Download Fog and Clouds

AT350 Introduction to Weather and Climate
Fog and Clouds
Condensation: Dew, Fog and Clouds
Please read Chapter 5 in Ahrens
Condensation
• Condensation is the phase
transformation of water vapor to liquid
water
• Water does not easily condense without
a surface present
– Vegetation, soil, buildings provide surface
for dew and frost formation
– Particles act as sites for cloud and fog drop
formation
Scott Denning
CSU Atmospheric Science
Spring 2008
Dew
• Surfaces cool strongly
at night by radiative
cooling
– Strongest on clear,
calm nights
• The dew point is the
temperature at which
the air is saturated
with water vapor
• If a surface cools
below the dew point,
water condenses on
the surface and dew
drops are formed
• Dew does not “fall”
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AT350 Introduction to Weather and Climate
Fog and Clouds
Frost
Cloud and fog drop formation
• If the temperature is
below freezing, the
dew point is called the
frost point
• If the surface
temperature falls
below the frost point
water vapor is
deposited directly as
ice crystals
• If the air temperature cools below the dew
point (RH > 100%), water vapor will tend to
condense and form cloud/fog drops
• Drop formation occurs on particles known
as cloud condensation nuclei (CCN)
• The most effective CCN are water soluble.
• Without particles clouds would not form in
the atmosphere
– deposition
– RH of several hundred percent required for
pure water drop formation
• The resulting crystals
are known as frost,
hoarfrost, or white
frost
Typical Sizes
Very Small Drops Tend to Evaporate!
• Surface of
small drops are
strongly curved
• Stronger
curvature
produces a
higher esat
• Very high RH
required for
equilibrium with
small drops
– ~300% RH for
a 0.1 µm pure
water drop
Scott Denning
CSU Atmospheric Science
Spring 2008
If small drops evaporate, how
can we ever get large drops?!
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AT350 Introduction to Weather and Climate
Fog and Clouds
The Solute Effect
Homogeneous Nucleation
• Condensation of water on
soluble CCN dissolves particle
• Formation of a pure water drop
without a condensation nucleus
is termed “homogeneous
nucleation”
• Random collision of water vapor
molecules can form a small
drop embryo
– Water actually condenses on
many atmospheric salt particles
at RH ~70%
• Some solute particles will be
present at drop surface
– Displace water molecules
– Reduce likelihood of water
molecules escaping to vapor
– Reduce esat from value for pure
water drop
– Collision likelihood limits maximum
embryo size to < 0.01 µm
• esat for embryo is several
hundred percent
Water molecule
– Embryo evaporates since
environmental RH < 100.5%
Solute molecule
Relative Humidity %
Curvature and Solute Effects Compete
Curves are equilibrium
droplet growth curves. Peak
of each curve is critical point
for cloud drop activation.
Can define corresponding
critical RH.
10-15
10-16 g g
Numbers
in red are
the mass
of the salt
condensation
nucleus
10-14 g
Steps in Cloud/Fog Formation
• Air parcel cools causing RH to increase
– Radiative cooling at surface (fog)
– Expansion in rising parcel (cloud)
• CCN (tenths of µm) take up water vapor
as RH increases
– Depends on particle size and composition
• IF RH exceeds critical value, drops are
activated and grow readily into cloud
drops (10’s of µm)
Droplet Radius (micrometers)
Scott Denning
CSU Atmospheric Science
Spring 2008
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AT350 Introduction to Weather and Climate
Fog and Clouds
Where do CCN come from?
• Not all atmospheric particles are cloud condensation nuclei
(CCN)
• Good CCN are hygroscopic (“like” water, in a chemical sense)
• Many hygroscopic salt and acid particles are found in the
atmosphere
• Natural CCN
– Sea salt particles (NaCl)
– Particles produced from biogenic sulfur emissions
– Products of vegetation burning
More About CCN
Sulfate aerosol (SO42-)
SO2  H2SO4
• CCN from human activity
– Pollutants from fossil fuel combustion react in the atmosphere to
form acids and salts
– Sulfur dioxide reacts to form particulate sulfuric acid and
ammonium sulfate salts
– Nitrogen oxides react to form gaseous nitric acid which can
combine with ammonia to form ammonium nitrate particles
Fogs
Scott Denning
• CCN concentrations vary in
time and space
– Typically 100-1000 per cubic
centimeter
– High in polluted
environments
– Higher CCN concentrations
give rise to greater cloud
drop concentrations
• Climate impacts of these
more reflective clouds?
• Surface radiation and conduction of heat away from
the overlying air cool air temperatures near the
ground
• A layer of air near the ground becomes saturated and
fog forms
• Fog deepens as radiative cooling from the fog top
continues overnight
• Solar heating warms the ground and causes the fog to
“burn off” from the ground up
• What type of meteorological conditions would favor
radiation fog?
Radiation fog
Advection fog
Upslope fog
Evaporation (mixing)
fog
CSU Atmospheric Science
– Clouds ingest sulfur dioxide
(SO2)
– Chemical reactions in the
cloud drops convert
dissolved SO2 to soluble
forms of sulfate (SO4 2-)
– When the cloud drops
evaporate, soluble sulfate
particles are left behind
Radiation Fog
• Fogs are clouds in
contact with the
ground
• Several types of
fogs commonly form
–
–
–
–
SO2
• Clouds even contribute to
CCN production
Spring 2008
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AT350 Introduction to Weather and Climate
Fog and Clouds
Advection Fog
•
•
•
•
Warm air moves (is advected) over cold surface
Cold surface cools warm air
If saturation is reached, fog forms
Common on west coast of U.S.
• Evaporation (mixing) fog
– Warm moist air from Pacific is advected over upwelling cold coastal waters
– As foggy air moves ashore, solar heating warms the ground and overlying
surface
• Fog evaporates near ground
– Coastal advection fogs are key moisture sources for California Redwoods
Fogs and Visibility
• Light scattering by fog drops (geometric
scatterers) degrades visibility, leading to
– Traffic fatalities
– Airport accidents and closures
• Remedies
– Fog monitoring and warning (optical sensors)
– Fog dispersal (expensive and of limited utility)
Scott Denning
CSU Atmospheric Science
Other Fog Types
Spring 2008
– Mixing of warm, moist air
with colder air produces
saturated air parcel
– Examples
• Exhale on a cold day
• Evaporation of water
from relatively warm, wet
surface and mixing with
colder air above.
• (Smokestack plume,
contrails)
• Upslope fog
– Moist air flows up along
sloped plain, hill or
mountain
– Expansion of rising air
causes cooling and RH
increases
Clouds
• Clouds result when air
becomes saturated
away from the ground
• They can
– be thick or thin, large or
small
– contain water drops
and/or ice crystals
– form high or low in the
troposphere
– even form in the
stratosphere (important
for the ozone hole!)
• Clouds impact the
environment in many
ways
– Radiative balance,
water cycle, pollutant
processing, earthatmosphere charge
balance, etc….
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AT350 Introduction to Weather and Climate
Fog and Clouds
Cloud Classification
Cloud type summary
• Clouds are categorized by their height,
appearance and vertical development
– High Clouds - generally above 16,000 ft at middle
latitudes
• Main types - Cirrus, Cirrostratus, Cirrocumulus
– Middle Clouds – 7,000-23,000 feet
• Main types – Altostratus, Altocumulus
– Low Clouds - below 7,000 ft
• Main types – Stratus, stratocumulus,
nimbostratus
– Vertically “developed” clouds (via convection)
• Main types – Cumulus, Cumulonimbus
High Clouds
Cirrus
• High clouds
– White in day;
red/orange/yellow at
sunrise and sunset
– Made of ice crystals
– Cirrus
• Thin and wispy
• Move west to east
• Indicate fair weather
– Cirrostratus
– Cirrocumulus
• Less common than cirrus
• Small, rounded white
puffs individually or in
long rows (fish scales;
mackerel sky)
Scott Denning
CSU Atmospheric Science
• Thin and sheetlike
• Sun and moon clearly
visible through them
• Halo common
• Often precede
precipitation
Spring 2008
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AT350 Introduction to Weather and Climate
Fog and Clouds
Cirrus
Cirrocumulus
Cirrus Display at Dawn
Cirrocumulus
Cirrostratus
Cirrocumulus at Sunset
Cirrostratus with Halo
Scott Denning
CSU Atmospheric Science
Spring 2008
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AT350 Introduction to Weather and Climate
Fog and Clouds
Contrails
Middle Clouds
• Altocumulus
–
–
–
–
<1 km thick
mostly water drops
Gray, puffy
Differences from
cirrocumulus
• Larger puffs
• More dark/light
contrast
• Altostratus
– Gray, blue-gray
– Often covers entire sky
– Sun or moon may show
through dimly
• Usually no shadows
Altostratus
Alto Stratus Castellanus
Scott Denning
CSU Atmospheric Science
Spring 2008
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AT350 Introduction to Weather and Climate
Fog and Clouds
Altocumulus
Altocumulus
Alto Cumulus Radiatus
Alto Cumulus
Low Clouds
• Stratus
– Uniform, gray
– Resembles fog that does
not reach the ground
– Usually no precipitation, but
light mist/drizzle possible
• Stratocumulus
– Low lumpy clouds
– Breaks (usually) between
cloud elements
– Lower base and larger
elements than altostratus
• Nimbostratus
Alto Cumulus Undulatus
Scott Denning
CSU Atmospheric Science
Spring 2008
– Dark gray
– Continuous light to
moderate rain or snow
– Evaporating rain below can
form stratus fractus
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AT350 Introduction to Weather and Climate
Fog and Clouds
Stratus fractus
Looking down on an eastern Atlantic
stratus deck
Scott Denning
CSU Atmospheric Science
Spring 2008
Stratiform cloud layers
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AT350 Introduction to Weather and Climate
Fog and Clouds
Stratocumulus cloud streets
Stratus
A Layer of Stratocumulus
Cloud viewed from above
Stratus undulatus
Vertically
“developed”
developed”
clouds
• Cumulus
– Puffy “cotton”
– Flat base, rounded top
– More space between cloud
elements than
stratocumulus
• Cumulonimbus
– Thunderstorm cloud
– Very tall, often reaching
tropopause
– Individual or grouped
– Large energy release from
water vapor condensation
Scott Denning
CSU Atmospheric Science
Cumulonimbus with Pileaus caps
Spring 2008
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AT350 Introduction to Weather and Climate
Fog and Clouds
Cumulonimbus Clouds Spawn Tornadoes
Satellite
Observations
• Satellites can be
– Geostationary
• Monitors fixed spot on
Earth’s surface
– Polar orbiting
• Orbit poles with Earth
revolving below
• Satellites observe
–
–
–
–
Scott Denning
CSU Atmospheric Science
Spring 2008
Clouds
Water vapor
Precipitation
Surface properties
(temperature, snow cover,
vegetation, etc…)
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AT350 Introduction to Weather and Climate
Fog and Clouds
Visible and Infrared Satellite Photos
Visible
Scott Denning
IR
CSU Atmospheric Science
Spring 2008
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