Chapter 5: Cloud Development and Forms
Introduction
• Clouds are instrumental to the Earth’s energy and moisture
balances
• Most clouds form as air parcels are lifted and cooled to
saturation
• Lowering temperature to dew point  cloud formation
Lifting Mechanisms
(initial uplift push)
1. Orographic Lifting
2. Frontal Lifting
3. Convergence
4. Localized Convection
Orographic Uplift
• results when air is displaced upward over topographic barriers
such as mountains
• leads to adiabatic cooling (expansion) on windward slope, may
reach saturation
• leeward slope – adiabatic warming (compression), rain shadow
• e.g. Sierra Nevada and Death Valley
Frontal Lifting
• occurs in transitional areas where large temperatures changes occur
over relatively short distances
• when air masses of unlike temperatures (fronts) meet  warmer
air is forced upward
• results in adiabatic cooling and cloud formation
• Cold fronts – warm air is displaced by cold air
• Warm fronts – warm air rises over cold air
Cold front
Warm front
Convergence
• atmospheric mass is not uniformly distributed over Earth
• large-scale atmospheric circulation results from these
pressure differences (Aleutian Low, Hawaiian High)
• air advects from areas of
HP to areas of LP
• leads to convergence, air
rises  adiabatic cooling
Localized Convection
• localized surface heating may lead to free convection
• heated air is less dense  rises  cools adiabatically
• clouds form and precipitation may occur
• limited in spatial extent
Cloud Types
• liquid droplets, ice crystals or both
• original scheme (1803):
• cirrus – thin, wispy clouds of ice.
• stratus – layered clouds.
• cumulus – clouds having vertical development.
• nimbus – rain-producing clouds.
• create classification scheme based on height and form:
• high clouds – cirrus, cirrostratus, cirrocumulus
• middle clouds – altostratus, altocumulus
• low clouds – stratus, stratocumulus, nimbostratus
• extensive vertical development – cumulus, cumulonimbus
Cloud types
High Clouds
• Bases above 6000 m, composed of ice
• Cirrus - most common, wispy appearance due to low water content and
cold temperatures
• Fall streaks – falling ice crystals
• Cirrostratus – result from thickening cirrus and stretch across the sky
• Cirrocumulus – puffy, billowy clouds associated with wind shear.
Cirrus
Cirrus with fall streaks
Middle Clouds
•
•
•
•
•
bases between 2000 and 6000 m
largely composed of liquid drops
carry the “alto” prefix
Altostratus – thick enough to obscure the sun/ moon, blanket the sky
Altocumulus – a series of puffy clouds arranged in rows.
Altocumulus
Low Clouds
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•
•
•
•
below 2000 m (6,000 ft), 500-1000 m thick, extensive coverage
normally composed of liquid water
stratus – result from lifting of extensive area of stable air
nimbostratus – produce light precipitation
Stratocumulus – low, layered clouds with some vertical development
stratocumulus
stratus
Clouds with Vertical Development
• High vertical velocities in air that is unstable or
conditionally unstable  cumulus
--- Cumulus humilis, or fair weather cumulus
• develop primarily from localized convection
• evaporate shortly after formation, vertically
limited
Formation of fair weather cumulus
Cumulus humilis
Clouds with Vertical Development
--- Cumulus congestus
• greater development, cloud towers appear
• towers are indicative of uplift cells
Cumulus congestus
Clouds with Vertical Development
--- Cumulonimbus
• Most violent of all clouds = thunderstorms
• Indicate unstable conditions
• may extend through the troposphere
• Anvil tops develop
Cumulonimbus
Unusual Clouds
• Lenticular clouds - form downwind of mountain ranges,
• Condensation on windward slope, evaporation on leeward slope
• Nacreous clouds – found in stratosphere, super-cooled water or
ice, mother of pearl clouds
• Noctilucent clouds – found in mesosphere, typically
illuminated after sunset
Cloud Coverage
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•
•
•
When clouds comprise more than 9/10th of the sky = overcast
When coverage is between 6/10th and 9/10th = broken
When coverage is between 1/10th and 6/10th = scattered
Cloud coverage less than 1/10th = clear
Chapter 6: Precipitation Processes:
Why does it rain on us???
Introduction
• Not all clouds precipitate due to the small size and
slow fall rates of average cloud drops
• Rapid cloud drop growth rates are required for
precipitation to form
Growth of Cloud Droplets
• Gravity and frictional drag  balance to achieve terminal velocity
• terminal velocities for cloud drops, due to their small size, cannot
exceed even weak updrafts
• volume of cloud drop must be 1,000,000 times greater than
average drop to overcome updrafts
1. Growth by Condensation
• Initially condensation occurs around
condensation nuclei – forms most cloud
drops.
• Growth limited to a radii of ~ 20 μm due to limited
amount of water vapor available for condensation.
Also with so many droplets competing for a limited
amount of water, none can grow very large.
• Insufficient volume to overcome updraft and
generate precipitation
2. Growth in Warm Clouds
• Clouds with temperatures >0oC dominate tropics and
mid-latitudes during the warm season
• Collision-coalescence generates precipitation
• Process begins with large collector drops (i.e. the
largest droplet) which have high terminal velocities
Collision
• Collector drops collide with smaller drops
• compressed air beneath falling drop forces
small drops aside
• collector drops ‘capture’ fairly large cloud
drops
Coalescence
• When collisions occur, drops either bounce apart
or coalesce into one larger drop
• Coalescence efficiency is very high indicating that
most collisions result in coalescence
3. Growth in Cool and Cold Clouds
• Cool and cool clouds: T < 0oC
• Clouds may be composed of: liquid water, super-cooled water, and/or ice
• If ice nuclei are present, condensation leads to ice; if ice nuclei is absent,
condensation leads to liquid water, i.e., super-cooled water.
• Coexistence of ice and super-cooled water is critical to the creation of cool
cloud precipitation - the Bergeron Process
Bergeron Process
• The Bergeron process
relies on the fact that
the saturation vapor
pressure with respect
to ice is less than the
saturation vapor
pressure with respect
to water.
RH = vapor / saturated vapor
Tempera
ture
RH wrt*
H2O(liq)
RH wrt
H2O(ice)
0°C
100%
100%
-05°C
100%
105%
-10°C
100%
110%
-15°C
100%
115%
-20°C
100%
121%
*wrt = with respect to
differences in saturation vapor
pressures of water
• From the perspective of the supercooled droplets, the air
is in equilibrium at saturation, but from the perspective
of the ice crystals, the air is supersaturated. Therefore,
water vapor will sublimate on the ice crystals.
• Since the amount of
water vapor in the air
has decreased, and
from the perspective
of the supercooled
water droplet, the air
is subsaturated, the
supercooled water will
evaporate until the air
once again reaches
saturation.
Bergeron Process
• When ice and water are present, water will be
deposited directly onto ice
• Ice crystals grow rapidly at the expense of supercooled drops
• Collisions between falling crystals and drops
causes growth through riming and aggregation
• Riming = liquid water freezes onto ice crystals
producing rapid growth when ice crystals fall through a
cloud and collide with super-cooled droplets.
• Aggregation = the joining of two ice crystals to form a
single, large one.
• Collision combined with riming and aggregation allow
formation of precipitation within 1/2 hour of initial
formation
Forms of Precipitation
• Snow results from the Bergeron process, riming, and aggregation
Snowflakes  variable shapes/sizes
Dendrite ice crystals
Plate ice crystal
• distribution related to north-south alignment of mountain ranges and
Great Lakes
• convergence leads to uplift, adiabatic cooling and snowfall =>
Orographic uplift.
•Lake effect
snows develop
on leeside of
water bodies,
e.g. Great
Lakes
Lake effect snows develop on leeside of
water bodies, e.g. Great Lakes
• As cold air from the north or northwest flows over the
lake, heat and water vapor are transferred upward and
make air moist and unstable. As the air passes over the
shore, the wind slow down due to large friction =>
convergence = > air rising => clouds = > heavy snows.
(a) an initial mechanism for uplift
(b) unstable air
(c ) sufficient moisture.
• Rain: always associated with warm clouds and
sometimes cool clouds (T > 0oC)
• Rain showers – episodic precipitation events
associated with convective activity and cumulus
clouds
• Drops tend to be large and widely spaced to begin,
then smaller drops become more prolific => larger
raindrops have a faster terminal velocity.
• Graupel – ice crystals that undergo
extensive riming
• Lose six sided shape and smooth out
• Either falls to the ground or provides
a nucleus for hail
Hail – consists of ice pellets formed in roughly concentric layers.
- Initially, an updraft carries a graupel pellet or water droplet or water
Droplet above the freezing level to form the core of a hailstone.
- When the cores falls, it collides with water liquid droplet that coat it
with a film of liquid water.
- The updraft carries the pellet aloft, and the liquid water freezes to
form a second layer of ice.
- Process repeats, and finally hailstones are very heavy, and updraft can
not resist it.
• Hailstones are very heavy – high
density
• Capable of tremendous amounts of
damage
• Great Plains = highest frequency of
hail events
Annual hail frequency
• Sleet begins as ice crystals which melt into rain through a
mid-level inversion refreeze near surface .
• Freezing Rain forms similarly to sleet, however, the drop
does not completely solidify before striking the surface
Sleet formation involves
a mid-level inversion