Lecture 5.2: Stability

Are you stable or unstable?

Does it depend on the situation?

Content

• Types of stability

• Stability in the atmosphere

• Convection

STABLE UNSTABLE NEUTRAL

Thermodynamics

• Stability:– Stable

– Unstable

– Neutral Stability

Stability & MovementStability & Movement

Figure 7.1Figure 7.1

A rock, like a parcel of air, that is in stable equilibrium will return A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed.to its original position when pushed.

If the rock instead departs in the direction of the push, it was in If the rock instead departs in the direction of the push, it was in unstable equilibrium.unstable equilibrium.

A Human and a Basketball

• Human pushes the ball on a flat surface.• Ball stops when it is no longer being pushed.• This is considered an neutrally stable situation.• You disturb the position of the ball and it neither

returns nor moves away from it’s original position.

Movement & TemperatureMovement & TemperatureRising air Rising air expands, using expands, using energy to push out, energy to push out, which slows and which slows and adiabatically cools adiabatically cools the air.the air.

A parcel of air A parcel of air may be forced to may be forced to rise or sink, and rise or sink, and change change temperature temperature relative to relative to environmental air, environmental air, which is sampled which is sampled using radiosonde using radiosonde balloons.balloons.

Figure 7.2Figure 7.2

Stable Atmospheric ConditionsStable Atmospheric Conditions

Cold air below warm air is considered stable because cold air has a Cold air below warm air is considered stable because cold air has a higher density, and when displaced will gravitate back below the higher density, and when displaced will gravitate back below the warm, lighter, air.warm, lighter, air.

A morning atmosphere has this stable vertical profile, keeping fog A morning atmosphere has this stable vertical profile, keeping fog unmixed.unmixed.

Figure 7.4Figure 7.4

0

500

1000

1500

2000

2500

3000

3500

4000

4500

-10 0 10 20 30Temperature (C)

Altitude(m)

Thermodynamics

Dry AdiabaticLapse Rate

EnvironmentalSounding

Parcel

• Stable Sounding

Thermodynamics

• If we (somehow) lift the parcel:– It will cool at the dry adiabatic lapse

rate.

– The parcel will find itself cooler than the environmental (sounding) temperature.

– At the same pressure, a cooler parcel will be more dense than the environment.

– Being denser, the parcel will descend back to where it came from.

• STABLE!!!!!!!

Dry & Moist Adiabatic RatesDry & Moist Adiabatic Rates

Figure 7.3AFigure 7.3A Figure 7.3BFigure 7.3B

A parcel of rising dry air cools at approximately 10° C/km, while A parcel of rising dry air cools at approximately 10° C/km, while latent heat released into the parcel by condensing vapor keeps the latent heat released into the parcel by condensing vapor keeps the moist adiabatic lapse rate nearer to 6° C/km.moist adiabatic lapse rate nearer to 6° C/km.Because the environmental air cools more slowly, the parcel of air Because the environmental air cools more slowly, the parcel of air held by the helicopter is always cooler and sinks back to its held by the helicopter is always cooler and sinks back to its original position.original position.

Thermodynamics

• Stable Atmosphere

100C

00C

Tenv.=100C2000 m

3000 m Tenv.=140C

Lift ParcelDry

Adabatically

Since the parcel is cooler than the environmentit will return to it’s original level.

Compare LiftedParcel Temperatureto the Environmental

Temperature

0

500

1000

1500

2000

2500

3000

3500

4000

4500

-10 0 10 20 30Temperature (C)

Altitude(m)

Thermodynamics

Dry AdiabaticLapse Rate

EnvironmentalSounding

Parcel

• Unstable Sounding

Thermodynamics

• If we (somehow) lift the parcel:– It will cool at the dry adiabatic lapse rate.– The parcel will find itself warmer than the

environmental (sounding) temperature.– At the same pressure, a warmer parcel will

be less dense than the environment.– Being less dense, the parcel will ascend and

move farther from where it came from.

• UNSTABLE!!!!!!!

Thermodynamics

• Unstable Atmosphere

100C

00C

Tenv.= 100C 2000 m

3000 m Tenv.= -60C

Lift ParcelDry

Adiabatically

Since the parceltemperature is warmer than theenvironment theparcel willcontinue to moveaway from it’soriginal level.

Compare LiftedParcel Temperatureto the Environmental

Temperature

Absolutely Unstable AtmosphereAbsolutely Unstable Atmosphere

Figure 7.6AFigure 7.6A

Absolutely unstable Absolutely unstable conditions indicate that a conditions indicate that a lifted parcel of air, lifted parcel of air, whether dry or moist, whether dry or moist, will be warmer than the will be warmer than the surrounding surrounding environmental air, and environmental air, and hence continue to rise.hence continue to rise.

Conditionally UnstableConditionally Unstable

Conditions are initially stable Conditions are initially stable when the environmental lapse when the environmental lapse rate (LR) is between the dry rate (LR) is between the dry and moist adiabatic LR.and moist adiabatic LR.

Air that is dry is always cooler Air that is dry is always cooler and sinks, but when the and sinks, but when the parcel's dew point is reached parcel's dew point is reached it cools more slowly, and may it cools more slowly, and may become warmer than the become warmer than the surrounding air, creating surrounding air, creating instability.instability.

Figure 7.7AFigure 7.7A

Stabililty SummaryStabililty Summary

Environmental Environmental temperatures determine temperatures determine stability for rising parcels stability for rising parcels of air.of air.

The atmosphere is The atmosphere is absolutely unstable when absolutely unstable when the environmental lapse the environmental lapse rate (ELR) is steeper than rate (ELR) is steeper than the dry LR, absolutely the dry LR, absolutely stable when the ELR is less stable when the ELR is less steep than the moist LR, steep than the moist LR, and conditionally unstable and conditionally unstable otherwise.otherwise.

Figure 7.8Figure 7.8

0

500

1000

1500

2000

2500

3000

3500

4000

4500

-10 0 10 20 30Temperature (C)

Altitude(m)

Thermodynamics

Dry AdiabaticLapse Rate

EnvironmentalSounding

Parcel

• (Dry) Neutral Atmosphere

Thermodynamics

• If we (somehow) lift the parcel:– It will cool at the dry adiabatic lapse rate.– The parcel will find itself at the same temperature

as the environmental (sounding) temperature.– Being the same density, the parcel will not be

accelerated in any direction and will remain where it is.

• NEUTRAL STABILITY!!!!!!! -- Dry Neutral

Thermodynamics

• (Dry) Neutrally Stable Atmosphere

100C

00C

Tenv.= 100C 2000 m

3000 m Tenv.= 00C

Lift ParcelDry

Adiabatically

Compare LiftedParcel Temperatureto the Environmental

Temperature

Since the parceltemperature is the same as theenvironment theparcel will neithercontinue to moveaway from norreturn to it’soriginal level.