Download Moisture, Fog, and Clouds

Moisture, Fog, and Clouds
AT 351
Lab 6
February 28, 2008
Moisture Is Important!!!
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Properties of Water
Physical States
 Solid
 Liquid
 Gas (Water Vapor)
 Invisible
 Only natural substance that occurs
naturally in all three states on Earth’s
surface
Properties of Water
 Heat Capacity
 Highest of all common solids and liquids
 Compressibility
 Virtually incompressible as a liquid
 Density
 Recall, density of seawater is controlled by temperature,
salinity, and pressure
 Liquid water has maximum density at +4°C
 Solid phase has lower density since it must form crystal
structure
 Radiative Properties
 Transparent to visible; Absorbs infrared
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Phase Changes
 Above critical temperature and
pressure, it becomes a
supercritical fluid.
 No gas/liquid boundary
 Can diffuse through solids like
a gas
 Dissolve materials like a liquid
 Submarine volcanoes & Venus
 Since it never gets too cold,
there is always vapor present
 Lots of energy is required to
change the phases of water.
 Equal amounts for:
 Freezing and Melting
 Evaporating and Condensing
 Sublimation and Deposition
Evaporation
Very high surface tension
 Takes energy to break the hydrogen bonds
on a water surface
 Molecules attracted by (-) charge on Oxygen
and (+) charge on Hydrogen
When temperatures are increases,
molecules move faster and can break
the surface tension more easily
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Evaporation
 Depends on droplet size: larger droplets are flatter.
 Flat drops: Each water molecule attracts its neighbor
creating “surface tension” that holds molecules
together, resisting evaporation.
 Curved (small) drops: surface exposes molecules
more readily to the air, promote evaporation, and
reduces the surface tension in droplets.
 More curvature also makes it more difficult to
produce condensation.
Saturation
 If we evaporate water in a closed
container, eventually the evaporated
water vapor will condense back into
the liquid.
 The air above the water is said to be
saturated with water vapor when the
evaporation and condensation rates
reach equilibrium.
 If this set up is heated, more water
will have to be evaporated, and the
amount of water vapor saturating the
air will be greater.
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Sublimation
 Recall, water vapor can also sublimate off of
the surface of ice
 This takes the combined energy of both melting
AND evaporation
 Therefore, at the same temperature, there is
more water vapor in saturated air over water
than there is over ice.
 The same temperature energy will still only do
the same amount of work.
Condensation
 Depends on temperature, but not for the reasons you
might think…
 For condensation to be really effective, water vapor needs
something to condense onto.
 We call these things in air Condensation Nuclei.
 Dust, smoke, salts, other particles…
 When air is warm and molecules move fast, water
vapor may bounce off the CCN.
 When air is cold and molecules move more slowly,
water vapor is more likely to stick.
 This shows, again, that you are more likely to have
more water in the vapor form in warm air than in
cold air.
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Condensation
 Salt particles are “hygroscopic”, having an
affinity for water
 They attract water from the air at relative
humidities as low as 75%
 Once a salty droplet has formed, more
benefits:
 salt adds mass to the water
 it occupies spaces exposed to air that would otherwise be
water molecules; exposing less water to the air
 Promotes condensation even with very little
water vapor present
So, we have all this really important
water vapor in the air all of the time.
It would be really helpful if we could
keep track of it.
Let’s review how we measure water
vapor in the atmosphere.
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Absolute Humidity
 If we were able to remove all of the water
vapor in a parcel of air with a known volume
and measure its mass,
Absolute Humidity =
mass of water vapor
volume of air
 It’s like water vapor density (mass/volume)
 Usually measured in g m-3
 But, since air moves up and down a lot in the
atmosphere, its volume changes, too.
 This makes absolute humidity variable.
Absolute Humidity
 The actual amount of water vapor is the
same, but the absolute humidity changes.
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Specific Humidity (q)
Specific Humidity =
mass of water vapor
total mass of air
Mixing Ratio (r)
Mixing Ratio =
mass of water vapor
mass of dry air
1 g kg-1 = For every one kilogram of dry
air, there is an additional one gram of
water vapor in it
Very similar to specific humidity
 Uses only dry air, where specific humidity
uses the dry air PLUS the water vapor
itself
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Surface Specific Humidity
Zonally Averaged Specific Humidity
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Vapor Pressure (e)
 Dalton’s Law
 The total pressure exerted by the gases in a
mixture is equal to the sum of the partial
pressures of each individual component in a gas
mixture.
 For 1000 mb of air:
 78% N2 = 780 mb
 21% O2 = 210 mb
 1% H2O(v) = 10 mb ---> actual vapor pressure
 More air = more pressure
 More H2O(v) = more vapor pressure
Saturation Vapor Pressure (es)
 Recall: when evaporation and condensation
are in equilibrium the air is saturated with
water vapor.
 Saturation vapor pressure describes the
amount of water vapor that would be in
saturated air at a given temperature
 It is the pressure that that amount of vapor would
exert.
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Note the
exponential
dependence
on
temperature
Relative Humidity (RH)
RH =
water vapor content
water vapor capacity
RH is not the actual amount of water
vapor in the air.
100% = saturated
>100% = supersaturated
RH =
actual vapor pressure
! 100%
saturation vapor pressure
RH =
actual mixing ratio
! 100%
saturation mixing ratio
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Changing RH
 Increase vapor content
 Higher RH at same Temp
 Increase Temperature
 Lower RH for same vapor content
 Hot = fast = less likely to condense = lower RH
Dew Point Temperature
This is a measure of moisture content.
Temperature to which the air must cool
to reach saturation with respect to
water.
Frost Point
 Temperature to which the air must cool to
reach saturation with respect to ice.
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July Dew Point Averages
Zonally Averaged RH
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RH and Discomfort
 When the air is dry, we may feel that it is
cooler than it actually is because RH is so low
that sweat can evaporate and cool.
 Wet bulb temperature
 When it is moist, a high RH will prevent
evaporation, or even allow condensation to
make us feel warmer
 Heat Index
 “Apparent Temperature”
Heat Index
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Formation of Dew and Frost
 As the surface and low air cool, the air may become
saturated and condense water vapor onto a surface
as dew.
 If saturation occurs with respect to ice, you get frost
by deposition.
 These mainly occur on clear, calm nights.
 Why need it be clear? What time of day would you get the
most dew or frost?
 If it is very dry, temps may fall below freezing but
not reach frost point.
 This is called freeze or black frost.
 Very damaging to crops.
What if a larger layer of air near
the surface is cooled?
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Then Vapor Condenses in Air
 Homogeneous nucleation
 Condensation directly from the vapor phase
without the presence of a nucleus
 In order for the drop to condense and grow, the
environment must be supersaturated
 Amount of supersaturation depends on the size of the
droplet formed (1-400%)
 Nucleation can occur in a subsaturated volume,
however the drop will not grow.
Cloud Condensation Nuclei
 Aerosol: a fine suspended solid or liquid particle in a gas
 Cloud droplets can form on both insoluble and soluble particles
 A particle that will serve as CCN is called “hygroscopic” or
hydrophillic
 Vapor may condense at RH <100%
 A particle that will not serve as a CCN is called hydrophobic.
 Vapor usually will condense on these for RH >100%
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Cloud Condensation Nuclei
CCN are described by the size of the
particle
CCN
 Sources are dust, volcanoes, factory smoke,
forest fires, sea salt
 Over Ocean: 300-600 cm-3
 Over land: 103 – 107 cm-3
 More in urban areas, less in rural
 Aerosol concentrations decrease with height
 Very light, stay suspended for a long time
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Haze
Dry haze
 Just the small particles themselves, maybe
a little bit of condensation
Wet haze
 Begins at RH ~ 75%
 Much more light scattering than dry (3x)
Fog
 Going above 70% RH to 100%
 Condensation on less active nuclei
 Essentially, a thick wet haze on the ground
 We call it fog when visibility is less than 1 km
 The Solute Effect and the Curvature Effect
help the droplets grow
 They can grow to as large as 25 microns
 Fall at 5 cm s-1
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Fog
 Type of CN affects fog
 Over the ocean
 Fewer, larger drops
 Over urban areas
 More, smaller drops
 Lower visibility
 London Fog
 Chemical reactions can cause fog to become
acidic
Radiation/Ground Fog




Surface radiatively cools
Light breeze helps more air contact cold surface
Common in the fall and winter (highs)
Common in low-lying areas
 River valleys
 Form upward from the ground
 Deepest around sunrise
 May intensify after sunrise (dew evaporation)
 “Burns Off” with more insolation
 Dissipates from bottom up
 Dissipates easily around edges (thin, mixing)
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Advection Fog
 Warm air moves over a cold
surface.
 Breeze required
 Pacific Coast Cold Current
 Gulf Stream and the
Labrador Current
 Gulf Stream and British Isles
 Ice Surface
 May combine with radiation
fog
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Upslope Fog
Moist air rises up the side of a mountain
Just like a cloud forming, but touching
the surface
Would be seen on western slopes, not in
Fort Collins so much
Can last for days under
the right conditions
Evaporation (Mixing) Fog
 Just breathing out when it’s cold
 Cold air over warm water (steam fog)
 Seen over a pool or spa
 Increased moisture raises the dew point
 Maintained through vapor pressure difference and
mixing
 Steam devils on a lake
 Caribou fog
 Precipitation Fog
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Foggy Weather
Annual number of days with fog in the US
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
Form in the stratosphere (important for creation
of the the ozone hole!)
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Cloud Classification
Cirriform Clouds
Usually exist above 16,000 feet
Generally thin, sometimes partially translucent
Comprised of ice crystals
Absorb longwave radiation, but are bright and
reflective (have a high albedo)
 Rarely precipitate




 Virga
 Cirrus (Ci)
 Called “mares tails”
 Cirrocumulus (Cc)
 Called “fish scales” or “mackerel sky”
 Cirrostratus (Cs)
 Usually present when halos around the sun are observed
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Cirrus
Wispy
Cirrostratus
Halo
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Cirrocumulus
Stratiform clouds
Characterized by a horizontally uniform
base
May or may not precipitate
May exist at any level
Layered
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Stratus
Uniform and gray, maybe lifted fog
Stratocumulus
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Nimbostratus
Patchier and rainier than regular stratus
Marine stratus
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Cumuloform clouds




Large in vertical extent
May or may not precipitate
Result from vertical motion
Cumulus Humilis
 “fair weather cumulus”
 Cumulus Congestus
 Towering Cumulus
 Cumulonimbus
 “anvil cloud”
“Fair weather” cumulus
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Cumulus Congestus
Cumulunimbus
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Other cloud types
 Mammatus
 Pileus
 Fractus
 Pyrocumulus
 Contrails
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 Mammatus
clouds
Precipitation
evaporates out of
the anvil
Evaporation cools
the parcels of air
and it sinks
If drops are large,
mammatus will be
long lived
Fractus
 Also known as
scud
 Low,
detached
clouds caught
in the
outflow of a
thunderstorm
 Can also be
seen below
stratus
clouds.
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 Pyrocumulus
Caused by fire,
volcano or industry
Caused by intense
heating of moist air
Only forms in calm
wind situations.
 Contrails
Condensed
exhaust from jet
aircraft
Important when
considering
climate effects of
clouds
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Cloud Formations
 Lenticular
 Kelvin-Helmholtz
waves
 Cloud streets
 Wall Clouds
 Shelf Clouds
 Lenticular Clouds
Stationary, lensshaped clouds over
mountains
High altitude
Stable, moist air
flows over mountain,
creating a largescale standing waves
Indicates region of
turbulence
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 Kelvin-Helmholtz
waves
Form when 2 parallel
layers of air are moving
at different speeds and
in different directions
Upper layer usually
faster
Short lived
 Cloud Streets
Form due to horizontal
rolls in the atmosphere
Due to uneven surface
heating
Clouds form over
updraft in the rolls
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 Shelf and Roll
Clouds
Low, horizontal,
wedge-like cloud
Shelf: attached
to parent storm
Roll: removed
from parent storm
Due to gust front
from
thunderstorms
Really High Clouds
 Nacreous Clouds (mother of pearl)
 Form in stratosphere
 Seen best at polar latiudes in winter
 Composition not well known
 Noctilucent Clouds
 Sometimes seen in the mesosphere
 Stars shine through them
 Made of tiny ice crystals
 Water may be from meteorites
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Nacreous Clouds
Noctilucent Clouds
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Rainbow Schematics
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