Atmospheric Stability Terminology I Hydrostatic Equilibrium –Balance, in the vertical, between PGF and gravity –The general state of the atmosphere –Net force (in the vertical) on air parcels is very near zero over a large area

Atmospheric Stability: Terminology II  Stable Equilibrium (“Stable” atmosphere) –When a parcel is moved upward or downward, forces act to return it to it’s original altitude (bowl with ball in it) bowl with ball in itbowl with ball in it –Vertical motions are restricted –Stratiform cloudiness (stratus)

Atmospheric Stability Terminology III  Unstable Equilibrium (“Unstable” atmosphere) –When a parcel is moved upward or downward, forces act to accelerate it away from it’s original altitude (upside-down bowl with ball on top) upside-down bowl with ball on topupside-down bowl with ball on top –Vertical motions enhanced –Cumuliform cloudiness (cumulus)

Atmospheric Stability Terminology IV  Lapse Rate: the rate of decrease of air temperature with increasing elevation  Environmental Lapse Rate: the lapse rate of the atmosphere. We use the environmental lapse rate to determine atmospheric stability.

Atmospheric Stability Terminology V  Dry Adiabatic Lapse Rate –The rate at which a rising, unsaturated parcel cools –5.5 deg F per 1000 feet of ascent (10 deg C per km) –The rate at which a sinking, unsaturated parcel warms (conservation of energy)

Atmospheric Stability Terminology VI  Moist Adiabatic Lapse Rate –The rate at which a rising, saturated parcel cools –Approximately 3.3 deg F per 1000 feet of ascent (6 deg C per km) –Why is the moist rate less than the dry rate?  CONDENSATION releases energy into rising parcel –The rate at which a sinking, saturated parcel warms, although sinking parcels are rarely saturated  EVAPORATION requires energy (cooling process), so the rate of warming is slowed as parcel descends

Atmospheric Stability  Rising (sinking) air MUST cool (warm) at either the dry or moist lapse rates- it’s the law (of physics)!  Think about a hot-air balloon and why it rises

Stability and the Environmental Lapse Rate: Unstable Equilibrium  Unstable Equilibrium: rapid cooling with height –Environmental temperature decreases faster than 5.5 deg F per 1000 feet (10 deg C per 1000 m) –If parcels forced to rise they remain warmer than their surroundings  Parcel and surrounding environment at same pressure  Parcel will be warmer and thus less dense than air around it and will be buoyant  The larger the temperature difference (parcel warmer) the faster it will rise, because it will be much less dense than its surroundings

Stability and the Environmental Lapse Rate: Stable Equilibrium I  Stable Equilibrium: slow or no cooling with height –Environmental temperature decreases slower than 3.3 deg F per 1000 feet (6 deg C per 1000 m) –If parcels forced to rise they become colder than their surroundings  Parcel and surrounding environment at same pressure  Parcel will be colder and thus more dense than air around it and will sink back to original position if upward forcing (convergence into low pressure, orography) ceases  This does not mean that air in a stable atmosphere will not rise (or sink)!

Stability and the Environmental Lapse Rate: Stable Equilibrium II  Inversion: extremely stable- warming with increasing height –Environmental temperature increases as altitude increases –If parcels forced to rise they become much colder than their surroundings  At ground level –Nocturnal: ground cools faster than air above (clear, calm night) –Cool bodies of water (daytime, spring and summer)  Aloft (above the ground) –Tropopause: boundary between troposphere and stratosphere –Subsidence (high pressure, sinking air)

Graphical Examples

Stability and Mixing (Air Pollution)  Instability (unstable) –Promotes rising and sinking air (vertical motion) –Mixes air from aloft down to ground, which means generally cleaner air (unless there are tall smokestacks around  )  Stability (stable) –Promotes stratification –Little mixing, so junk put into the air stays in area –Also, rapid changes in wind speed and direction can occur over small vertical distance

Stability and Mixing (Wind Speed)  Stability: Typical Clear Early Morning –Coolest near ground, still relatively warm aloft. Promotes stratification (no vertical mixing) –With little mixing, air in contact with ground slows (winds die down) –Also, rapid changes in wind speed and direction can occur over small vertical distance  Instability: Typical Sunny Afternoon –Very warm near ground, still relatively cool aloft. Promotes rising and sinking air (vertical mixing) –Mixes fast moving air from above (no friction) with slow-moving air near ground

Key Figures  8.6, 8.7, 8.8, 8.15, 8.17, 8.20, 8.21, 8.26, 8.27, 8.51, 8.54  Cloud Atlas, Wave Clouds Cloud AtlasWave Clouds Cloud AtlasWave Clouds