Chapter 5: Cloud
Development and
Precipitation
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Atmospheric Stability
Determining stability
Cloud development and stability
Precipitation processes
Precipitation types
Measuring precipitation
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Atmospheric Stability
Q: Why does the air rise on some occasions and not on others?
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
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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.
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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)
Q: 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
Q: 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
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Q: 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
Q: What does a radiosonde measure?
 dry adiabatic lapse rate;
 moist lapse rate;
 environmental lapse rate
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Absolutely Stable Atmosphere
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.
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Q: For a rising surface dry
parcel with dew point T of
20C, its temperature at 2000
m height is: a) 10C, b) 14C,
c) 18 C, d) 20C
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Absolutely 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.
Q: For a descending saturated
parcel at 1000 m height
(with T = 24C), its T at
surface is: a) 28C, b) 30C, c)
32C, d) 34C
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Conditionally Unstable Air
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Conditional
instability:
environmental
lapse rate between
dry and moist
lapse rates
Lifting
condensation level
is the cloud base
Level of free
convection
Q: Between what heights in the figure would Tp = Te?
a) 0-1 km, b) 1-2 km, c) 2-3 km
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Q: If environmental lapse is greater than dry lapse rate, the
atmosphere is
a) stable, b) unstable, c) conditionally unstable
Q: If environmental lapse is less than moist lapse rate, the
atmosphere is
a) stable, b) unstable, c) conditionally unstable
Q: The earth’s atmosphere is ordinarily
a) stable, b) unstable, c) conditionally unstable
Q: The air T in an unsaturated parcel follows the
a) dry lapse rate, b) moist lapse rate,
c) environmental lapse rate
Q: If air T increases with height, the air is surely stable. If it
decreases with height, the air is:
a) stable, b), unstable, c) conditionally unstable, d) undecided9
Q: For stable condition, if
clouds exist, they are usually
a) cumuliform clouds,
b) stratiform clouds
Q: For conditionally unstable
conditions, if clouds exist,
they are usually
a) cumuliform clouds,
b) stratiform clouds
Q: Do you usually expect to
see layered clouds in
Tucson during the day in
summer?
a) yes, b) no
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Four Ways for Cumulus Development
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surface heating
and free
convection
uplift along
topography
convergence of
air
lifting along
weather fronts
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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.
Q: why are there large clear
areas between cumulus
clouds?
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Topography and Clouds
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rain shadow: 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
The diameter increases by 2
orders of magnitude from
condensation nucleus to
cloud droplet, and from
cloud droplet to raindrop
Growth of cloud droplets by
condensation is too slow (a
few days), but rain drops can
develop in < 1 hr in nature
Q: 0.0002 mm is: a) 20 μm,
b) 2 μm, c) 0.2 μm, d) 0.02
μm
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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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Q: Do larger drops fall faster
in a vacuum? a) yes, b) no
Q: Does coalescence exist for
uniform droplets? a) yes, b) no
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Ice Crystal Process
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cold clouds (ice crystals and liquid droplets coexist)
supercooled water droplets due to lack of ice nuclei
Q: what are the
two reasons for the
existence of the
anvil?
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Saturation
vapor pressures
over liquid water is higher than
over ice
This causes water vapor
molecules to diffuse from water
droplet towards the ice crystals
Ice crystals grow at the
expense of water droplets. It is
called ice-crystal (or Bergeron)
process.
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Snow pellets and snowflakes
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Accretion: ice crystals grow by colliding with supercooled
water droplets to form snow pellets
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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 (AgI): as cloud-seeding agent because it has a
crystalline structure similar to an ice crystal; it acts as an
effective ice nucleus at T = -4˚C and lower.
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?
Q: What are the preferable clouds for seeding?
A: existence of supercooled liquid droplets;
low ratio of ice crystals to droplets.
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Q: 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
Q: Cloud liquid droplet collision is called
a) coalescence, b) accretion
Q: 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
Q: Cloud (liquid) droplet does not exist for temperature:
a) warmer than 0˚C, b) warmer than −20˚C,
c) warmer than −40˚C, d) colder than −40˚C
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Precipitation Types
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Rain
Snow
Sleet and freezing rain
Hail
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Rain
Rain: drop diameter of
0.5 ~ 6mm
 Drizzle: drop diameter
< 0.5mm
 Virga: rainfall not
reaching surface
 Rain drop shape
#1: tear-shaped
#2: spherical shape for
small raindrops with
diameter <2 mm
#3: For large raindrops
with diameter > 2 mm
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Snow
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Fallstreaks: ice crystals and
snowflakes from high cirrus
clouds that usually do not
reach surface
Flurries: light snow falling
from cumulus clouds
snow storm: heavy snowfall
Blizzard: low T and strong
wind bearing large amounts
of snow, reducing visibility
to a few meters
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Snow
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Dendrite: Snowflake shape
depends on both temperature
and relative humidity
Annual snowfall in U.S. and
Canada
Mt. Rainier in
Washington
receives an annual
average snowfall
of 17 m!
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Sleet and Freezing Rain
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Sleet: frozen raindrop (called
ice pellet); makes a tapping
sound when falling on glass;
bounce when striking the
ground
Freezing rain: supercooled
liquid drops spread out and
freeze on cold surface
Q: what is the necessary vertical temperature profile for sleet?
A: a below-cloud (above freezing) layer to melt snowflake;
deep freezing layer to freeze the raindrop
Q: what is the necessary vertical temperature profile for sleet?
A: shallow below-cloud layer (so that supercooled raindrop can
not be frozen)
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Sleet and Freezing Rain
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Ice storm: substantial
accumulation of freezing rain
Rime: white granular ice,
formed by freezing small,
supercooled cloud or fog
droplets
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Snow Grains and Snow Pellets
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snow grains: snow equivalent of drizzle
snow pellets: larger than snow grains and bounce on surface;
formed as ice crystals collide with supercooled water droplets;
usually from cumulus congestus clouds
Graupel:
when snow pellets accumulate a heavy coating of rime,
they are called graupel
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Hail
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hail is produced when graupel grows
by accumulating supercooled liquid
droplets, a process called accretion.
Strong updrafts are needed.
• A hailstone can be sliced open to
reveal accretion rings, one for each
updraft cycle.
18.75 inch in diameter
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Q: what is the shape of a small rain droplet?
a) tear-shaped, b) spherical, c) mushroom
Q: 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)
Q: When ice crystals collide with supercooled water droplets,
what would be formed?
a) snow pellets, b) graupel, c) hail
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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
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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
 Polarimetric radar:
identify rain from snow
(by using horizontal
and vertical pulses)
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Measuring precipitation
from space.
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Q: Can you claim from your rain gauge that precipitation
rate is 0.001 in/hour? a) yes, b) no
Q: Surface temperature is above 0C for
a) sleet, b) frozen rain, c) hail
Q: how to transform a stable atmosphere near surface
to an unstable atmosphere?
Q: for a thick nimbostratus cloud with ice crystals and
supercooled cloud droplets of about the same size, which
precipitation process would be most important in
producing rain from the cloud? Why?
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