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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’ssurface
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 crystalstructure
Radiative Properties Transparent to visible; Absorbs infrared
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Phase Changes
Above critical temperature andpressure, it becomes asupercritical 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 tochange 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 breakthe 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 moleculestogether, resisting evaporation.
Curved (small) drops: surface exposes moleculesmore readily to the air, promote evaporation, andreduces the surface tension in droplets.
More curvature also makes it more difficult toproduce condensation.
Saturation
If we evaporate water in a closedcontainer, eventually the evaporatedwater vapor will condense back intothe liquid.
The air above the water is said to besaturated with water vapor when theevaporation and condensation ratesreach equilibrium.
If this set up is heated, more waterwill have to be evaporated, and theamount of water vapor saturating theair will be greater.
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Sublimation
Recall, water vapor can also sublimate off ofthe 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 waterthan there is over ice.
The same temperature energy will still only dothe same amount of work.
Condensation
Depends on temperature, but not for the reasons youmight 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, watervapor 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 havemore water in the vapor form in warm air than incold air.
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Condensation
Salt particles are “hygroscopic”, having anaffinity for water
They attract water from the air at relativehumidities as low as 75%
Once a salty droplet has formed, morebenefits: 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 littlewater vapor present
So, we have all this really importantwater vapor in the air all of the time.
It would be really helpful if we couldkeep track of it.
Let’s review how we measure watervapor in the atmosphere.
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Absolute Humidity
If we were able to remove all of the watervapor in a parcel of air with a known volumeand measure its mass,
It’s like water vapor density (mass/volume) Usually measured in g m-3
But, since air moves up and down a lot in theatmosphere, its volume changes, too. This makes absolute humidity variable.
Absolute Humidity =mass of water vapor
volume of air
Absolute Humidity
The actual amount of water vapor is thesame, 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)
1 g kg-1 = For every one kilogram of dryair, there is an additional one gram ofwater vapor in it
Very similar to specific humidity Uses only dry air, where specific humidity
uses the dry air PLUS the water vaporitself
Mixing Ratio =mass of water vapor
mass of dry air
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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 partialpressures of each individual component in a gasmixture.
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 condensationare in equilibrium the air is saturated withwater vapor.
Saturation vapor pressure describes theamount of water vapor that would be insaturated air at a given temperature It is the pressure that that amount of vapor would
exert.
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Note theexponentialdependenceontemperature
Relative Humidity (RH)
RH is not the actual amount of watervapor in the air.
100% = saturated
>100% = supersaturated
RH =water vapor content
water vapor capacity
RH =actual vapor pressure
saturation vapor pressure!100%
RH =actual mixing ratio
saturation mixing ratio!100%
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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 towater.
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 iscooler than it actually is because RH is so lowthat sweat can evaporate and cool. Wet bulb temperature
When it is moist, a high RH will preventevaporation, or even allow condensation tomake 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 becomesaturated and condense water vapor onto a surfaceas dew.
If saturation occurs with respect to ice, you get frostby 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 butnot reach frost point. This is called freeze or black frost. Very damaging to crops.
What if a larger layer of air nearthe 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 theparticle
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 becomeacidic
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 coldsurface.
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 mountainJust like a cloud forming, but touching
the surfaceWould be seen on western slopes, not in
Fort Collins so muchCan 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 saturatedaway 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 uniformbase
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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MammatuscloudsPrecipitationevaporates out ofthe anvilEvaporation coolsthe parcels of airand it sinksIf drops are large,mammatus will belong lived
Fractus Also known as
scud Low,
detachedclouds caughtin theoutflow of athunderstorm
Can also beseen belowstratusclouds.
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PyrocumulusCaused by fire,volcano or industryCaused by intenseheating of moist airOnly forms in calmwind situations.
ContrailsCondensedexhaust from jetaircraftImportant whenconsideringclimate effects ofclouds
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Cloud Formations
Lenticular Kelvin-Helmholtz waves Cloud streets Wall Clouds Shelf Clouds
Lenticular CloudsStationary, lens-shaped clouds overmountainsHigh altitudeStable, moist airflows over mountain,creating a large-scale standing wavesIndicates region ofturbulence
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Kelvin-HelmholtzwavesForm when 2 parallellayers of air are movingat different speeds andin different directionsUpper layer usuallyfasterShort lived
Cloud StreetsForm due to horizontalrolls in the atmosphereDue to uneven surfaceheatingClouds form overupdraft in the rolls
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Shelf and RollCloudsLow, horizontal,wedge-like cloudShelf: attachedto parent stormRoll: removedfrom parent stormDue to gust frontfromthunderstorms
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