Cloud formation occurs through adiabatic cooling or lifting of air parcels to their dew point temperature. Clouds are classified based on their height and form. High clouds like cirrus are made of ice crystals while low clouds like stratus are uniform layers near the surface. Fog forms through different cooling processes like radiation, advection, or evaporation. Precipitation forms through the Bergeron process using ice crystals or collision-coalescence of water droplets. Rain, snow, sleet, hail, and freezing rain are different types of precipitation. Weather modification techniques like cloud seeding are used to artificially influence precipitation and other weather phenomena.

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Forms of Condensation and
Precipitation Chapter 5

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5.1 Cloud Formation

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Cloud Formation
A cloud is a visible aggregate of small water
droplets or ice crystals that are suspended in
the atmosphere above the earth’s surface
Lifting condensation level
 The ascending parcel of air is cooled to its dew
point temperature and triggers condensation

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Cloud Formation
Condensation aloft:
Adiabatic cooling causes clouds to form as
water vapor condenses in the atmosphere.
The air becomes saturated and there must be a
surface.
Condensation nuclei act as surfaces, on which
the water vapor can condense.

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Cloud Formation
Growth of cloud droplets:
Hygroscopic (water-seeking) nuclei are most
effective for condensation.
Growth is rapid at first, then slows as water vapor
is consumed.
Hygrophobic (water-fearing) nuclei are not
efficient condensation nuclei but could droplets
will form on them when the relative humidity
reaches 100%

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5.2 Cloud Classification

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Cloud Classification
Clouds are classified on the basis of two criteria.
1. Form:
 Cirrusare high, white, and thin forming delicate veil
like patches or wisplike strands often have a feathery
appearance
 Cumulus clouds are globular, usually exhibiting a flat
base and appear as rising domes or towers.
 Stratus
clouds are best described as layers or sheets
covering much of the sky.

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Cloud Classification
2. Height:
 High clouds are above 6000 m.
 Middle clouds range between 2000–6000 m.
 Low clouds are at altitudes of less than 2000 m.
 Clouds of vertical development extend upward to
span more than one height range.

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Cloud Classification

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Cloud Classification

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Cloud Classification- High Clouds
Low temp and small amount of water vapor at
high altitudes equal high clouds, thin, white
and made of ice crystals
Cirrus
 Delicate,
icy filaments. Winds cause the
filaments to bend or curl

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Cloud Classification- High Clouds
Cirrostratus
 Transparent,whitish cloud veils with fibrous or
sometimes smooth appearance covering whole
sky.
 Produce halos around the the sun or moon

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Cloud Classification- High Clouds
Cirrocumulus
 White patches composed of small cells or
ripples resembling fish scales

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Cloud Classification- Medium
Clouds
Altocumulus
 Tend to form in large patches composed of
rounded masses or rolls that may or may not
merge
 Composed of water not ice droplets
 The individual cells have a more distinct outline

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Cloud Classification- Medium
Clouds
Altostratus
 Formless layer of grayish clouds that cover all
or portions of the sky.
 Sun is visible as a big bright spot with the edge
of it’s disc not discernible; no halos
 Infrequent snow or drizzle.
 Commonly associated with approaching warm
fronts thicken into nimbostratus (which = lots of
rain)

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Cloud Classification- Low Clouds
Stratus
 Uniform layer covers much of the sky
 Ones with scalloped bottom and appears as long
parallel rolls or broken patches are called
stratocumulus

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Cloud
Classification
- Low Clouds
 Nimbostratus
 Rain clouds

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Clouds of Vertical Development
Cumulus humilis
 Dense, billowly clouds with tops resembling
cauliflower. Often form on clear days
 Known as fair weather clouds
 Humilis are smaller

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Clouds of Vertical Development
Cumulonimbus
 Large dense billowly clouds of considerable
vertical extent in the form of huge vertically
towers
 Tops spread out into an anvil shape
 Towers produce heavy precipitation with
lightening, thunder, and occasionally hail

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Cloud Classification
Cloud varieties:
 Uncinus are hooked shaped clouds, often
precursors to bad weather.
 Fractus or fractured clouds are stratus or
cumulus clouds that appear broken.
 Mammatus clouds have udder-shaped
protuberances on their bottom surfaces and are
associated with stormy weather.
 Lenticular clouds are lens shaped and are
common in rugged or mountainous
topographies.

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Cloud Classification

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5.3 Types of Fog

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Types of Fog
Fog is defined as a cloud with its base at or
very near the ground.
Fog results from cooling or when air becomes
saturated through the addition of water vapor
 Radiation
 Advection
 Upslope
 Evaporative

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Fog Formed by Cooling
 Radiation
fog results from radiation cooling of the
ground and adjacent air.
 Night time phenomena requiring clear skies and
relatively high humidity
 The high humidity can cause a small amount of
cooling to lower the temperature to the dew point.
 To be extensive, there should be a slight breeze.
 It is usually thickest in valleys.

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Fog Formed by Cooling
 Advection fog is a blanket of fog caused by warm,
moist air blowing over a cold surface.
 Some turbulence is needed (10–30 kph winds).
 Turbulence facilitates cooling through a thicker
layer of air and carries it to greater heights
 Wintertime fog in the midwest
 This fog is thick and produces hazardous driving
conditions

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Fog Formed by Cooling
Upslope fog is created when relatively humid
air moves up a sloping landform or up the
steep slopes of a mountain.
 Theupward flow causes the air to expand and
cool adiabatically resulting in fog.

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Fog Formed by Cooling
There are two types of evaporation fog.
 Frontal (precipitation) fog
 Occurs when rain droplets falling from
relatively warm air above a frontal surface
evaporates into the cooler air below and
causes it to become saturated.

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Fog Formed by Cooling
Steam fog
 Occurs when cool air moves over warm water.
 Moisture evaporates and saturates the air above
it.
 Rising water vapor meets the cool air,
condenses and rises. Looks like steam.
 Steam fog is very common over lakes.

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5.4 How Precipitation Forms

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How Precipitation Forms
 Clouddroplets are about 20 micrometers in
diameter. A human hair is about 75
micrometers
 A rain
droplet is about 2 mm or 100 times the
average cloud droplet
 Volume is 1 million times greater

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How Precipitation Forms

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How Precipitation Forms
 Process that generates precipitation in the middle
latitudes
 The Bergeron Process depends on the process of
the coexistence of water vapor, liquid cloud
droplets and ice crystals.
 Cloud droplets do not freeze at 0°C. It freezes at
-40°C. it is super cooled.
 The saturation vapor pressure above ice crystals is
slightly lower than above super cooled liquid droplet

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How Precipitation Forms
So, imagine a cloud at -10°C where each ice
crystal (snow crystal) is surrounded by thousands
of liquid droplets. Because air is saturated at 100%
with respect to liquid water, it will be super
saturated (above 100%) with respect to the newly
formed ice crystals.
At this result of this supersaturation, the ice
crystals collect water, lowering relative humidity,
the water droplets shrink to replenish what was
lost

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How Precipitation Forms
So the growth of ice crystals is fed by
continued evaporation of liquid droplets
When ice gets large enough they fall, they
grow as they intercept cloud droplets on their
fall. A chain reaction ensues and produces
many snow crystals= snow flakes

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How Precipitation Forms
The type of precipitation (snow, sleet, rain, or
freezing rain) depends on the temperature in
the lower few km of the atmosphere
When the surface temperature is above 39°F
snow usually melts before it hits the ground

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How Precipitation Forms
The Bergeron process
(precipitation from cold
clouds) depends on the
coexistence of water
vapor, liquid cloud
droplets, and ice crystals

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How Precipitation Forms
The collision-coalescence process
(precipitation from warm clouds) occurs as
copious rainfall associated with clouds located
below the freezing level (called warm clouds),
especially in the tropics.
 Small droplets hit other droplets and become
larger.
 They collide with more droplets and their
falling velocity increases.

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How Precipitation Forms

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How Precipitation Forms

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5.5 Forms of Precipitation

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Forms of Precipitation

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Forms of Precipitation
Rainis restricted to droplets of water with a
diameter at least 0.5mm.
Cloudbursts are unusually heavy rainfalls.
Drizzleare fine, uniform droplets with a
diameter less than 0.5mm. (not really
considered rain)

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Forms of Precipitation
Virga is rain that evaporates above ground.
Fallstreaks are ice crystals that sublime in the
dry air below
Mist contains the smallest droplets.

54. +Virga

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Forms of Precipitation
 Snow is precipitation in the form of ice crystals or
aggregates of ice crystals.
 Size,
shape and concentration depend to great
extent on the temp at which they form
 Low temperature, the moisture is low = very light
fluffy snow made up of individual 6 sided crystals
 Warmer temperature at about 23°F, ice crystals join
together into large clumps of tangled crystals

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Forms of Precipitation
Sleet is clear to
translucent particles of
ice, rain drops freeze
while falling.
Freezing rain (glaze) are
rain drops that become
super-cooled, hit a
surface, and freeze
immediately.

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Forms of Precipitation
Hail is precipitation in the
form of hard, rounded
pellets of ice.
 Itis produced in
cumulonimbus clouds.
 Hail stones begin as small
ice pellets and grow as they
are propelled by updrafts
and downdrafts through the
cloud.

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Forms of Precipitation
Rime is a deposit of ice crystals, formed on
surface objects by super-cooled fog or cloud
droplets.
 Itoccurs when the surface temperature of an
object is below freezing.

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5.6 Precipitation Measurement

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Precipitation Measurement
Standard instruments:
 A standard rain gauge catches rain water and
conducts it through a narrow opening into a
cylindrical measuring tube.
 The gauge is 20 cm in diameter and can
measure rainfall to the nearest 0.025 cm.
 < 0.025 cm = trace

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Precipitation Measurement
A tipping bucket has two compartments (or
buckets). When one bucket fills, it tips and
empties its water and the other bucket takes its
place at the funnel.
A weighing gauge collects rain fall in a
cylinder that rests on a spring balance. As the
cylinder fills, the movement is transmitted to a
pen that records the data.

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Precipitation Measurement
 Whenmeasuring snowfall, two measurements are
normally taken.
 The depth is measured with a calibrated stick.
 To obtain the water equivalent, snow is melted and
then weighed or measured as rain.
 Weather radar uses radio waves to measure
precipitation.
 The radio waves penetrate small droplets, but are
reflected off larger ones. Echoes are sent back and
displayed.

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5.7 Intentional Weather
Modification

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Intentional Weather Modification
Intentional weather modification, such as
cloud seeding, is deliberate human
intervention to influence processes that
constitute the weather.
 Snow and rain making
 Silver iodide crystals act as freezing nuceli.
 Fog and cloud dispersal
 Cloud seeding with dry ice into super-cooled
fog or stratus clouds helps to disperse them to
improve visibility.

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Intentional Weather Modification
 Hailsuppression has been shown to be
ineffective.
 Anti-hail cannons produced a loud whistling
noise and a large smoke ring thought to
suppress hail.
 Cloud seeding with silver iodide crystals was
also employed to disrupt the growth of
hailstones.

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Intentional Weather Modification
Several methods of frost prevention are being
used.
 Water sprinklers add heat from water and from
the latent heat of fusion when the water freezes.
 Air mixing uses wind machines to mix warm
and cool air.
 Orchard heaters produce the most successful
results, but fuel cost can be significant.