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Chapter 5 – Clouds, Precipitation and Optical Effects
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Overview
 Cloud particles consist of:
 Water droplets.
 Ice crystals.
 Or a mixture of both.
 Classification by type, based on appearance.
 Classification based on altitude of bases.
 Clouds of vertical development.
 Some special cases.
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What are Clouds?
 Clouds are made up of tiny particles suspended in the
atmosphere.
 Water clouds are made up of small water droplets.
 Ice clouds are made up of small ice crystals.
 They form when water vapor condenses on particles called
condensation nuclei when either:
 The air temperature is reduced to the dew point, or
 Water vapor is added until the dew point rises to equal the air
temperature.
 The average size of cloud particles is 0.02mm, smaller than
the width of a human hair.
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How are Clouds Classified?
 By the height of their bases.
 Low: less than 6,500 ft (2,000 m).
 Middle: 6,500 to less than 20,000 ft.
 High: 20,000 ft. (7,000 m) or above.
 By their appearance.
 Cumuliform (lumpy)
 Stratiform (flat, uniform)
 Cirriform (wispy)
 A separate category is Clouds of Vertical Development.
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Summary of Cloud Types (1 of 3)
 Low Clouds (water droplets):
 Cumulus (Cu). Cumuliform clouds of small to moderate vertical
extent.
 Stratus (St). Stratiform clouds that look grey from below.
 Stratocumulus (Sc). Layers of lumpy, fuzzy clouds.
 Nimbostratus (Ns). Dark, stratiform clouds with precipitation.
 Middle Clouds (mostly water, some ice):
 Altostratus (As). Similar to stratus. Often thin.
 Altocumulus (Ac). Similar to stratocumulus. Sometimes in rows.
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Summary of Cloud Types (2 of 3)
 High Clouds (ice particles):
 Cirrus (Ci). Wispy, thin, often with narrow streaks.
 Cirrostratus (Cs).Thin, uniform layer or in clumps (e.g. anvil tops).
 Cirrocumulus (Cc). Layer of uniform, or irregular, small cumulus.
 Clouds of Vertical Development:
 They are cumuliform.
 More detail later.
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Summary of Cloud Types (3 of 3)
Types/
Heights
Low
(<6,500 ft.)
Middle
(6,500-20,00 ft.)
High
(>20,000 ft.)
Cumuliform
Cumulus (Cu)
Stratocumulus (Sc)
Altocumulus
(Ac)
Cirrocumulus
(Cc)
Stratiform
Stratus (St)
Nimbostratus (Ns)
Altostratus (As)
Cirrostratus (Cs)
Cirriform
None
None
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Cirrus (Ci)
Clouds of
Vertical
Development
Cumulus
Congestus or
Towering
Cumulus
(Cu)
Also as (Tcu)
Cumulonimbus
(Cb)
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Cumulus (Cu) Clouds
 Ordinary cumulus are small,
puffy clouds (“fair weather
cumulus” or cumulus
humilus).
 They have little vertical
development (see also
clouds of vertical
development).
 They very rarely produce
precipitation.
Courtesy of NOAA
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Stratus (St) Clouds
 Stratus (St) clouds are flat




and uniform.
They can be thin or thick.
When seen from below, they
are grey—the thicker, the
darker.
When seen from above, they
are white (daytime).
They do not produce
precipitation (see Ns).
Courtesy of NOAA
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Stratocumulus (Sc) Clouds
 Stratocumulus (Sc) have an
appearance between stratus
and cumulus.
 They are usually layered like
stratus.
 They have a somewhat puffy
shape.
 The edges, unlike Cu, are
fuzzy.
Courtesy of NOAA
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Nimbostratus (Ns) Clouds
 Nimbostratus (Ns) clouds are
simply stratus clouds that
have thickened and produce
precipitation.
 The are typically fairly dark
as seen from below.
 Shafts of rain can usually be
seen below them.
Courtesy of NOAA
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Altocumulus (Ac) Clouds
 Altocumulus (Ac) clouds
appear similar to Sc, with
clumps of cumuliform
pieces.
 They can be arranged
randomly, or,
 In rows, like “streets”.
 They do not precipitate.
 The “streets” show the
Courtesy of NOAA
direction of upper winds.
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Altostratus (As) Clouds
 Altostratus (As) clouds look
very much like stratus
clouds.
 They can be:
 Thick (can’t see the sun) or
 Thin (sun is partly visible).
 When thin, there will be a
corona around the sun or
moon (see section 3 on
optical phenomena).
Courtesy of NOAA
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Cirrus (Ci) Clouds
 Cirrus (Ci) clouds are formed
of ice particles.
 Their appearance is thin and
wispy.
 Usually have thin filaments.
 If these are curved, they are
called “mares tails.”
 They are often mixed with
cirrostratus clouds.
Courtesy of NOAA
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Cirrostratus (Cs) Clouds
 Cirrostratus (Cs) clouds look
very much like Altostratus
clouds.
 They are ice clouds.
 Principle way to tell the
difference is if the sun or
moon is behind them.
 As clouds show coronas.
 Cs clouds show halos.
 See Section 3
Courtesy of NOAA
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Cirrocumulus (Cc) Clouds
 Cirrocumulus (Cc) clouds are
similar to Altocumulus
clouds, but the puffs look
smaller, since they are
farther away.
 Almost always ice.
 Large patches are called
“mackerel skies.”
 They show the upper level is
unstable.
Courtesy of NOAA
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Clouds of Vertical Development
 Cumulus clouds whose tops grow above the low cloud
region (<6,500 ft.) are called clouds of vertical
development.
 If the layer of unstable air is moderately thick, cumulus
clouds can develop into towering cumulus, normally higher
than they are wide.
 If the layer is unstable throughout most of the troposphere,
they grow into cumulonimbus (Cb).
 Main difference is that Cb produce precipitation. They can
produce violent weather, as discussed in the next chapter.
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Cumulonimbus (Cb) Clouds
 Cumulonimbus (Cb) are
cumuliform clouds that
have grown very tall.
 They are almost always
thunderstorms (produce
lightning).
 They can have rounded
tops or flat, anvil-shaped
tops of cirrostratus.
 Much more in Chapter 6.
Courtesy of NOAA
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Interesting Examples (1 of 3)
 A very beautiful form of
altostratus is the lensshaped clouds that top
mountains.
 These “lenticularis” clouds
form when there is a:
 Deep layer of steady wind.
 Stable atmosphere.
 Sufficient moisture.
 Mountain or range of
mountains to provide uplift.
Courtesy of NOAA
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Interesting Examples (2 of 3)
 Cumulonimbus mammatus
clouds are mature
thunderstorms whose base is
lumpy, rather than flat.
 It is a sign that the cloud is
about to start dissipating.
 It is not, as was once thought,
a sign of impending tornados.
Courtesy of NOAA
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Interesting Examples (3 of 3)
 One feature of thunder-
storms that does portend
the formation of a tornado is
the wall cloud at the base.
 It can often be seen as
beginning the rotation that
will lead to the tornado.
Courtesy of NOAA
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Overview
 Precipitation—How does it form?
 What are the basic types?
 Ice on surfaces.
 Station model—the final look.
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Precipitation
 Precipitation can be in the
form of water drops or ice,
just like clouds.
 It can be rain, drizzle, snow,
sleet, ice pellets, freezing rain,
etc.
 The average size of a rain drop
is 2 mm (compare to cloud
particle in the picture).
 Drizzle is less than 0.5 mm.
©1997, USA Today. Reprinted with permission
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How Does Precipitation Form?
 There are two accepted theories of how cloud particles
become precipitation.
 The first, applicable to warm tropical air, is called the collision-
coalescence process. It requires many salt particles in the air.
 The second, applicable to mid-latitude air, which is well below
freezing at higher altitudes, is called the Bergeron process.
 In the collision-coalescence process, water droplets are
moved around in the cloud by winds and convection.
 When they collide, they may coalesce into larger droplets.
 The process repeats until the drops are large enough to fall.
 They then grow larger by collision with cloud droplets.
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The Bergeron Process
 The Bergeron process relies on two facts of physics:
 Pure water will not freeze until approximately -40°C.



With freezing nuclei, it can freeze well above -40°C.
There are not many freezing nuclei in the mid-latitudes at the
heights where precipitation forms.
Water below 0°C is called super-cooled.
 Vapor pressure of ice is less than that of water at a given
temperature.


A few ice particles in the cloud will therefore grow at the expense
of the water droplets.
Once the ice particles are large enough to fall, they begin
collecting the super-cooled water droplets, which freeze
instantly.
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The Bergeron Process (cont.)
 Cold cloud particles are a mixture of water and ice.
 Between 0°C and -10°C, they are virtually all super-cooled water.
 Between -10°C and -20°C, they are a mixture of water and ice.
 Below -20°C, they are normally all ice particles.
 Therefore, in the higher levels of the atmosphere, precipitation
is in the form of ice pellets or snow.
 Changes depend on the vertical temperature structure.
 It can remain as frozen precipitation.
 It can melt into rain or drizzle.
 It can melt and then re-cool to sleet or super-cooled water.
 The next slide shows these processes in a warm front.
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Forms of Precipitation
©1997, USA Today. Reprinted with permission
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Precipitation Rates
 Drizzle drops are 0.5 mm or less (by definition), and the rate of
fall is measured by the visibility:
 Light: Visibility greater then 1 km (5/8 mile).
 Moderate: Visibility between 1 and 0.5 km.
 Heavy: Visibility 0.5 km (5/16 mile) or less.
 Rain drops average 2 mm, and the rate of fall is measured in
inches per hour:
 Light: 0.1 inches per hour or less.
 Moderate: 0.1 to 0.3 inches per hour.
 Heavy: Greater than 0.3 inches per hour.
 For snow, bounds are the same as drizzle.
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Other types of Precipitation
 Snow pellets are snow partially melted and re-frozen. They




are brittle and can be crushed with the fingers.
Snow grains are the frozen equivalent of drizzle.
Ice crystals (“diamond dust”) are tiny single crystals.
Sleet (ice pellets) are re-frozen rain or ice unchanged after
formation (never melted).
Freezing rain/drizzle is ice or snow melted and re-cooled.
 Freezing rain forms ice sheets (glaze ice) on cold objects.
 Freezing drizzle usually forms rough ice (rime ice).
 Hail is formed in cumulonimbus clouds (next chapter).
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Final Station Model
 This completes the station model
(for our purposes).
 To the left of the circle: Visibility




of 1 mile with steady light rain.
Below: Nimbostratus clouds with
bases 200 to 599 ft (coded as 2).
Above: Middle cloud is thick
altostratus.
The very top: High cloud is cirrus.
Center: Total cloud cover is 6/8.
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965
1
-23\
49
2
 What is the weather situation?
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Overview
 Types of meteors.
 How light interacts with matter.
 Absorption.
 Reflection.
 Scattering.
 Refraction.
 Halos and coronas.
 Sun dogs and sun pillars.
 Rainbows.
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Meteors
 Things in the atmosphere are called meteors.
 Hydrometeors are things like clouds and precipitation.
 Lithometeors are things like dust, aerosols, and soot.
(“Shooting stars” are meteors only while in the atmosphere).
 Photometeors are not objects, but are interactions of light
with the molecules of the air and with other meteors.
 Photometeors, such as those below, tell us something
about the state of the atmosphere.
 Halos and coronas.
 Sun pillars and sun dogs.
 Rainbows.
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Interaction of Light with Matter
 When light from the sun or moon enters the atmosphere, it
can interact in four distinct ways:
 It can be absorbed, changing its electromagnetic energy into
thermal energy (heat).
 It can be reflected, which changes only the propagation direction
without affecting the light in any other way.
 It can be scattered (form of diffraction), dispersing the energy in
all directions, like river water striking a rock.
 It can be refracted, changing direction and dispersing the
colors—i.e., bending depends on wavelength.
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Scattering
 Scattering disperses the light
spatially in all directions.
 More energy is scattered near
Particle Sizes
Direction
of propagation
the forward direction in most
cases.
 The amount scattered
decreases with angle from the
forward direction
 The pattern depends on the
ratio of the wavelength to the
size of the particle.
 Air molecules scatter blue light
Very Small
(air molecules)
Small
Large
more than red.
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Refraction
 Refraction bends light in one
direction or a small range of
directions.
 Long wavelengths (red)
bend less than short (blue)
 The degree of bending is
influenced by:


The optical properties of the
material.
The angle of incidence.
©2008, US Power Squadrons. Reprinted with permission
 An example is a prism.
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Why is the Sky Blue?
 Air molecules scatter blue
light more strongly than
other colors.
 The blue light is scattered
many times, so it reaches
our eyes from all directions.
 A very blue sky is a sign of
clean air (no smog or haze).
Courtesy of NOAA
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Why are Clouds Red at Sunset?
 A low sun angle means the
light travels through more
atmosphere.
 Even more blue light (and
most green) is removed
from the ray.
 Only red light remains to
illuminate the cloud.
©2008, US Power Squadrons. Reprinted with permission
 This effect is even more
spectacular after a volcanic
eruption.
Courtesy of NOAA
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Halos
 Halos around the sun or moon
are caused by refraction in
hexagonal ice crystals.
 The angle from the sun is 22°
on all sides.
 Faint color rings can
sometimes be seen—less
often in lunar halos.
©2008, US Power Squadrons. Reprinted with permission
 Halos in the west can indicate
an approaching front.
Courtesy of NOAA
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Coronas
 Coronas are caused by the
scattering of sunlight or
moonlight by water drops in
thin stratiform (usually As)
clouds.
 Because light is scattered
more in the forward
direction, halos are brighter
near the source.
 They can also signal an
approaching front.
Courtesy of NOAA
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Sun Pillars and Sun Dogs
 These are also caused by ice
crystals.
 Near sunrise and sunset.
 Sun pillars, above and below
the sun, are reflected from
flat, hexagonal crystals.
 Sun dogs (mock suns) are on
either side of the sun and are
refracted by hexagonal
crystals (like halos).
 They are sometimes seen
together.
Courtesy of NOAA
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Rainbows
 Rainbows are caused by
refraction and reflection in
water drops.
 Primary bows—reflected once.
 Secondary are reflected twice.
The colors are reversed and
dimmer.
• Must have the sun behind you,
shining on rain ahead of you.
• (Rare tertiary bows are seen in
the opposite direction).
©1997, USA Today. Reprinted with permission
Courtesy of NOAA
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Summary (1 of 3)
 Clouds are collections of very small water or ice particles.
 Must have condensation nuclei and saturated air.
 About 0.02 mm (i.e. 20 microns) in size.
 Clouds are organized into three basic types:
 Cumuliform—puffy (Cu, Sc, Ac, Cc, Cb).
 Stratiform—flat and uniform (St, Ns, As, Cs).
 Cirriform—wispy (Ci).
 Clouds are also categorized by height of their bases.
 Low—up to 2000 meters (6,500 ft.).
 Middle—prefix Alto, from 2,000 to 7,000 meters (20,000 ft.).
 High—prefix Cirro, above 7,000 meters.
 Prefix Nimbo or suffix nimbus—precipitating (Ns, Cb).
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Summary (2 of 3)
 Precipitation can be water or ice.
 Rain, drizzle, freezing rain, etc.
 Snow, sleet, hail, etc.
 It forms by one of two processes:
 Collision-coalescence operates in warm air (e.g. tropics).
 Bergeron process operates in air well below freezing.



Cloud droplets are super-cooled.
The few ice particles present grow at the expense of the droplets.
Water at 0° C to -10° C; Mix at -10° C to -20° C; Ice below -20° C.
 Ice or snow can change by melting (rain, drizzle), or melting and
cooling (freezing rain), or re-freezing (sleet).
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Summary (3 of 3)
 Light interacts with molecules and meteors (particles) in the
atmosphere by absorption, reflection, scattering, and refraction.
 Scattering is a form of diffraction and spreads energy in all directions.
 Refraction bends the light and disperses the wavelengths (colors).
 The sky is blue because blue light is scattered more strongly.
 Sunsets are red because the blue and green are scattered away.
 Halos are caused by scattering in water clouds.
 Coronas are caused by refraction in ice clouds.
 Rainbows are caused by refraction/reflection in rain drops.
 Primary rainbows have one internal reflection.
 Secondary rainbows have two reflections and the colors are reversed.
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Chapter 5 Questions
QUESTION
ANSWER
“-nimbus” added to a cloud name means:
Clouds with a horizontal, layered
appearance are:
It is precipitating.
Stratiform.
Clouds with a wispy appearance are:
Cirriform.
Clouds with a lumpy appearance are:
Cumuliform.
Middle clouds are those that have bases
between what altitudes?
6,500 ft and 20,000 feet (2 to 7 km).
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Chapter 5 Questions
QUESTION
ANSWER
What type of cloud would you associate
with the phrase “mare’s tails”?
Cirrus (Ci).
What type of cloud would you associate
with the phrase “mackerel sky”?
Cirrocumulus (Cc).
Cirrus clouds consist of:
Ice crystals.
A wintertime deposit that can affect the
stability of vessels is:
Clouds that form “caps” over mountains
are called:
Glaze ice.
Lenticularis (or cap clouds).
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Chapter 5 Questions
QUESTION
ANSWER
The reason the clear sky is blue is due to
what fact?
Clouds in the west at sunset are red due to
what fact?
The atmosphere preferentially
scatters blue light.
The atmosphere scatters red light the
least.
Halos are caused by what?
Ice crystals.
Rainbows require:
Rain drops and direct sunlight.
A cyclone is an example of what size of
meteorological scale?
Mesoscale.
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End of Chapter 5
 Are there any questions?
 Chapter 6 covers Severe
Weather.
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