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Precipitation and Cloud
Topic
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What is cloud
Cloud Classification
Cloud Formation
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Overview of Cloud Formation
Cloud formation by Bounyant Lifting
Cloud formation by Forced lifting
Cloud formation due to Cooling by the Earth’s
surface
What is cloud?
A cloud is a visible mass of condensed droplet,
frozen crystals suspended in the atmosphere
above the surface of the Earth
The condensing substance is typically
water vapor, which forms small droplets or
ice crystals, typically 0.01 mm in diameter.
When surrounded by billions of other
droplets or crystal they become visible as
clouds.
Cloud base or condensation level
Cloud Classification
Cloud Classification
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Low Clouds
Middle Clouds
High Clouds
Cloud Vertical Development
Low Clouds
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Low Clouds
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Stratus
Stratocumulus
Nimbostatus
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Low cloud base height are within 2000 m.
from earth surface.
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Generally compose of water droplets or
supercool droplets.
Stratocumulus are low, layered clouds with some vertical development.
Their darkness varies when seen from below because their thickness
varies across the cloud. Thicker sections appear dark, and thinner areas
appear as bright spots.
Low clouds have bases below 2000 m. Stratus are layered
clouds that form when extensive areas of stable air are lifted.
Usually the rate of uplift producing a stratus cloud is only a few
tens of centimeters per second, and its water content is low.
Low, layered clouds that yield light precipitation are called nimbostratus.
Seen from below, these clouds look very much like stratus,
except for the presence of precipitation.
Middle Clouds
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Middle Clouds
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Altostratus
Altocumulus
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Middle-cloud base occur between 2000 to
7000 m. above ground.
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Cloud temperature below 0 C. As a result,
middle-cloud compose of supercool droplets
or ice crystals.
Altostratus clouds are the middle-level counterparts to cirrostratus.
They are more extensive and composed primarily of liquid water.
Altostratus scatter a large proportion of incoming sunlight back to space.
The insolation that does make its way to the surface consists primarily or
exclusively as diffuse radiation. When viewing the Sun or Moon behind
altostratus, one sees a bright spot behind the clouds instead of a halo.
Altocumulus are layered clouds that form long bands or contain
a series of puffy clouds arranged in rows. They are often gray in
color, although one part of the cloud may be darker than the rest
and consist mainly of liquid droplets rather than ice crystals.
High Clouds
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High Clouds
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Cirrus
Cirrostratus
Cirrocumulus
High cloud typically form between 6000 to
10000 m. above the ground depend of the
season and the latitude (higher where
warmer).
High cloud compose of entirely ice crystals.
High clouds are generally above 6000 m (19,000 ft).
The simplest of the high clouds are cirrus,
which are wispy aggregations of ice crystals.
Cirrostratus clouds are composed entirely
of ice but tend to be more extensive
horizontally and have a lower concentration
of crystals. When viewed through a layer
of cirrostratus, the Moon or Sun has a
whitish, milky appearance but a clear
outline. A characteristic feature of
cirrostratus clouds is the halo,
a circular arc around the Sun or Moon
formed by the refraction (bending) of
light as it passes through the ice crystals.
Cirrocumulus are composed of ice crystals that arrange
themselves into long rows of individual, puffy clouds.
Cirrocumulus form during episodes of wind shear, a condition
in which the wind speed and/or direction changes with height.
Wind shear often occurs ahead of advancing storm systems,
so cirrocumulus clouds are often a precursor to precipitation.
Because of their resemblance to fish scales, cirrocumulus
clouds are associated with the term “mackerel sky.”
Clouds vertical development
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Clouds vertical development
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Cumulus
Cumulonimbus
Cloud base height are lower than 2000 m. but their
top may reach top of tropopause.
Cumuliform clouds are those that have substantial vertical
development and occur when the air is absolutely or conditionally
unstable. Fair-weather cumulus (above) called cumulus humilis,
do not yield precipitation and they evaporate soon after formation.
Cumulonimbus are the most violent of all clouds and produce the most
intense thunderstorms. In warm, humid, and unstable air, they can have
bases just a few hundred meters above the surface and tops extending
into the lower stratosphere. A cumulonimbus is distinguished by the
presence of an anvil composed entirely of ice crystals formed by the high
winds of the lower stratosphere that extend the cloud forward.
Mechanism of Cloud Formation: Overview
Clouds
Aggregations of
Liquid droplets
Droplets &
ice crystals
Ice crystals
Low clouds
Middle clouds
High clouds
Formed when air becomes
saturated
Due to
Modify from: Danielson, et al, 2003, Meteorology
Mechanism of Cloud Formation: Overview
Saturation Process
Cooling
Moisture gain
Mixing
Adiabatic ascent
Contact
with
Forces (stable) earth’s
surface
ascent
Buoyant
(unstable)
ascent
Orogr Fron
-aphic -tal
Earth’s surface Evaporating
precipitation
Parcel
Vertical
More than one mechanism may be archive in the same
cloud
Modify from: Danielson, et al, 2003, Meteorology
Mechanism of Cloud Formation: Overview
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More than one mechanism may be archive in
the same cloud.
Upward Motion as a Cooling Mechanism
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Adiabatic Changes in Rising Air Parcel
Graphic Dry Adiabatic Processes
Moist Adiabatic Processed
Adiabatic Changes in Rising Air Parcel
Change in density,
temperature and pressure
P4, T4, 4
Height
P e = P4
P3, T3, 3
P2, T2, 2
Pe
P1, T1, 1
Time
Pressure
Assume that during this displacement, no exchange of heat energy occur between
air parcel and environment
Adiabatic Changes in Rising Air Parcel
Changes in Density
Density =
mass
volume
Density decrease due to
greater volume air parcel
Changes in Temperature
Assume adiabatic process,
there is no external source
of energy for air parcel.
10C/km
Air parcel require energy
for expansion, so energy
come from parcel’s
component molecular.
2
1mv1
=
2
Tp1
Rate of temperature change
is known as dry adiabatic
lapse rate
2
1mv2 + work
2
Tp2
Air parcel expansion
Adiabatic Changes in Rising Air Parcel
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Change in Pressure
According to gas law;
Pressure = constant  density  temperature
P = RT
Pressure will decrease until air parcel pressure equals the
surrounding air pressure; otherwise, a pressure gradient will
exist, and the parcel will continue to expand.
Why cloud form in rising air
Graphing Dry Adiabatic Processes
Variables of State
Temperature-pressure plotting chart
Graphing Dry Adiabatic Processes
Dry
adiabatic
lapse rate
10C/km
Dry adiabatic temperature change
ascent
decent
Graphing Dry Adiabatic Processes
Dewpoint Changes
Saturated Mixed ratio line
There is water
vapor 2 grams
within each
kilogram of dry
air
Moisture change can be accounted.
The saturation mixing ratio value
corresponding to the dewpoint
gives the actual amount of water
vapor present in the air.
The dewpoint temperature
follows a line of constant
mixing ratio. Therefore,
dewpoint temperature
decrease much more slowly
with height than the air
temperature.
Graphing Dry Adiabatic Processes
Determining Relative Humidity
RH =
Mixing ratio  100
Saturation mixing ratio
= 2 g/kg  100%
8 g/kg
= 25%
Determining the Lifting
Condensation Level
LCL is level where dewpoint
and rising air temperature
become equal.
Lifting Condensation Level
LCL commonly marks the
cumulus cloud base.
The first wisps of cloud
beginning to form.
Moist Adiabatic Processes
Is it possible that air parcel
continuously move follow
dry adiabatic lapse rate?
Moist Adiabatic Processes
Is it possible that air parcel
continuously move follow
dry adiabatic lapse rate?
Impossible because the air
would be highly
supersaturated, an occurrence
not observed.
RH =
Mixing ratio  100
Saturation mixing ratio
= 2 g/kg  100%
0.5 g/kg
= 400%
Moist Adiabatic Processes
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Relative humidity in the cloud typically is never far
from 100% that means temperature and dewpoint
temperature equal to each other as the saturated air
continues to ascent.
As water vapor condenses to droplet, it changes
conditions in the parcel in two important ways;
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The water molecule condense from gas to liquid. The air’s
parcel mixing ratio decrease.
The water releases considerable latent heat, thus warming
air parcel
Moist Adiabatic Processes
Mixing ratio = 1g/kg at “D”
Mixing ratio of air parcel = 2g/kg
The air parcel lost 2 – 1 = 1 gram
of water vapor from each kilogram
of dry air.
The lost water vapor become
droplet of ice crystal forming.
Cloud formation by Buoyant Lifting
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Heating of the earth’s surface may lead to convection, and
convection consists in part of rising air currents. Thus the
rising parcel is part of a convective circulation. As the earth’s
surface becomes warmer, more air is carried aloft
convectively, cooling as it rise until its water vapor begins to
condense to form cumulus clouds.
However, the above explanation is incomplete.
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How height the rising air will go?
Will it stop rising before reaching the LCL and form no cloud at all?
Will it reach LCL and form a cumulus humilis, or will it grow to a
giant cumulonimbus?
Parcel versus the Environment
Parcel = a specific group of gas
molecules that does not mix with
the surrounding air but changes
temperature with altitude at either
the dry or moist adiabatic rates.
Environment = consist of different
air molecules at each level whose
temperature and humidity values
may differ erratically from one
level to the next.
Radiosonde data
Cloud Formation Due to Static Instability
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Stability of Unsaturated air
Stability of Saturated Air
Stability of Lapse Rate
Changes in Stability
Stability of Unsaturated air
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Air parcel Environment
temperature temperature
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New Mexico, Late afternoon
Tp < Te
From
P = RT
= P
RT
Air parcel density less than
environment. Therefore, being
denser, the parcel will sink
back toward the surface from
which it was perturbed; air is
stable.
Stability of Unsaturated air
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Environmental
temperature
Air parcel
temperature
Tp > Te
From
P = RT
= P
RT
Air parcel density greater than
environment. Therefore, lower
density, the parcel will rising
from the surface; air is stable.
New Mexico, Early spring morning
Stability of Unsaturated air
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Cumulus clouds occur with greater frequently in the
afternoon than in the morning.
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In late afternoon, the earth’s surface is at its warmest, the
near-surface environment is at its most unstable, and
convective currents leading to cumulus formation are
therefore most likely to exist.
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Near sunrise, the ground is coldest, causing the near-surface
atmosphere to be at its most stable, with a corresponding
absence of deep convection and cumulus cloud development.
Stability of Saturated Air
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At 800 mbar, air is
saturated.
Stability and Lapse Rate
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Absolutely unstable; any
environment whose lapse
rate is greater than dry
adiabatic.
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A lifting air parcel,
whether it cools dry or
moist adiabatically, will
remain warmer than the
environment and hence
will be buoyant and
continue to rise.
Stability and Lapse Rate
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Conditionally unstable; any
environment with lapse
rate between dry and moist
adiabatic.
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Parcel stability is depend
on the air’s humidity.
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If it is saturated, a parcel
will unstable.
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If it is unsaturated, a parcel
is stable.
Stability and Lapse Rate
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Absolutely stable;
environment lapse rate less
than the moist adiabatic
rate.
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Parcel dose not depend on
the air’s humidity: whether
the parcel rise dry or moist
adiabatically.
Cumulus Cloud Growth
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Cumulus formation results from buoyant
forces acting on a perturbed air parcel.
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The parcel, being warmer and less dense than
the environment, rises and cools at dry
adiabatic rate.
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Eventually, condensation occurs; thereafter,
cooling proceeds at the moist adiabatic rate as
the parcel continues to rise.
Cumulus Cloud Growth
Cloud Formation by Forced Lifting
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Stratiform clouds
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Some external mechanism forces the rntire
layer to ascend.
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Orographic Lifting
Lifting in Fronts and Low Pressure Centers
Orographic Lifting
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Unlike buoyant lifting, in which individual cells of air rise, the entire
lowest layer of the atmosphere has ascended by moving uphill. The result
is the uniform sheet of clouds.
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Orographic lifting of stable air causes stratiform and lenticular clouds.
The air, perturbed upward orographically, receives no additional lift from
buoyancy and settles back to lower altitude once past the perturbing
influence if high terrain.
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If the orographic lifted air is unstable, however, then cumulus clouds
result from the combined influences of buoyancy and orographic lifting.
The high frequency of cumulus clouds and thunderstorms in the
mountains is a result of orographic lifting and buoyancy working together.
Lifting in Fronts and Low Pressure Ceneter
The front rises steeply, about1 km over 50 km distance, with
showers and thunderstorms at the front.
Lifting in Fronts and Low Pressure Ceneter
The cross-sectional view shows the gentle slope of overrunning
warm air (shallow slope 1:300) , a typical temperature inversion
(32oF isotherm bends back), and the shifting winds.
Occluded Fronts
Fast moving cold fronts may overtake the slower
moving warm front, particularly when they are
influenced by cyclonic winds.
Cold occlusion describes this scenario with very cold
air, as compared with the warm occlusion.
Figure 12.19
Formation of a cold occluded
front: The faster moving cold
front (a) catches up to the
slower moving warm front (b),
and forces it and the warm air
mass to rise off the ground (c).
[Green shading represents
precipitation]
(c)
(b)
(a)
Idealized life cycle
of a wave cyclone
Tropical Cyclone (TC).
A tropical cyclone is a warm-core, low pressure
system without any "front" attached, that develops
over the tropical or subtropical waters, and has an
organized circulation. Depending upon location,
tropical cyclones have different names around the
world. In the:
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Atlantic/Eastern Pacific Oceans - hurricanes
Western Pacific - typhoons
Indian Ocean - cyclones
Regardless of what they are called, there are several favorable
environmental conditions that must be in place before a tropical
cyclone can form. They are:
 Warm ocean waters (27?C) throughout a depth of about 46 m.
 An atmosphere which cools fast enough with height such that
it is potentially unstable to moist convection.
 Relatively moist air near the mid-level of the troposphere
(4,900 m).
 Generally a minimum distance of at least 480 km from the
equator.
 A pre-existing near-surface disturbance.
 Low values (less than about 37 km/h) of vertical wind shear
between the surface and the upper troposphere. Vertical wind
shear is the change in wind speed with height.
1. Formation
1.
2.
3.
Sea surface temperature at
least 80?F (27?C)
Calm air over the sea
Coriolis Force, the force
that causes the cyclone to
spin.
2.Prematurity
As the tropical low becomes further organized
and the surface winds reach gale force it is then
declared a tropical cyclone according to international
convention. Satellite and radar observations of the
system show the distinctive spiral banding pattern.
3. Full Maturity
If the ocean and atmosphere environment continues
to be favorable the cyclone may continue to intensify
as it moves poleward. The cloud system becomes
more circular in shape and develops a distinct eye.
This is the severe cyclone stage where the cyclone is at
its most dangerous. Approximately half of the cyclones
that form progress to full maturity.
4. Decay
Tropical cyclones normally decay when they move into a less
favourable environment, either over land or the cooler waters
in higher latitudes. The rate of decay varies with the
circumstances. A tropical cyclone moving into mid-latitude
westerlies may be quickly sheared apart by strong upper winds,
or it may react with a frontal system and persists for several more
days. Similarly, a cyclone moving over land normally dissipates
rapidly due to loss of its energy source, namely the warm ocean
surface. However in northern Australia cyclones moving inland
are frequently observed to persist as rain depressions for a
number of days bringing widespread flood rains, and may even
redevelop if they move over the ocean once more.
Tropical Cyclone Structure
4
1
2
3
Tropical Cyclone Classification
63 km/h or less
63 - 118 km/h
119 km/h or more
"tropical depressions".
"tropical storm"
the cyclone is called:
•hurricane in the North Atlantic Ocean, the Northeast
Pacific Ocean east of the dateline, and the South Pacific
Ocean east of 160?E, (The word hurricane comes from the
Carib Indians of the West Indies, who called this storm a
huracan. Supposedly, the ancient Tainos tribe of Central
America called their god of evil "Huracan". Spanish colonists
modified the word to hurricane.),
•typhoon in the Northwest Pacific Ocean west of the dateline
(super typhoon if the maximum sustained winds are at least
150 mph / 241 km/h)
Cloud Development
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How are cloud droplets with typical diameters of
10-100 mm formed?
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surface tension keeps the surface area of a liquid to a
minimum,
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thus it is difficult for molecules to stick together when
the droplets are small
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this makes it impossible to start growing, or nucleating
droplets of pure water
Average Cloud droplet
r = 10 mm,
N = 106 number per liter,
and v = 1 cm/s
Average rain drop
r = 50 mm,
N = 103 number per liter,
and v = 27 cm/s
Average Cloud condensation nuclei
r = 0.1 mm,
N = 106 number per liter,
and v = 0.0001 cm/s
Cloud Development
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Why cloud droplets are observed to form in the atmosphere
when ascending air just reaches equilibrium saturation?
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Answer: Atmosphere contains significant concentrations of
particles of micron or submicron size which have affinity for
water and serve as center for condensation. These particles
are called condensation nuclei
Cloud Development
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condensation nuclei (tiny solid and liquid particles,
which are impurities in the air) of diameters 0.1-10
mm provide relatively large surfaces on which
condensation of water vapour can readily occur;
frozen nuclei help in the growth of ice crystals
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hygroscopic (water attracting) nuclei also help in the
formation of cloud droplets
Two important processes to form
raindrops..…
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Collision-Coalescence Process
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Bergeron Process
Collision-Coalescence process
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a large cloud droplet falls at a
larger terminal velocity than a
small droplet
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the terminal velocity characterizes
the fall speed of an object, such as
a paratrooper, where the
downward force of gravity is
balanced by the upward force of
air resistance
Collision-Coalescence process
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as the large droplet falls through a cloud, it sweeps up and collects smaller
droplets through collision and coalescence
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the process becomes effective only when the cloud droplets reach about 40
mm
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but.....
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cloud droplets do not usually grow larger than 20 mm in diameter
through condensation nuclei alone
thus we need the.....
Bergeron process
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operates in clouds at temperatures below 0oC, and requires the co-existence
of water vapour, ice crystals and supercooled liquid water droplets (i.e.,
unfrozen water droplets below 0oC)
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at the same temperature, the saturation vapour prssure over a liquid surface
is greater than over an ice surface, i.e., water molecules vaporize more readily
from liquid water than from solid ice, at subfreezing temperatures
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thus a vapour pressure that is saturated for water droplets is supersaturated
for ice crystals
Bergeron process
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the net result is that the ice
crystals grow at the expense of
the supercooled water droplets
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as the ice crystals grow larger,
they become heavier and the
terminal velocity increases; they
fall out of the cloud base and the
coalescence process takes over