Department of Atmospheric

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Shashi K PathakPhD, CPhys (London)

• Department of Atmospheric Oceanic Sciences
• Office BH - 831
• Ph [phone]
• Email shashi.pathak_at_mcgill.ca

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Chapter 6 Condensation Dew, Fog and Clouds
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Brief Introduction

• Dew, frost, and fog
• Dew, frost - forms when objects on the surface
  cool below the airs dew-point temperature.
• Fog - forms as the air cools, or as water
  evaporates and mixes with drier air.
• Condensation
• Condensation nuclei are important in the
  atmosphere because they serve as surfaces on
  which water vapor condenses.
• Hence - Cloud Formation.

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Brief Introduction

• Cloud classification - according to their height
  and their physical appearance.
• Clouds are usually divided into four main groups
  high, middle, low, and clouds with vertical
  development.
• Identification of clouds.
• Satellites not only photograph clouds, they
  provide scientists with a great deal of physical
  information about the Earth and its atmosphere.

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Dew

• Figure 1 Dew forms on clear nights when objects
  on the surface cool to a temperature below the
  dew point. If these beads of water should freeze,
  they would become frozen dew.

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Frost

• Figure 2 These are the delicate ice-crystal
  patterns that frost exhibits on a window during a
  cold winter morning.

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Fog Formation

• Formation of fog is a progressive process that
  starts at relative humidities less than 100 when
  water vapor begins to condense onto hygroscopic
  condensation nuclei in the air.
• Cloud droplets scatter light and a cloud layer
  that forms near the ground is officially
  designated haze or fog depending on the reduction
  in visibility that it causes.

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Fog Formation

• Fog can be produced under a variety of
  situations.
• Either by cooling moist air to saturation or by
  evaporating or mixing water vapor into the air.

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Haze

• Figure 3 The high relative humidity of the cold
  air above the lake is causing a layer of haze to
  form on a still winter morning.

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Fog

• Figure 4 Radiation fog nestled in a valley.

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Fog

• Figure 5 Advection fog rolling in past the
  Golden Gate Bridge in San Francisco. As fog moves
  inland, the air warms and the fog lifts above the
  surface. Eventually, the air becomes warm enough
  to totally evaporate the fog.

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Fog

• Figure 6 Tiny drops, each one made from many fog
  droplets, drip from the needles of this tree and
  provide a valuable source of moisture during the
  otherwise dry summer along the coast of
  California.

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Fog

• Figure 7 Even in summer, warm air rising above
  thermal pools in Yellowstone National Park
  condenses into a type of steam fog.

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Fog

• Figure 8 Average annual number of days with
  dense fog throughout the United States.

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Cloud Droplets

• Typical Sizes (Radius)
• CCN - 0.0002 mm
• Typical Cloud Droplet - 0.02 mm
• Typical Rain Drop - 2 mm (100 times of a cloud
  droplet)
• (In the tropics, a cloud can form, grow, and
  produce rain in 30 minutes.)

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Classification of Clouds

• First classification French naturalist Jean
  Lamarck (1801)
• Luke Howard, an English scientist (1803)
• International Meteorological Commission (1929)
• Cirrus
• Cumulus
• Stratus

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Classification of Clouds

• Today's Classification has Four Main Divisions

High Clouds 20,000 to 40,000 ft (6,100 12,200
m) Intermediate Clouds 6,500 to 20,000 ft
(1,980 6,100 m) Low Clouds near ground level
to 6,500 ft (1,980 m) and Clouds with
Vertical Development1,600 ft to over 20,000 ft
(490 6,100 m
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Classification of Clouds
Group Height Types High
Clouds tropics 6000 -18000m
Cirrus mid-latitudes 5000 -13000m
Cirrostratus polar region 3000 -8000m
Cirrocumulus
Middle Clouds tropics 2000 -8000m
Altostratus mid-latitudes 2000 -7000m
Altocumulus polar region 2000 -4000m
http//www.windows.ucar.edu/
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Classification of Clouds
Group Height Types
Low Clouds tropics surface -2000m
Stratus mid-latitudes surface -2000m
Stratocumulus polar region surface -2000m
Nimbostratus
Clouds with tropics up to 12000m
Cumulus Vertical Growth mid-latitudes up to
12000m Cumulonimbus polar region
up to 12000m
http//www.windows.ucar.edu/
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Clouds Cirrus and Cirrocumulus

• Figure 9(left) Cirrus clouds.
• Figure 10(right) Cirrocumulus clouds.

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Clouds Cirrostratus

• Figure 11 Cirrostratus clouds with a faint halo.

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Clouds Altocumulus and Altostratus

• Figure 12(left) Altocumulus clouds.
• Figure 13(right) Altostratus cloud. The
  appearance of a dimly visible "watery sun"
  through a deck of gray clouds is usually a good
  indication that the clouds are altostratus.

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Clouds Nimbostratus and Stratocumulus

• Figure 14 The nimbostratus is the sheetlike
  cloud from which light rain is falling. The
  ragged-appearing cloud beneath the nimbostratus
  is stratus fractus, or scud.
• Figure 15 Stratocumulus clouds. Notice that the
  rounded masses are larger than those of the
  altocumulus.

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Clouds Stratus

• Figure 16 A layer of low-lying stratus clouds.

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Clouds Cumulus and Cumulus Congestus

• Figure 17 Cumulus clouds. Small cumulus clouds
  such as these are sometimes called fair weather
  cumulus, or cumulus humilis.
• Figure 18 Cumulus congestus. This line of
  cumulus congestus clouds is building along
  Maryland's eastern shore.

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Clouds Cumulonimbus

• Figure 19 A cumulonimbus cloud. Strong
  upper-level winds blowing from right to left
  produce a well-defined anvil. Sunlight scattered
  by falling ice crystals produces the white
  (bright) area beneath the anvil. Notice the heavy
  rain shower falling from the base of the cloud.

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Figure 20 A generalized illustration of basic
cloud types based on height above the surface and
vertical development.
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Clouds Lenticular

• Figure 21 Lenticular clouds forming one on top
  of the other on the eastern side of the Sierra
  Nevada.

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Clouds Banner and Pileus

• Figure 22 The cloud forming over and downwind of
  the Rainier is called a banner cloud.
• Figure 23 A pileus cloud forming above a
  developing cumulus cloud.

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Clouds Mammatus

• Figure 24 Mammatus clouds forming beneath a
  thunderstorm.

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Clouds

• Figure 25 A contrail forming behind a jet
  aircraft.

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Clouds Nacreous

• Figure 26 The clouds in this photograph are
  nacreous clouds. They form in the stratosphere
  and are most easily seen at high latitudes.

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Clouds Noctilucent

• Figure 27 The wavy clouds in this photograph are
  noctilucent clouds. They are usually observed at
  high latitudes, at altitudes between 75 and 90 km
  above the earth's surface.

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Clouds

• Figure 28 Clouds on the horizon appear closer
  together than clouds overhead. Note that the
  amount of clear space between each cloud is the
  same. To the observer, however, there appears to
  be more space between clouds 1 and 2 than between
  clouds 3 and 4.

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Satellite Observations

• Figure 29(left) The geostationary satellite
  moves through space at the same rate that the
  earth rotates, so it remains above a fixed spot
  on the equator and monitors one area constantly.
• Figure 30(right) Polar-orbiting satellites scan
  from north to south, and on each successive orbit
  the satellite scans an area farther to the west.

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Satellite Observations

• Figure 31 Generally, the lower the cloud, the
  warmer its top. Warm objects emit more infrared
  energy than do cold objects. Thus, an infrared
  satellite picture can distinguish warm, low
  (gray) clouds from cold, high (white) clouds.

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Satellite Observations Visible Image

• Figure 32a A visible image of the eastern
  Pacific taken on the same day at just about the
  same time as figure (b).

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Satellite Observations Infrared Image

• Figure 32b An infrared image of the eastern
  Pacific taken on the same day at just about the
  same time as figure (a).

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Satellite Observations Enhanced Infrared Image

• Figure 33 An enhanced infrared image of the
  eastern Pacific taken on the same day as the
  images shown in the previous two figures.

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Satellite Observations Infrared Water Vapor Image

• Figure 34 Infrared water vapor image. The darker
  areas represent dry air aloft the brighter the
  gray, the more moist the air in the middle or
  upper troposphere. Bright white areas represent
  dense cirrus clouds or the tops of thunderstorms.
  The area in color represents the coldest cloud
  tops.

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Summary

• Condensation Nuclei
• Cloud Condensation Nuclei
• Aitken Nuclei
• Hygroscopic Nuclei
• Hydrophobic Nuclei
• Dew Point
• Frost Point
• Deposition
• Haze
• Fog
• Acid Fog

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Summary

• Radiation (Ground) Fog
• Valley Fog
• Advection Fog
• Evaporation Fog
• Steam Fog
• Supercooled Fog
• Stratus
• Cumulus
• Cirrus
• Nimbus
• Cirrus Clouds

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Summary

• Cirrocumulus Clouds
• Cirrostratus Clouds
• Altocumulus Clouds
• Altostratus Clouds
• Nimbostratus Clouds
• Stratus Fractus
• Stratocumulus Clouds
• Stratus Clouds
• Cumulus Clouds
• Cumulus Humilis
• Cumulus Fractus

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Summary

• Cumulus Congestus
• Towering Cumulus
• Cumulonimbus Clouds
• Cumulonimbus Incus
• Lenticular Clouds
• Pileus Clouds
• Mammatus Clouds
• Contrails
• Mother-of-pearl Clouds
• Noctilucent Clouds
• Geostationary Satellites
• Geosynchronous Satellites