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Chapter 5
Condensation: Dew, Fog and
Clouds
Dew/Frozen Dew
• Clear calm night
• Objects close to the ground cool rapidly by emitting ____
radiation
• ground and objects become colder than surrounding air
• air in contact with these surfaces cools by conduction
• given enough time, air will cool to the Dew point
• water then condenses to the cold surfaces
• dew will freeze if temperature drops below freezing
Frost
• Formation similar to dew or frozen dew
• Dew point starts out below freezing (frost point)
• water vapor changes directly to ice without becoming a
liquid
• Deposition occurs
Formation of Dew & Frost
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.
Condensation Nuclei
• Must be airborne particles on which water vapor can
condense to produce cloud droplets
• although air looks clean, it never truly is
• a volume of air (about the size of an index finger) contains
between 1,000 and 150,000 particles
• many of these particles serve as condensation nuclei
• small condensation nuclei (<0.2micrometer) - Aitken Nuclei
• larger condensation nuclei (0.2 - 1.0 micrometer) are called
giant nuclei
• condensation nuclei most favorable for producing clouds
(>1.0 micrometer) are called cloud condensation nuclei
(CCN)
Condensation Nuclei (cont.)
• usually between 100 - 1000 CCN exist in a cubic
centimeter of air
• these particles come from… dust, volcanoes, smoke, salt,
sulfate particles emitted by phytoplankton
• condensation can occur when RH is lower than 100%
• these particles are called hygroscopic (water-seeking)
• Salt is the most common hygroscopic particle
Fog
• air close to the ground becomes saturated
• millions of CCN present to form a cloud
• fog over cities usually more dense than fog over the ocean
• fog is formed in two ways
- by cooling (air is cooled to its dew point)
- by evaporation and mixing
Radiation Fog
• fog produced by the earth’s radiational cooling
• sometimes called ground fog
• forms best on clear night when a shallow layer of moist air
is below a larger layer of dry air
• the shallow layer does not absorb much of the outgoing IR
radiation from the surface
• the ground, therefore, cools very rapidly
• the air above the layer also cools very rapidly and a
surface inversion forms
• the moist layer is chilled by the cold surface and becomes
saturated
Ground Based Radiation Fog
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.
Advection Fog
• surface air is cooled my moving warm moist air over a cold
surface
• example: Pacific coast during summer
- surface water near the coast is much colder than
surface water offshore
- warm, moist air from the Pacific Ocean is carried by
westerly winds over the cold, coastal waters
• also forms over land
- warm, moist air from the Gulf of Mexico moves
Northward over colder, elevated land
- this fog is called advection-radiation fog
Advection Fog
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.
Other kinds of fog
Upslope fog - moist air flows up along an elevated plain, hill,
or mountain
Evaporation fog - mixing warm unsaturated air with cool
unsaturated air from evaporation
Hail fog
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.
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.
Figure 6.8
As a result, there are
several fog dispersal
experiments.
Clouds
• Clouds are water droplets (or ice crystals) suspended in
the atmosphere
• clouds are grouped by their elevations as high (20-60kft),
middle(6500-26,000ft), and low (sfc - 6500ft), as well as
those that stretch (vertically) across many altitudes
• these 4 groups contain several cloud types
Cirrus Clouds
High clouds (above 6000 m in middle
latitudes) that are thin and wispy and
comprised mostly of ice crystals.
Stratus Clouds
Low clouds that resembles a fog, but does not reach the ground, and
can generate a light mist or drizzle.
Cumulus Clouds
Clouds with vertical development that take a variety of shapes,
separated by sinking air and blue sky.
Shredded sections are called cumulus fractus.
Cirrocumulus Clouds
High clouds
that are
rounded
puffs,
possibly in
rows, are less
common
than cirrus.
Cirrostratus Clouds
High clouds
that thinly
cover the
entire sky
with ice
crystals.
Light passing
through
these crystals
may form a
halo.
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.
Altostratus Clouds
Middle
clouds that
cover the
entire sky
and may
create a
dimly visible
or watery
sun and
diminish
formation of
shadows.
Nimbostratus Cloud
Low clouds
(below 2000m)
with
precipitation
that reaches the
ground.
Shredded parts
of these clouds
are called stratus
fractus or scud.
Stratocumulus Clouds
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.
Cumulus Congestus Clouds
Clouds with vertical development that become larger in height, with
tops taking a ragged shape similar to cauliflower.
Cumulonimbus Cloud
Clouds with vertical development that have grown into a towering
thunderstorm cloud with a variety of key features, including the anvil
top.
Summary of Cloud Types
Lenticular Clouds
An unusual
cloud that
has a lens
shape and
forms in the
crest of a
wave.
Banner “Cap” Cloud
A lenticular cloud that forms downwind of a mountain peak and is
regularly replenished by condensing water vapor.
Pileus Cloud
An unusual
cloud that
forms above
a building
cumulus by
deflected
moist winds.
Mammatus Clouds
An unusual cloud that hang like sacks, formed by sinking air with a
high water content.
Wall Cloud
Funnel Cloud
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.
Nacreous Clouds
An unusual cloud best viewed at winter in the poles and forms in the
stratosphere.
Noctilucent Clouds
An unusual wavy cloud that is best viewed at the poles and forms in
the upper mesosphere.