Clouds

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Clouds S.K. Satheesh Centre for Atmospheric
Oceanic Sciences Indian Institute of
Science Bangalore.
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General Features Cloud Condensation
Nuclei Cloud Droplets and Rain
Droples Detection of Clouds using Satellites
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What is a cloud?

• There are lots of drops of water floating
  together in the air. We cant see each drop, but
  we see them together as a cloud.

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How do clouds form?

• First there is a body of water. This could be a
  lake, ocean, or river.
• Sun warms up the water from the lake or ocean,
  and turns it into water vapor.

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Water Vapor

• Water vapor rises into the air. It then forms
  tiny drops of water.

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Clouds

• These tiny drops join together to make clouds.

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Rain

• The drops get so big that they can no longer stay
  in the air. Then they fall down to earth as Rain.
• Rain comes down to earth again, and the cycle
  starts all over.
• This is called HYDROLOGICAL CYCLE.

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Now What?

• Why some clouds are white and puffy?
• Why some are producing rain while others are not?
• Why some are dark?

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There are many different types of clouds. Clouds
are classified by their shape, altitude and
characteristics.
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Clouds are visible aggregates of minute water
droplets or tiny crystals of ice. Clouds are
classified on the basis of their appearance and
height. There are three basic cloud types.
Cirrus, Stratus, and Cumulus. Cirrus clouds are
high, white and thin. The cumulus consists of
individual cloud masses with large vertical
extent. Stratus clouds are sheets or layers that
cover much or all of the sky.
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Cloud Classification
Clouds
Cirrus
Cumulus
Stratus
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Cloud Types
Height Type High clouds Cirrus above
6 kms Cirrocumulus Cirrostratus Medium
Clouds Altostratus 2 - 6 kms Altocumulus Lo
w Clouds Stratocumulus below 2
kms Stratus Nimbostratus Clouds with
Cumulus vertical extent Cumulonimbus
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Because of low temperature and small quantities
of water vapour available at high altitudes, all
of the high clouds are thin and white and are
made of ice crystals. Since most of water vapour
is available at lower altitudes, middle and low
clouds are denser and darker. Layered clouds
generally indicates that the air is stable. This
is because clouds would not grow vertical in
stable air. Some clouds do not fit into any of
the height categories. Such clouds have their
bases in the low height range and often extend
upward into the middle or high altitudes. These
are referred to as clouds of vertical development.
When upward air movement is intense, clouds of
large vertical extent are formed. As the cumulus
grow vertically, its top exceed the low altitude
range and is called cumulus congestus. When it
becomes even larger and rain begins, it is called
cumulonimbus.
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Cloud Classifications
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Cloud Classifications
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Cloud Classifications
Types of Cumulus Clouds
Cumulus
Altocumulus
Cumulonimbus
Cumulus Congestus
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Cirrus

• Thin, delicate, ice-crystal clouds in the form of
  thin
• elements that allow sunlight to pass through them

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Cirrus Clouds
cirrus
Fair weather cumulus
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Cirrostratus

• White ice crystal clouds that appear in the form
  of extensive sheets that may cover the whole sky.
• Sometimes produce halos around the Sun

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Cirrostratus Cloud Pictures
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Cirrocumulus

• High level cloud
• Appears as small rounded puffs arranged in rows
  or sheets

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Cirrocumulus Cloud Pictures
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Stratus

• Form in sheets or layers in the lower parts of
  the atmosphere.
• Sometimes cover the sky completely.
• May produce drizzle.
• Fog is classified as stratus clouds

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Stratus Cloud Pictures
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Nimbostratus

• Rain producing cloud
• Varies in thickness and layers
• Mostly occurring in a widespread sheet

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Nimbostratus Cloud Pictures
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Altostratus

• Forms in the middle levels of the atmosphere
• Always a sign of significant amounts of moisture
  in those layers
• Ranging from a thin, white veil of cloud to a
  dense gray mantle

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Altostratus Cloud Pictures
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Cumulus Congestus

• Cumulus clouds with great vertical extent with
  tops that resemble cauliflower
• Their heights exceed the dimensions of their bases

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Cumulus Congestus Pictures
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Stratocumulus

• Forms in layers
• Usually has a ragged upper surface while base is
  flat
• Stratocumulus clouds are the most common type of
  cloud

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Stratocumulus Cloud Pictures
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Altocumulus

• Cumulus in the middle levels of the atmosphere
  associated with the lifting of a large air mass

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Altocumulus Cloud Pictures
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Fair Weather Cumulus
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Cumulonimbus

• Relatively large
• Producing most of the damage associated with a
  thunderstorm
• Can produce large hail, flash floods, severe wind
  gusts, wall clouds and tornadoes

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Cumulonimbus Thunderstorm Clouds
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Water can occur in 3 states

• Gas - water vapour (invisible)
• Liquid - water droplets (visible)
• Solid - ice crystals, hail, snow

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How clouds are formed?

• Clouds are formed of tiny droplets of water or
  ice.
• Clouds form when water vapour cools and
  condenses.
• The temperature at which condensation occurs is
  called dew point.
• Condensation also requires nuclei (small
  particles) such as aerosols.

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What happens when air is cooled?

• Relative humidity increases.
• Dew point is reached (100 relative humidity).
• Water vapour (invisible) condenses to form water
  droplets (visible).
• Sometimes the water vapour may change straight
  into solid state called sublimation.

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Necessary Conditions for Condensation Condensatio
n Occurs when water vapour condense to form
liquid. The result of this process may be fog or
clouds. For any form of condensation to occur,
the air must be saturated. Saturation occurs by
two ways. The first is when air is cooled below
the dew point, which most commonly happens. The
second is when water vapour is added to the air.
Moreover there should be a surface on which water
vapour may condense. When dew occurs, objects
near the ground serve this purpose. When
condensation occurs in the air above the ground,
tiny bits of particulate matter known as
condensation nuclei serve as surfaces for
condensation of water vapour. The importance of
these nuclei is that if they are absent, a
relative humidity of about 400 is needed to
produce clouds.
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How air is cooled? During cloud formation and
often in the formation of fog, the air is cooled
to its dew point. Near the ground the heat is
exchanged readily between ground and air above.
This accounts for the cooling involved in the
formation of some types of fog. However as air is
a poor conductor of heat, this exchanges are not
possible above few thousand meters. Thus some
other mechanism must operate during cloud
formation. When air moves upward it passes
through regions of successively lower pressure.
As a result, an ascending air parcel expands and
cools adiabatically. Unsaturated air cools at a
constant rate of 1C per every 100 meters of
ascent.
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Conversely, descending air comes under
increasingly higher pressures, compresses and is
heated 1?C for every 100 meters of descent.
This rate of cooling or heating only applies to
an unsaturated air and is known as dry adiabatic
rate. If the air ascents long enough, it will
cool sufficiently to get saturated and to
condense. From this point the latent heat stored
in the water vapour will be released. Although
the air will continue to cool continuously after
condensation begins also, the released latent
heat works against the adiabatic process,
thereby reducing the rate at which air cools.
This slower rate of cooling is called wet
adiabatic rate.
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-8C
5000
-3C
4000
2C
Condensation level
Altitude (m)
3000
12C
2000
1000
22C
Surface
32C
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Stability of Atmosphere Stable When lapse rate
is less than wet adiabatic rate (6K per
km). Unstable When lapse rate is less than wet
adiabatic rate (9.8K per km). Conditionally
Stable When lapse rate is between wet and dry
rates
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How Air Ascents? Stable air will not ascent by
its own. There are mainly three mechanisms to
trigger vertical movement. (1) Convergence (2)
Orographic lifting (3) Frontal lifting
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Convergence When two air parcels meet in
opposite directions, convergence occurs and air
parcel is forced to rise.
Stable air
Stable air
Surface
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Orographic lifting Occurs when sloping terrain
such as mountain barriers the flow of air and
force air to ascent. Many rainiest places in the
world are located on windward mountain slopes.
In addition to lift the air, mountains further
remove in other ways by slowing its
horizontal movement. Thus by the time it reaches
the other side of mountain, much of its moisture
has been lost and if the air descends, it warms
making condensation even less likely. The
results is a rain shadow desert.
Warm, stable air
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Frontal lifting Occurs when cool air acts as a
barrier to the flow of warm air. As cold air is
denser than warm air, warm air rises. Usually
occurs when warm air pass through a lake or cold
water body.
Warm air
Cold air
Surface
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Cloud Droplet Growth
Collision-Coalescence
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Aerosols as Cloud Condensation Nuclei The cycle
of water in nature relies on the condensation of
water vapour to form cloud droplets, some which
return to earth in the form of rain. The
formation of an aerosol from volatile gases
initially requires a surface for condensation.
This surface could be a small cluster of vapour
molecules or a small particle of some other
material, called CONDENSATION NUCLEI. When
condensation of a vapour takes place on a cluster
of vapour molecules, it is called spontaneous
condensation or homogeneous nucleation.
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Saturation Ratio The saturation ratio of a
vapour is given by, S P / Ps(T) where P is
the partial pressure of the vapour in the gas
and Ps(T) is the saturated vapour pressure of
the vapour over a plane of the liquid at a
temperature T. When S gt 1, then the gas is
supersaturated with vapour When S 1, then the
gas is saturated with vapour When S lt 1, then the
gas is unsaturated with vapour.
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Homogeneous Nucleation Homogeneous nucleation
takes place in three steps. First the vapour must
be supersaturated to an extend that condensation
will takes place. Second small clusters of
vapour molecules called "embryos"
form. Finally, the vapour condenses on these
small clusters so that clusters grows into a
larger nucleus and subsequently becomes a
droplet. The spontaneous nucleation is not
significant until fairly high super saturation
are achieved. Experiments showed that super
saturations of about 6 are required for
spontaneous nucleation of water vapour in
particle free air.
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Heterogeneous Nucleation The vapour must be
supersaturated. Then, the vapour condense on
these small particles so that particle grows
into a larger particle, very often a liquid a
droplet. In real atmosphere, a number of
particles exist always. This makes extremely high
super saturations unnecessary for condensation.
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Soluble and Insoluble Nuclei The utilisation of
the nuclei for condensation also depends on the
chemical composition of the nuclei. There two
general classes of nuclei Soluble Nuclei and
Insoluble Nuclei. With soluble nuclei the
condensing vapour dissolves the nucleus changing
the properties of the nuclei. With insoluble
nuclei the surface characteristics are important,
and once the nucleus is coated with liquid it
behaves in a manner similar to a pure liquid
droplet.
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Insoluble Nuclei There are two extremes of
insoluble nuclei Nuclei which are activated
(wetted) easily and nuclei which are not easily
activated. Nuclei which are easily activated
rapidly get coated with liquid and subsequently
behave like a droplet and further grow in size
by condensation. In cases where nuclei surface
is not wettable, condensation proceeds with much
more difficulty. The surface of the nuclei tries
to make the condensing liquid into small
spheres. When the entire surface is covered with
these small spheres, a liquid coating can
form. Here after the nuclei behave like a normal
droplet and grow in size by condensation of
vapour. Laboratory experiments shows that very
high super saturations are required to activate
an insoluble nuclei.
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Soluble Nuclei When condensation takes place on
a soluble nuclei, it produces a solution
droplet. Example is the condensation of water
vapour in sodium chloride aerosol. Initially a
saturated sodium chloride solution is formed. As
condensation proceeds further, the solution
becomes more and more dilute until finally the
drop behaves in a manner similar to a pure
liquid.
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Relations between Aerosol and Cloud Droplet
Concentration The dependence of Nc on Na, can be
described as a power law relationship CCN
b Nan Sk where b is a constant, n 1.2 and k
0.76. S supersaturation. Nc f
CCN where f 0.8 to 1.0
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Cloud Drops and Rain Drops
Conventional boarder line between cloud drops
rain drops R100 ?m V 70 cm s-1
Typical Condensation Nucleus (CN) R 0.1 -
1?m N 1000 cm-3 V 0.001 mm s-1
Typical Cloud Drop R10 ?m N 300 - 500 cm-3 V
1 cm s-1
Larger Cloud Drop R50 ?m N 10 cm-3 V 27 cm
s-1
Typical Rain Drop R 1000 ?m or 1 mm N 0.01
cm-3 V 650 cm s-1
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Cloud Droplet Spectra The cloud droplet number
distribution usually follow modified gamma
distribution given by, where N is the number
density in cm-3, ? and ? are constants which
describe the slope of the distribution, a is a
normalising constant ensuring that the integral
of size distribution over all radii yields N.
The value of B is given by, where rmod is
the mod radius in ?m.
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Microstructure of Cumulus Clouds
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Droplet Size Distribution
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10
30
50
Droplet Size (mm)
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Cloud Detection by Satellites
Threshold Technique The simplest technique
used for extracting cloud information from
digital satellite images is Threshold technique.
Principle A visible brightness or infrared
brightness is set such that if a pixel is
brighter or colder than the temperature
threshold, the pixel is assumed to be cloud
covered. The fractional area covered by cloud is
simply the ratio of the number of cloudy
pixels to total number of pixels. The cloud
height can be determined by comparing the
infrared Brightness temperature with a sounding.
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A typical satellite image
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There are two complications Some clouds are
smaller than the satellite scan spot. How to set
a threshold? Lclr radiance from clear sky
scene Lcld radiance from cloudy scene Pixels
that are partly filled with clouds will have
radiances between Lclr and Lcld. If threshold is
set near Lclr, all partly filled pixels will be
counted as cloudy pixels. This will result in an
over estimate of cloud amount. If threshold is
set near Lcld, all partly filled pixels will be
counted as clear pixels. This will result in an
under estimate of cloud amount.
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The problem can be solved to some extend by
setting two thresholds one near Lclr and one
near Lcld. Pixels that are in between the two
thresholds, are counted as 50 cloud
covered. Setting the threshold is another chief
difficulty in threshold technique. The problem is
that threshold is a function of many
variables. Surface type (land, ocean, ice etc.),
surface conditions, season, time of the day and
even satellite-Sun-Earth geometry.
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Histogram Technique This technique serve as
alternative to threshold technique. Principle
Pixels with similar characteristics cluster
around same point when plotted as histogram.
Here visible albedo versus IR brightness
temperature are plotted. High clouds are white
and cold Thus having high albedo and low
IRBT. Low clouds which are dark, are hotter
Thus having low albedo And high IRBT.
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Pattern Recognition Technique Principle When
averaged over a small area, regions which are
completely clear and completely cloudy have low
standard deviation and partly cloudy pixels have
high standard deviation. completely cloudy
regions have higher albedo than clear sky regions
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Multi-Spectral Techniques Lvis N Rcld Esun
(1-N) Rclr Esun LIR (1-N) Lclr N ? Lcld
N(1- ?)Lclr The cloud top temperature
can be calculated from Lcld using Plank function.
N Lvis Rclr Esun / (Rcld-Rclr)Esun Lcld
LIR (1-N ?)Lclr / N ?
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Cloud Optical Thickness in the Arctic
June 2, 2001
tc
20
15
10
5
0
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Cloud Optical Thickness (M. D. King, S.
Platnick, M. Gray, E. Moody, et al. NASA GSFC,
UMBC)
April 2001
tc
20
16
12
8
4
0
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Effective Radius of Cloud Droplets, Reff ?
R3 N ( R) dR/ ? R2 N ( R) dR
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Cloud Effective Particle Radius (M. D. King, S.
Platnick, M. Gray, E. Moody, et al. NASA GSFC,
UMBC)
April 2001
re(µm)
40
34
28
22
16
10
4
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Thank you