# Lecture 5.2: Stability Are you stable or unstable? Does it depend on the situation?

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Lecture 5.2: Stability Are you stable or unstable? Does it depend on the situation?

Content Types of stability Stability in the atmosphere Convection

STABLEUNSTABLE NEUTRAL Thermodynamics Stability: –Stable –Unstable –Neutral Stability

Stability & Movement Figure 7.1 A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed. If the rock instead departs in the direction of the push, it was in 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 & Temperature Rising air expands, using energy to push out, which slows and adiabatically cools the air. A parcel of air may be forced to rise or sink, and change temperature relative to environmental air, which is sampled using radiosonde balloons. Figure 7.2

Stable Atmospheric Conditions Cold air below warm air is considered stable because cold air has a higher density, and when displaced will gravitate back below the warm, lighter, air. A morning atmosphere has this stable vertical profile, keeping fog unmixed. Figure 7.4

Temperature (  C) Altitude (m) Thermodynamics Dry Adiabatic Lapse Rate Environmental Sounding 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 Rates Figure 7.3A Figure 7.3B A parcel of rising dry air cools at approximately 10° C/km, while latent heat released into the parcel by condensing vapor keeps the moist adiabatic lapse rate nearer to 6° C/km. Because the environmental air cools more slowly, the parcel of air held by the helicopter is always cooler and sinks back to its original position.

Thermodynamics Stable Atmosphere 10 0 C 00C00C T env. =10 0 C 2000 m 3000 m T env. =14 0 C Lift Parcel Dry Adabatically Since the parcel is cooler than the environment it will return to it’s original level. Compare Lifted Parcel Temperature to the Environmental Temperature

Temperature (  C) Altitude (m) Thermodynamics Dry Adiabatic Lapse Rate Environmental Sounding 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 10 0 C 00C00C T env. = 10 0 C 2000 m 3000 m T env. = -6 0 C Lift Parcel Dry Adiabatically Since the parcel temperature is warmer than the environment the parcel will continue to move away from it’s original level. Compare Lifted Parcel Temperature to the Environmental Temperature

Absolutely Unstable Atmosphere Figure 7.6A Absolutely unstable conditions indicate that a lifted parcel of air, whether dry or moist, will be warmer than the surrounding environmental air, and hence continue to rise.

Conditionally Unstable Conditions are initially stable when the environmental lapse rate (LR) is between the dry and moist adiabatic LR. Air that is dry is always cooler and sinks, but when the parcel's dew point is reached it cools more slowly, and may become warmer than the surrounding air, creating instability. Figure 7.7A

Stabililty Summary Environmental temperatures determine stability for rising parcels of air. The atmosphere is absolutely unstable when the environmental lapse rate (ELR) is steeper than the dry LR, absolutely stable when the ELR is less steep than the moist LR, and conditionally unstable otherwise. Figure 7.8

Temperature (  C) Altitude (m) Thermodynamics Dry Adiabatic Lapse Rate Environmental Sounding 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 10 0 C 00C00C T env. = 10 0 C 2000 m 3000 m T env. = 0 0 C Lift Parcel Dry Adiabatically Compare Lifted Parcel Temperature to the Environmental Temperature Since the parcel temperature is the same as the environment the parcel will neither continue to move away from nor return to it’s original level.

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