Chapter 4Chapter 4Moisture and Atmospheric Moisture and Atmospheric

StabilityStabilityThis chapter covers:This chapter covers: WaterWater

states of matterstates of matter heat capacity and latent heatheat capacity and latent heat

Humidity and dew pointHumidity and dew point Adiabatic temperature changes in Adiabatic temperature changes in

the atmospherethe atmosphere Atmospheric stabilityAtmospheric stability

The Hydrologic CycleThe Hydrologic Cycle

States of Matter: Solid, Liquid and VaporStates of Matter: Solid, Liquid and Vapor

Water’s Changes of State

Gas (Vapor)Gas (Vapor)

widely spaced moleculeswidely spaced molecules no bonding between moleculesno bonding between molecules molecules move at high speedsmolecules move at high speeds very compressiblevery compressible

LiquidLiquid Closely spaced moleculesClosely spaced molecules Moderate bonding between Moderate bonding between

moleculesmolecules molecules move at medium molecules move at medium

speedsspeeds Slightly compressibleSlightly compressible

Solid (i.e., ice)Solid (i.e., ice)

closely spaced moleculesclosely spaced molecules Strong, rigid bonding between Strong, rigid bonding between

moleculesmolecules No molecule movement – only No molecule movement – only

vibrationsvibrations Fairly incompressibleFairly incompressible

Solid Water: Ice

Liquid Water

Water Vapor

Heat Capacity and Latent Heat of Water

SaturationSaturation

Condition in which the air is holding Condition in which the air is holding the maximum amount of water the maximum amount of water vapor possiblevapor possible

Amount of water vapor present at Amount of water vapor present at saturation depends onsaturation depends on Temperature; more vapor at higher Temperature; more vapor at higher

temp. Very strong effecttemp. Very strong effect Pressure; more vapor at higher Pressure; more vapor at higher

pressure.pressure.

Absolute HumidityAbsolute Humidity

Amount of water vapor present in Amount of water vapor present in airair

Given as grams water vapor per Given as grams water vapor per cubic meter of aircubic meter of air

Value is affected by air pressureValue is affected by air pressure

Mixing RatioMixing Ratio

Amount of water vapor present Amount of water vapor present in air, but more useful than in air, but more useful than absolute humidityabsolute humidity

Given as grams water vapor per Given as grams water vapor per kilogram of airkilogram of airTypically ranges from 0 to 4%Typically ranges from 0 to 4%

Value is Value is notnot affected by air affected by air pressurepressure

SaturationSaturation

Air is limited in how much water Air is limited in how much water vapor it can hold without water vapor it can hold without water droplets formingdroplets forming

SaturationSaturation is the point at which air is the point at which air can’t hold more water vaporcan’t hold more water vapor

Mixing ratio at saturation depends on Mixing ratio at saturation depends on temperature, and somewhat on temperature, and somewhat on pressurepressure

Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated

with water vapor and condensation occurs

Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated

with water vapor and condensation occurs

Saturation Mixing-Ratio:Saturation Mixing-Ratio:How much water vapor can be present in air How much water vapor can be present in air

at different temperaturesat different temperatures

Relative HumidityRelative Humiditythe humidity we feelthe humidity we feel

Amount of water vapor in air relative to maximum possible

amount (saturation mixing ratio)

Example

Temperature: 20oC

Saturation mixing ratio=14g vapor per 1 kg air

Actual vapor content = 7 g per 1 kg air

Relative humidity = 7 g / 14 g x 100% = 50%

Dew PointDew Point Temperature to which air must Temperature to which air must

be cooled to become saturatedbe cooled to become saturated Assumes no change in mixing ratioAssumes no change in mixing ratio Relative humidity is 100% in air Relative humidity is 100% in air

that’s at its dew pointthat’s at its dew point Stating air’s dew point is Stating air’s dew point is

essentially the same as stating its essentially the same as stating its mixing ratiomixing ratio

Air’s saturation mixing ratio and relative humidity change with temperature

Which has larger mixing ratio?

Which has higher relative humidity?

Antarctica

Death Valley

Hotter:Higher mixing ratio,Lower relative humidity

Colder:Lower mixing ratio Higher relative humidity

Relative Humidity, Mixing Ratio Relative Humidity, Mixing Ratio and Air Temperatureand Air Temperature

Hotter air can hold much more Hotter air can hold much more

water vapor than cold airwater vapor than cold air

Hotter air can have more vapor in it Hotter air can have more vapor in it

than cold air, yet have lower than cold air, yet have lower

relative humidityrelative humidity

Relative Humidity Changes with Relative Humidity Changes with Temperature DailyTemperature Daily

Rel

ativ

e H

um

idit

y

Tem

per

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Air Temp, Dew Pt. & Relative Air Temp, Dew Pt. & Relative Humidity in HeberHumidity in Heber

Dew Point Temperatures

Adiabatic Temperature ChangesAdiabatic Temperature Changes

• Air cools when it expands, warms Air cools when it expands, warms when its compressedwhen its compressed

• Rising air expands and coolsRising air expands and cools

• Sinking air is compressed and warmsSinking air is compressed and warms

• Adiabatic refers to temperature Adiabatic refers to temperature changes w/o heat transferchanges w/o heat transfer

Very important!

Adiabatic Temperature ChangesAdiabatic Temperature Changes

Dry & Wet Adiabatic RatesDry & Wet Adiabatic Rates• Saturated air cools less as it rises

because condensation of water releases heat

• Dry adiabatic rate = 10Dry adiabatic rate = 10ooC / C / 1000m = 5.51000m = 5.5ooF / 1000 feetF / 1000 feet

• Wet adiabatic rate = 5 to Wet adiabatic rate = 5 to 99ooC / 1000m C / 1000m (2.75 to 5(2.75 to 5ooF/1000ft)F/1000ft)

Dry & Wet Adiabatic RatesDry & Wet Adiabatic Rates

Lifting Condensation LevelLifting Condensation Level As air rises, it expands and coolsAs air rises, it expands and cools Level (altitude) at which it is cooled Level (altitude) at which it is cooled

to its dew point is theto its dew point is the lifting lifting condensation levelcondensation level

– Clouds form Clouds form above this level above this level if air is risingif air is rising

Causes of LiftingCauses of Lifting Orographic – wind blows over Orographic – wind blows over

mountainsmountains Frontal wedging – warm air forced Frontal wedging – warm air forced

over colder airover colder air Convergence – winds blowing Convergence – winds blowing

togethertogether Convection – solar heating creates Convection – solar heating creates

hot air that riseshot air that rises

Orographic LiftingOrographic Lifting

Important along Wasatch Front, much of Western U.S.

Frontal WedgingFrontal Wedging“Storm Fronts”

ConvergenceConvergence

ConvectionConvection

Cause of Rain Shadow DesertCause of Rain Shadow Desert

Rising air cools at

wet adiabatic

rate

sinking air warms at

dry adiabatic rate

Atmospheric StabilityAtmospheric Stability

Stable Air = Air that tends to not Stable Air = Air that tends to not riserise

Unstable Air = Air that tends to Unstable Air = Air that tends to keep rising keep rising (regardless of orographics, (regardless of orographics, fronts, etc.)fronts, etc.)

Importance – rising air cools, Importance – rising air cools, makes clouds, precipitation, even makes clouds, precipitation, even tornadostornados

What Controls StabilityWhat Controls Stability

Depends on adiabatic cooling Depends on adiabatic cooling

rate (dry and wet) vs. rate (dry and wet) vs.

Environmental Lapse RateEnvironmental Lapse Rate

Environmental Lapse Rate = the Environmental Lapse Rate = the

actual, existing decrease in air actual, existing decrease in air

temperature with altitudetemperature with altitude

Atmospheric Stability, Atmospheric Stability, cont.cont.

Three types of stability:Three types of stability:Absolute stabilityAbsolute stability

Absolute instabilityAbsolute instabilityConditional instabilityConditional instability

Absolute StabilityAbsolute Stability

Environmental Lapse rate is Environmental Lapse rate is less than wet adiabatic rateless than wet adiabatic rate As air rises, it cools so much (even if As air rises, it cools so much (even if

its saturated) that it becomes cooler its saturated) that it becomes cooler than surrounding air so it stops than surrounding air so it stops risingrising

Absolute stabilityAbsolute stability

Absolute instabilityAbsolute instability Environmental lapse rate is Environmental lapse rate is

greater than dry adiabatic greater than dry adiabatic raterate As air rises, despite cooling at As air rises, despite cooling at

dry adiabatic rate, it becomes dry adiabatic rate, it becomes progressively warmer than progressively warmer than surrounding air and rises surrounding air and rises fasterfaster

Absolute InstabilityAbsolute Instability

Absolute instabilityAbsolute instability

Conditional InstabilityConditional Instability

Environmental Lapse rate is Environmental Lapse rate is greater than wet adiabatic rate, greater than wet adiabatic rate, less than dry adiabatic rateless than dry adiabatic rate As air rises, if it is unsaturated it As air rises, if it is unsaturated it

tends to not rise, but once its tends to not rise, but once its saturated it keeps risingsaturated it keeps rising

Conditional StabilityConditional Stability

Conditional InstabilityConditional Instability

NWS Storm Prediction NWS Storm Prediction CenterCenter

Focuses on dangerous thunderstormsFocuses on dangerous thunderstorms Produces estimates of convective stability Produces estimates of convective stability

for locations across the country twice dailyfor locations across the country twice daily Main website: Main website: http://www.spc.noaa.govhttp://www.spc.noaa.gov Soundings (weather balloon data which Soundings (weather balloon data which

provide information on environmental lapse provide information on environmental lapse rate and more) with stability analysis rate and more) with stability analysis (somewhat advanced scientifically): (somewhat advanced scientifically): http://http://www.spc.noaa.gov/exper/soundingswww.spc.noaa.gov/exper/soundings//

Chapter 4Chapter 4

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