Condensation: Dew, Fog, and Clouds

1
Condensation Dew, Fog, and Clouds
This chapter discusses 1. How dew, frost, haze,
fog, and clouds form from atmospheric
moisture 2. Classification of fog and cloud
types from observation
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Formation of Dew Frost
Figure 6.2
Figure 6.1
As air cools to its saturation, or dew point,
vapor molecules slow down and can adhere as dew
on the ground surface or as frost when air
temperature drops below freezing. Daily
temperature lows often occur by radiational
cooling, forming dew at night or early morning.
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Haze Water Seeking Nuclei
Figure 6.3
Above the ground surface, cooling and slowing
water vapor instead condenses upon condensation
nuclei. Hygroscopic particles such as salt or
dust seek condensing vapor, and can form a wet
white haze when relative humidity is above 75,
or a dry blue haze when drier.
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Ground Based Radiation Fog
Figure 6.4
Fog is condensed vapor droplets at a density that
severely restricts visibility. It may form by
radiation fog, which occurs at the ground when
dew point temperature is reached by radiational
cooling. Acid fog threatens humans health because
the droplets combine with gaseous pollutants.
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Advection Fog
Figure 6.5
Warm moist air that moves, or advects, above a
cold surface may become cooled to its dew point
temperature, creating an advection fog. This fog
often forms above the ocean due to mixing
currents, or when warm ocean air rolls into the
cooler waters at the Pacific Coastline.
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Condensation Moisture Sources
Fog that is filtered by tree branches, or
condensation of vapor directly onto vegetation or
the ground surface, provides an important water
source for ecological processes.
Figure 6.6
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Evaporation or Mixing Fog
Fog can form by mixing warm unsaturated air with
cool unsaturated air, which can occur during
evaporation. Steam fog is on example of this
mixing process and occurs when warm pools of
water are a source for vapor that condense into
the cooler air above.
Figure 6.7
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Fog Human Safety
Foggy days in the US have a predictable
distribution due to ocean and mountain
influences. Fog can help crop growth in
California, but can also cause severe automobile,
airplane, and boating accidents. As a result,
there are several fog dispersal experiments.
Figure 6.8
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Cloud Groups Types
Clouds are water droplets suspended in the
atmosphere. Clouds are grouped by their
elevation as high, middle, low, and those that
vertically stretch across many altitudes. There
are several cloud types in these 4 groups.
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Cirrus Clouds
High clouds (above 6000 m in middle latitudes)
that are thin and wispy and comprised mostly of
ice crystals.
Figure 6.9
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Cirrocumulus Clouds
High clouds that are rounded puffs, possibly in
rows, are less common than cirrus.
Figure 6.10
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Cirrostratus Clouds
High clouds that thinly cover the entire sky with
ice crystals. Light passing through these
crystals may form a halo.
Figure 6.11
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Altocumulus Clouds
Middle clouds (between 2000 and 7000m in middle
latitudes) that are puffy masses of white with
gray edges. With your hand overhead, they are
about the size of your fingernail.
Figure 6.12
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Altostratus Clouds
Middle clouds that cover the entire sky and may
create a dimly visible or watery sun and diminish
formation of shadows.
Figure 6.13
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Nimbostratus Cloud
Low clouds (below 2000m) with precipitation that
reaches the ground. Shredded parts of these
clouds are called stratus fractus or scud.
Figure 6.14
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Stratocumulus Clouds
Figure 6.15
Low clouds with rounded patches that range in
color from light to dark gray. With your hand
extended overhead, they are about the size of
your palm.
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Stratus Clouds
Figure 6.16
Low clouds that resembles a fog, but does not
reach the ground, and can generate a light mist
or drizzle.
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Cumulus Humilis Clouds
Figure 6.17
Clouds with vertical development that take a
variety of shapes, separated by sinking air and
blue sky. Shredded sections are called cumulus
fractus.
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Cumulus Congestus Clouds
Figure 6.18
Clouds with vertical development that become
larger in height, with tops taking a ragged shape
similar to cauliflower.
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Cumulonimbus Cloud
Figure 6.18
Clouds with vertical development that have grown
into a towering thunderstorm cloud with a variety
of key features, including the anvil top.
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Summary of Cloud Types
Figure 6.20
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Lenticular Clouds
An unusual cloud that has a lens shape and forms
in the crest of a wave.
Figure 6.21
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Banner Cloud
Figure 6.22
A lenticular cloud that forms downwind of a
mountain peak and is regularly replenished by
condensing water vapor.
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Pileus Cloud
An unusual cloud that forms above a building
cumulus by deflected moist winds.
Figure 6.23
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Mammatus Clouds
Figure 6.24
An unusual cloud that hang like sacks, formed by
sinking air with a high water content.
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Jet Contrails
Jet engine exhaust provides vapor and nuclei for
condensation trails (contrails), which evaporate
quickly in dry air, but linger with higher
relative humidities.
Figure 6.25
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Nacreous Clouds
Figure 6.26
An unusual cloud best viewed at winter in the
poles and forms in the stratosphere.
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Noctilucent Clouds
Figure 6.27
An unusual wavy cloud that is best viewed at the
poles and forms in the upper mesosphere.
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Ground based Viewing of Clouds
Figure 6.28
Clouds directly overhead will appear to be less
densely packed than clouds at the horizon due to
viewing angles.
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Satellite Observation of Clouds
Figure 6.29
Geostationary satellites are able to view clouds
from above. As they move in synchronicity with
earth's rotation, they can provide regular
coverage for a region and help in forecasting.
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Polar Orbiting Satellites
Figure 6.30
Satellites can detect more than clouds, and polar
orbits allow for lower altitudes and more image
resolution. These satellites take multiple
passes of the earth before returning to the same
location.
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Satellite Imaging Options
Sensors on board the satellite are able to detect
visible and other electromagnetic signals, such
as infrared, to determine cloud heat and infer
its height.
Figure 6.33
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Visible Infrared Images
Figure 6.32B
Figure 6.32A
Visible imagery captures all clouds as white, but
infrared imagery can distinguish differences.
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Computer Image Enhancement
Infrared imagery can be evaluated and enhanced by
a computer program that highlights changes in
temperature and cloud types.
Figure 6.33
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False Color Imagery
Figure 6.34
Satellite images can be viewed in false color to
more clearly distinguish areas of importance,
such as green coolest cirrus clouds and darker
gray areas of drier air.