Download Condensation: Dew, Fog and Clouds

Condensation: Dew, Fog and Clouds
Please read Chapter 6 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
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”
Frost
• 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
– deposition
• The resulting crystals
are known as frost,
hoarfrost, or white
frost
Cloud and fog drop formation
• 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
– RH of several hundred percent required for
pure water drop formation
– Terms: Hygroscopic, Hydrophobic (likes
dissolve)
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
If small drops evaporate, how
can we ever get large drops?!
Homogeneous Nucleation
• 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
– Collision likelihood limits maximum
embryo size to < 0.01 µm
• esat for embryo is several
hundred percent
– Embryo evaporates since
environmental RH < 100.5%
The Solute Effect
• Condensation of water on
soluble CCN dissolves particle
– 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
Water molecule
Solute molecule
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.
10-16 g
10-15
g
10-14 g
Droplet Radius (micrometers)
Numbers
in red are
the mass
of the salt
condensation
nucleus
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)
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 (Na+, Cl-, SO4=, K+, Mg2+)
– Particles produced from biogenic sulfur emissions
– Products of vegetation burning
• 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
More About CCN
Sulfate aerosol (SO42-)
SO2 Æ H2SO4
SO2
• Clouds even contribute to
CCN production
– Clouds ingest sulfur dioxide
(SO2)
– Chemical reactions in the
cloud drops convert
dissolved SO2 to soluble
forms of sulfate (SO42-)
– When the cloud drops
evaporate, soluble sulfate
particles are left behind
• 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?
Fogs
• Fogs are clouds in
contact with the
ground
• Several types of fogs
commonly form
–
–
–
–
Radiation fog
Advection fog
Upslope fog
Evaporation (mixing) fog
Radiation Fog
• 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?
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.
– 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 and in
coastal deserts, e.g. Peru and Namibia
Other Fog Types
• Evaporation (mixing) fog
– Mixing of warm, moist air
with colder air can produce
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
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)
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….
Cloud Classification
• 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
Cloud type summary
High Clouds
• 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
– Cirrocumulus
• Less common than cirrus
• Small, rounded white
puffs individually or in
long rows (fish scales;
mackerel sky)
– Cirrostratus
• Thin and sheetlike
• Sun and moon clearly
visible through them
• Halo common
• Often precede
precipitation
Cirrus
Cirrus
Cirrus Display at Dawn
Cirrocumulus
Cirrocumulus
Cirrocumulus at Sunset
Cirrostratus
Cirrostratus with Halo
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
Altocumulus
Altocumulus
Alto Cumulus Radiatus
Alto Cumulus
Alto Cumulus Undulatus
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
– Dark gray
– Continuous light to
moderate rain or snow
– Evaporating rain below can
form stratus fractus
Stratus fractus
Looking down on an eastern Atlantic
stratus deck
Stratiform cloud layers
Stratocumulus cloud streets
Stratus undulatus
Stratus
A Layer of Stratocumulus
Cloud viewed from above
Vertically
“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
Cumulonimbus with Pileaus caps
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
–
–
–
–
Clouds
Water vapor
Precipitation
Surface properties
(temperature, snow cover,
vegetation, etc…)
http://www.rap.ucar.edu/weather/satellite/
Visible and Infrared Satellite Photos
Visible
IR
Clouds - Why We Care
• Clouds transport energy from one area to
another
– Evaporation takes latent heat out of warm
surface waters
– Condensation releases same latent heat into
atmosphere in a different location
– Heat has been ‘carried’ by the water inside a
cloud
Latent Heat Release
An average thunderstorm
contains several thousand
metric tons of water
Condensing 1 kg of water
releases ~ 2.26x106 J of
latent heat energy
An average thunderstorm containing around 1500 tons of water will
release 3.45 billion Joules of energy
Clouds - Why We Care
• Clouds affect the radiation budget by
reflecting visible light from the sun (thus
cooling the planet) and by trapping infrared
radiation from the surface (thus warming
the planet)
• Reflection/trapping behavior depends on
type of cloud…
Cloud Radiative Effects
Salient Tidbits
• In the infrared
– Clouds absorb radiation from below, re-emit at the
temperature of the cloud
• Cirrus - very cold, emit little radiation
• Stratus - very warm, basically emit like the surface
• In the visible
– Clouds reflect radiation based on the amount of cloud
droplets
• Cirrus - thin, not as reflective as thicker water clouds
• Stratus - thicker, many water droplets, highly reflective
High Clouds
Thin, cold ice clouds reflect
less sunlight
Extremely cold, emits
infrared at colder
temperatures, prevents
warmer surface infrared
from escaping to space
NET EFFECT: Warming
Low Clouds
Very thick water clouds reflect large
amounts of sunlight
Very near the surface, temperature of
the cloud effectively the same as
surface. Infrared radiation is
therefore about the same - almost
like the cloud wasn’t there!
NET EFFECT: Cooling
Challenges
• Climate modeling with clouds - need to get both
type and amount correct, which is difficult
– Overestimating high clouds - too much warming
– Overestimation low clouds - not enough
• Understanding cloud feedbacks
– What does CO2 warming do to cloud populations?
– How does increased aerosol pollution affect cloud
types and amounts?
• Observations
– Layered cloud structures not seen from satellites