Chapter 4Moisture in the AtmosphereChapter 4Moisture in the Atmosphere

Water on the Earth has three common states

solid, liquid, and vapor

Each with its own unique properties

Earth is the only planet we know of that has all three forms

Water on the Earth has three common states

solid, liquid, and vapor

Each with its own unique properties

Earth is the only planet we know of that has all three forms

Phase Change Heat Transfer Type of Heat

Liquid to Vapor 540-590 cal absorbed

Latent heat of vaporization

Solid to liquid 80 cal absorbed Latent heat of fusion

Solid to vapor 680 cal absorbed Latent heat of sublimation

Vapor to liquid 540-590 cal released Latent heat of condensation

Liquid to solid 80 cal released Latent heat of fusion

Vapor to solid 680 cal released Latent heat of sublimation

Rain, ice, snow

The water cycleThe water cycle

Distribution of water within the hydrological cycle

Atmospheric moisture• Water in the atmosphere

• Requires

- vapor pressure- the amount of pressurecontributed by any volatile substance > e.g., water

- air capable of "holding" vapor> dependent on temperature

• Evaporation-

- more water is becoming vapor than iscondensing (becoming liquid)

• Condensation

- opposite effect

Relative humidity (RH)• Water in atmosphere is dependent on temp.

• Saturation Vapor Pressure=

- the maximum amount of water vapor that aircan "hold"- temperature dependent- warm air "holds" more than cold air

RH = Vapor Pressure

Saturation Vapor Pressure

• measured by a number of devices

Saturation vapor pressure vs temperature

Condensation phenomena• As RH goes to 100% water vapor condenses

- i.e., it changes from vapor to liquid or solid

• forms clouds, rain, snow/ice, fog, dew

• must get air mass to reach saturation (approx)

- accomplished by lifting & cooling, cooling, orincreasing amount of water being vaporized

• usually have to have something for the waterto condense onto...such as:

- aerosols -dust particles and large molecules

Condensation FactorsReleases latent heat stored during vaporization

Environmental lapse rate6.5° C per 1000 m = Avg. lapse rate

dependent upon the local environmental conditions

i.e., empirical = derived by measuring the avg temp of the air mass at the surface (TS) and at the top of the troposphere (TT) and the elevation difference between surface and troposphere (HST)

(( TS - TT ) / (HST))

Different from another more important lapse rate called Adiabatic lapse rate

Cloud formation (1:2)• lifting of an air mass cooling due to adiabatic process

- ADIABATIC - no energy lost or gained byexchanging with air that has different

characteristics-

Dry lapse rate (not at saturation)> 10° C per 1000 meters (5.5 F / 1000 ft)

- Wet lapse rate (at saturation point)> Heat (latent) gained as water condenses> air does not cool as fast> 6° C per 1000 meters (3.3 F / 1000 ft)

Adiabatic lapse ratesAdiabatic lapse rates

Means of lifting• Heating (aka convectional lifting)

- warmed air rises (can also havea lot of water vapor)

• Orographic Lifting (mountains)

- air encouters a barrier andgoes over the top of it

• Frontal Lifting (air masses withdifferent densities

- cool air is more dense thanwarm air> slides underneath warm air,lifting it

Frontal lifting

• Warm Front

• Cold Front

Cloud terminology is descriptiveCloud terminology is descriptiveBased on cloud form or shapeBased on cloud form or shape

• • Cirrus = feathered or wispyCirrus = feathered or wispy

• • Stratus= layeredStratus= layered

• • Cumulus=puffyCumulus=puffy

also linked to elevationalso linked to elevation• • low, middle, high, and vertically developedlow, middle, high, and vertically developed

- - Alto = middleAlto = middle

also linked to precipitationalso linked to precipitation• • nimbo (-us) = rainnimbo (-us) = rain

sometimes linked to temperaturesometimes linked to temperature• • warm vs. cold cloudswarm vs. cold clouds

Cirrus Clouds - High CloudsCirrus Clouds - High Clouds• • "Feathers" or "Wisps" or small "Puffs""Feathers" or "Wisps" or small "Puffs"

• • almost always High cloudsalmost always High clouds

• • Usually Ice crystalsUsually Ice crystals- - a "mackerel"a "mackerel"sky with small,sky with small,puffed, cirruspuffed, cirrus

clouds known asclouds known ascirrocumuluscirrocumulus- - and almostand almostlayeredlayeredcirrostratuscirrostratuscloudsclouds

Perspective is everythingPerspective is everything• • from below, the low clouds look like stratusfrom below, the low clouds look like stratus

• • from above they look like cumulusfrom above they look like cumulus

• • in reality they are some combination of both!in reality they are some combination of both!

• • from below, the low clouds look like stratusfrom below, the low clouds look like stratus

• • from above they look like cumulusfrom above they look like cumulus

• • in reality they are some combination of both!in reality they are some combination of both!

- - three layersthree layersare visibleare visibleherehere- - cirrus cirrus > > v. highv. high- - altocirrusaltocirrus> > middlemiddle- - stratocumulusstratocumulus> > lowerlower

- - three layersthree layersare visibleare visibleherehere- - cirrus cirrus > > v. highv. high- - altocirrusaltocirrus> > middlemiddle- - stratocumulusstratocumulus> > lowerlower

Stratus clouds• layered clouds

• occur at all elevations

• can be thick and rain/snow producing

- Stratusclouds looklike a graysky

- these areNimbostratusclouds whichproduce rain

Cumulus• puffy clouds with somevertical development

- these cumulus cloud examplesdeveloped due to heating ofland which caused the air toheat and rise into cooler air

Cumulonimbus Clouds• continued vertical development will eventually lead to

Cumulonimbus clouds

- these produce heavy local rains, strong winds, andthunderstorms

• Characterized by a tall, often flat-topped, puffycloud form

Orographic lifting creating a lenticular cloud

Precipitation • includes rain,drizzle, freezing rain, snow,hail, ice pellets etc.

• occurs when the weight of the water whichhas condensed in the air overcomes theinfluences of the winds keeping it aloft

- falls under the influence of Earth's Gravity- velocity doesn't continue to increase

> air resistance slows it down as it falls- Max Velocity called Terminal Velocity

> varies with size of the water droplet

Precipitation • includes rain,drizzle, freezing rain, snow,hail, ice pellets etc.

• occurs when the weight of the water whichhas condensed in the air overcomes theinfluences of the winds keeping it aloft

- falls under the influence of Earth's Gravity- velocity doesn't continue to increase

> air resistance slows it down as it falls- Max Velocity called Terminal Velocity

> varies with size of the water droplet

• most precip forms by a processes known as

Bergeron process

• most precip forms by a processes known as

Bergeron process

Precipitation (2:2)• Rain = drops > 0.5 mm

• Drizzle = droplets ~ 0.2 & 0.5 mm

• Freezing rain = liquid water(sometimes supercooled) which fallsonto freezing (or near freezing)surfaces

• Snow = ice crystal in dendrites,plates, columns, or needles form

• Pellets = aka, sleet; water dropletsfreeze

• Hail = big ice pellets that have been"recycled" through a cloud severaltimes

• Rain = drops > 0.5 mm

• Drizzle = droplets ~ 0.2 & 0.5 mm

• Freezing rain = liquid water(sometimes supercooled) which fallsonto freezing (or near freezing)surfaces

• Snow = ice crystal in dendrites,plates, columns, or needles form

• Pellets = aka, sleet; water dropletsfreeze

• Hail = big ice pellets that have been"recycled" through a cloud severaltimes

Dew/Frost• saturated air near the ground gets cooledto the saturation point, and water collectson any surface available

- Frost is frozen condensation- Dew is liquid condensation

• Dewpoint is the temperature that the air mustbe cooled to (under constant pressure) to reachsaturation and cause condensation

• Dewpoint is the temperature that the air mustbe cooled to (under constant pressure) to reachsaturation and cause condensation

Fog• Essentially low level clouds

• created when air is heated orcooled in contrast to land or water

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