Atmospheric Stability Atmospheric Stability Terminology ITerminology I

Hydrostatic EquilibriumHydrostatic Equilibrium– Balance, in the vertical, Balance, in the vertical,

between PGF and gravitybetween PGF and gravity– The general state of the The general state of the

atmosphereatmosphere– Net forceNet force (in the vertical) (in the vertical)

on air parcels is very near on air parcels is very near zero over a large areazero over a large area

Atmospheric Stability: Terminology IIAtmospheric Stability: Terminology II

Stable Equilibrium (“Stable” atmosphere)Stable Equilibrium (“Stable” atmosphere)– When a parcel is moved upward or downward, When a parcel is moved upward or downward,

forces act to return it to it’s original altitude (forces act to return it to it’s original altitude (bowl with ball in itbowl with ball in it))

– Vertical motions are Vertical motions are restrictedrestricted

– Stratiform cloudiness Stratiform cloudiness (stratus)(stratus)

Atmospheric Stability Terminology IIIAtmospheric Stability Terminology III Unstable Equilibrium (“Unstable” atmosphere)Unstable Equilibrium (“Unstable” atmosphere)

– When a parcel is moved upward or downward, When a parcel is moved upward or downward, forces act to accelerate it away from it’s original forces act to accelerate it away from it’s original altitude (altitude (upside-down bowl with ball on topupside-down bowl with ball on top))

– Vertical motions enhancedVertical motions enhanced– Cumuliform cloudiness (cumulus)Cumuliform cloudiness (cumulus)

Atmospheric Stability Terminology IVAtmospheric Stability Terminology IV

Lapse Rate: the rate of decrease of air Lapse Rate: the rate of decrease of air temperature with increasing elevationtemperature with increasing elevation

Environmental Lapse Rate: the lapse rate of Environmental Lapse Rate: the lapse rate of the atmosphere. the atmosphere. We use the environmental We use the environmental lapse rate to determine atmospheric lapse rate to determine atmospheric stability.stability.

Atmospheric Stability Terminology VAtmospheric Stability Terminology V

Dry Adiabatic Lapse RateDry Adiabatic Lapse Rate– The rate at which a rising, The rate at which a rising, unsaturatedunsaturated parcel cools parcel cools– 5.5 deg F per 1000 feet of ascent (10 deg C per km)5.5 deg F per 1000 feet of ascent (10 deg C per km)– The rate at which a sinking, The rate at which a sinking, unsaturatedunsaturated parcel parcel

warms (conservation warms (conservation of energy)of energy)

Atmospheric Stability Terminology VIAtmospheric Stability Terminology VI

Moist Adiabatic Lapse RateMoist Adiabatic Lapse Rate– The rate at which a rising, The rate at which a rising, saturatedsaturated parcel cools parcel cools– Approximately 3.3 deg F per 1000 feet of ascent (6 Approximately 3.3 deg F per 1000 feet of ascent (6

deg C per km)deg C per km)– Why is the moist rate less than the dry rate?Why is the moist rate less than the dry rate?

CONDENSATION CONDENSATION releases energy into rising parcelreleases energy into rising parcel

– The rate at which a sinking, The rate at which a sinking, saturatedsaturated parcel parcel warms, although sinking parcels are rarely warms, although sinking parcels are rarely saturatedsaturated EVAPORATIONEVAPORATION requires energy (cooling process), so requires energy (cooling process), so

the rate of warming is slowed as parcel descendsthe rate of warming is slowed as parcel descends

Atmospheric StabilityAtmospheric Stability

Rising (sinking) air Rising (sinking) air MUSTMUST cool (warm) at cool (warm) at either the dry or moist either the dry or moist lapse rates- it’s the law lapse rates- it’s the law (of physics)!(of physics)!

Think about a hot-air Think about a hot-air balloon and why it risesballoon and why it rises

Stability and the Environmental Lapse Stability and the Environmental Lapse Rate: Unstable EquilibriumRate: Unstable Equilibrium

Unstable Equilibrium: rapid cooling with heightUnstable Equilibrium: rapid cooling with height– Environmental temperature decreases faster than Environmental temperature decreases faster than

5.5 deg F per 1000 feet (10 deg C per 1000 m)5.5 deg F per 1000 feet (10 deg C per 1000 m)– If parcels forced to rise they remain warmer than If parcels forced to rise they remain warmer than

their surroundingstheir surroundings Parcel and surrounding environment at same pressureParcel and surrounding environment at same pressure Parcel will be warmer and thus less dense than air Parcel will be warmer and thus less dense than air

around it and will be buoyantaround it and will be buoyant The larger the temperature difference (parcel warmer) the The larger the temperature difference (parcel warmer) the

faster it will rise, because it will be much less dense than faster it will rise, because it will be much less dense than its surroundingsits surroundings

Stability and the Environmental Lapse Stability and the Environmental Lapse Rate: Stable Equilibrium IRate: Stable Equilibrium I

Stable Equilibrium:Stable Equilibrium: slow or no cooling with height slow or no cooling with height– Environmental temperature decreases slower than Environmental temperature decreases slower than

3.3 deg F per 1000 feet (6 deg C per 1000 m)3.3 deg F per 1000 feet (6 deg C per 1000 m)– If parcels forced to rise they become colder than If parcels forced to rise they become colder than

their surroundingstheir surroundings Parcel and surrounding environment at same pressureParcel and surrounding environment at same pressure Parcel will be colder and thus more dense than air around Parcel will be colder and thus more dense than air around

it and will sink back to original position if upward forcing it and will sink back to original position if upward forcing (convergence into low pressure, orography) ceases(convergence into low pressure, orography) ceases

This does not mean that air in a stable atmosphere will This does not mean that air in a stable atmosphere will not rise (or sink)!not rise (or sink)!

Stability and the Environmental Lapse Stability and the Environmental Lapse Rate: Stable Equilibrium IIRate: Stable Equilibrium II

Inversion: extremely stable- warming with Inversion: extremely stable- warming with increasing heightincreasing height– Environmental temperature increases as altitude Environmental temperature increases as altitude

increasesincreases– If parcels forced to rise they become much colder If parcels forced to rise they become much colder

than their surroundingsthan their surroundings At ground levelAt ground level

– Nocturnal: ground cools faster than air above (clear, calm night)Nocturnal: ground cools faster than air above (clear, calm night)– Cool bodies of water (daytime, spring and summer)Cool bodies of water (daytime, spring and summer)

Aloft (above the ground)Aloft (above the ground)– Tropopause: boundary between troposphere and stratosphereTropopause: boundary between troposphere and stratosphere– Subsidence (high pressure, sinking air)Subsidence (high pressure, sinking air)

Graphical ExamplesGraphical Examples

Stability and Mixing (Air Stability and Mixing (Air Pollution)Pollution)

Instability (unstable)Instability (unstable)– Promotes rising and sinking Promotes rising and sinking

air (vertical motion)air (vertical motion)– Mixes air from aloft down to Mixes air from aloft down to

ground, which means ground, which means generally cleaner air generally cleaner air (unless there are tall (unless there are tall smokestacks around smokestacks around ))

Stability (stable)Stability (stable)– Promotes Promotes stratificationstratification– Little mixing, so junk put Little mixing, so junk put

into the air stays in areainto the air stays in area– Also, rapid changes in wind Also, rapid changes in wind

speed and direction can speed and direction can occur over small vertical occur over small vertical distancedistance

Stability and Mixing (Wind Speed)Stability and Mixing (Wind Speed) Stability: Typical Clear Early MorningStability: Typical Clear Early Morning

– Coolest near ground, still relatively warm aloft. Promotes Coolest near ground, still relatively warm aloft. Promotes stratificationstratification (no vertical mixing)(no vertical mixing)

– With little mixing, air in contact with ground slows (winds die down)With little mixing, air in contact with ground slows (winds die down)– Also, rapid changes in wind speed and direction can occur over small Also, rapid changes in wind speed and direction can occur over small

vertical distancevertical distance Instability: Typical Sunny AfternoonInstability: Typical Sunny Afternoon

– Very warm near ground, still relatively cool aloft. Promotes rising and Very warm near ground, still relatively cool aloft. Promotes rising and sinking air (vertical mixing)sinking air (vertical mixing)

– Mixes fast moving air from above (no friction) with slow-moving air near Mixes fast moving air from above (no friction) with slow-moving air near groundground

Key FiguresKey Figures

8.6, 8.7, 8.8, 8.15, 8.17, 8.20, 8.21, 8.26, 8.6, 8.7, 8.8, 8.15, 8.17, 8.20, 8.21, 8.26, 8.27, 8.51, 8.548.27, 8.51, 8.54

Cloud AtlasCloud Atlas, , Wave CloudsWave Clouds