is the moist lapse rate lower than dry lapse rate?
a)
because condensation occurs in the moist adiabatic process
b)
because evaporation occurs in the dry adiabatic process
c)
because condensation occurs in the dry adiabatic process
Q3: why is the moist lapse rate lower over tropics (higher T)
than over polar regions (lower T)?
 because warm saturated air contains more liquid water for
condensation;
 because cold saturated air contains more liquid water for
condensation
Q4: What does a radiosonde measure?
 dry adiabatic lapse rate;
 moist lapse rate;
 environmental lapse rate
Determining Stability
Stability analysis: assume a parcel rises following the dry
or moist adiabatic process; then compare its Tp with the
environmental Te (following environmental lapse rate);
colder T means denser air.
Stable condition: If a rising parcel’s Tp < Te, it is denser
and would sink back.
Unstable condition: If the rising parcel’s Tp > Te, it is less
dense and will continue to rise
• 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.
A Stable Atmosphere
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stabilizing processes
nighttime surface radiational
cooling;
warm air advected to cold
surface;
air aloft warming (e.g.,
subsidence inversions)
• Stable air provides ideal
conditions for high pollution
levels.
An Unstable Atmosphere
destabilizing processes
daytime solar heating of
surface air;
cold air advected to warm
surface
 superadiabatic lapse rates
(> 10 C/km)
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• Unstable air tends to be
well-mixed.
Conditionally Unstable Air
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Conditional
instability:
environmental
lapse rate
between dry and
moist lapse rates
Condensation
level
cloud base
Q5: exactly at what height in the figure would Tp = Te?
a) above 2 km, b) at 2 km, c) at 1.33 km
Q6: If environmental lapse is greater than dry lapse rate, the
atmosphere is
a) stable, b) unstable, c) conditionally unstable
Q7: If environmental lapse is less than moist lapse rate, the
atmosphere is
a) stable, b) unstable, c) conditionally unstable
Q8: The earth’s atmosphere is ordinarily
a) stable, b) unstable, c) conditionally unstable
Q9: The air T in an unsaturated parcel follows the
a) dry lapse rate, b) moist lapse rate,
c) environmental lapse rate
Q10: If air T increases with height, the air is surely stable. If it
decreases with height, the air is:
a) stable, b), unstable, c) undecided
Cloud Development and Stability
•
•
Layered clouds tend to form in a stable atmosphere;
Cumuliform clouds tend to form in an unstable or
conditionally unstable atmosphere
Q11: Do you usually expect to see layered clouds in
Tucson during the day in summer?
a) yes, b) no
Q12: Do you usually expect to see cumulus clouds at
night over midlatitudes?
a) yes, b) no
Four Ways for Cumulus Development
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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
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.
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Precipitation
Processes
Pay attention to the 2
orders of magnitude
change in diameter
Growth of cloud
droplets by
condensation is too
slow (a few days), but
rain drops can
develop in < 1 hr in
nature
Collision and Coalescence Process
Warm clouds (with T above freezing)
 terminal velocity
large drops fall faster than
small drops
 coalescence:
the merging of a large cloud
droplet with small droplets
by collision
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Q13: Do larger drops fall
faster in a vacuum?
a) yes, b) no
Ice Crystal Process
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cold clouds (ice crystals and liquid drop coexist)
supercooled water droplets due to lack of ice nuclei
Saturation
vapor
pressures over
liquid water is
higher than over ice
(see Fig. 4.5 insert
on p. 83)
This causes water
vapor molecules to
diffuse from water
droplet towards the
ice crystals
Ice crystals grow
at the expense of
water droplets
Snow pellets and snowflakes
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Accretion: ice crystals grow by colliding with supercooled
water droplets to form snow pellets
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
silver iodide
as cloud-seeding agent because it has a crystalline structure
similar to an ice crystal
Very popular in some countries
• 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?
Q14: In warm clouds (with T above freezing), cloud droplets
grow to rain droplets primarily through
a) collision/coalescence process
b) Ice crystal process
c) Accretion
Q15: Cloud liquid droplet collision is called
a) coalescence, b) accretion
Q16: What is the purpose of using silver iodide for the
seeding of supercooled clouds?
a) Increase the number of ice nuclei
b) Increase the number of cloud nuclai
Precipitation Type: Rain
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Rain: drop diameter of
0.5 ~ 6mm
Drizzle: drop diameter
< 0.5mm
Virga: rainfall not
reaching surface
Rain drop shape
small to
medium
large droplet
(> 2 mm diameter)
Snow
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Snow
Fallstreaks: ice crystals and
snowflakes from high cirrus
clouds that usually do not
reach surface
Dendrite: Snowflake shape
depends on both temperature
and relative humidity
Blizzard: low T and strong
wind bearing large amounts
of snow, reducing visibility
to a few meters
Flurries: snow falling from
cumulus clouds
Sleet and Freezing Rain
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Sleet: frozen raindrop; makes
a ‘tap tap’ sound when falling
on glass
Freezing rain: supercooled
liquid drops spread out and
freeze on cold surface
Rime: white granular ice
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
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.
Q17: what is the shape of a small rain droplet?
a) tear, b) spherical, c) mushroom
Q18: What is the difference between sleet and freezing rain?
A: sleet refers to raindrops freezing through a deep cold layer
below cloud and could bounce on the ground, while
freezing rain refers to supercooled liquid drops spreading
out and freezing on cold surface (ground, trees, …) and
substantially affect driving conditions (both on the road
and on the wind shield)
Q19: When ice crystals collide with supercooled water
droplets, what would be formed?
a) snow pellets, b) graupel, c) hail
Precipitation Measurement Instruments
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standard rain gauge: 0.01 inch interval ; trace
tipping bucket rain gauge: used in ASOS
• difficult to capture rain in a bucket when wind blows strongly.
• Tipping bucket underestimates rainfall for heavy events
Doppler Radar and Precipitation
Radar: radio detection and ranging
 Doppler radar:
use Doppler shift
(e.g., a higher-pitched
whistle as a train
approaches you);
Provide precipitation
area and intensity;
Provide horizontal
speed of falling rain
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Q20: Can you claim from
your raingauge that
precipitaiton rate is 0.001
in/hour?
a) yes, b) no