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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)
– 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)
– 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