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Atmospheric Stability Terminology I

Atmospheric Stability Terminology I. Hydrostatic Equilibrium Balance, in the vertical, between PGF and gravity The general state of the atmosphere

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Atmospheric Stability Terminology I

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  1. 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

  2. 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) • Vertical motions are restricted • Stratiform cloudiness (stratus)

  3. 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) • Vertical motions enhanced • Cumuliform cloudiness (cumulus)

  4. 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.

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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)!

  10. 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)

  11. Graphical Examples

  12. 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

  13. 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

  14. 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

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