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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