Atmospheric Stability
and Cloud Formation
RECAP
• Mechanical
equilibrium: stable,
unstable, neutral.
• Adiabatic
expansion/compression: no
heat exchange.
• Adiabatic lapse rate
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♦ Dry adiabatic lapse rate ~ 10 K/km
♦ Moist adiabatic lapse rate ~ 6 K/km
♦ Remember: Dry > Moist always
Environmental lapse rate.
Atmospheric stability:
♦ Absolutely stable atmosphere
♦ Absolutely unstable atmosphere
♦ Neutrally stable atmosphere
♦ Conditionally unstable atmosphere
Midchapter summary
• The
air temperature in a rising parcel of unsaturated air decreases
at the dry adiabatic rate, while the air temperature in a rising parcel
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of saturated air decreases at the moist adiabatic rate.
The dry adiabatic rate and moist adiabatic rate of cooling are
different due to the fact that latent heat is released in a rising
parcel of saturated air.
In a stable atmosphere, a lifted parcel of air will be cooler (heavier)
than the air surrounding it, and will tend to sink back to its original
position.
In an unstable atmosphere, a lifted parcel of air will be warmer
(lighter) than the air surrounding it, and will continue to rise upward,
away from its original position.
The atmosphere becomes more stable (stabilizes) as the surface air
cools and/or the air aloft warms.
The atmosphere becomes more unstable (destabilizes) as the
surface air warms and/or the air aloft cools.
Layered clouds tend to form in a stable atmosphere, while
cumuliform clouds form in an unstable atmosphere.
Stable atmosphere
Unstable atmosphere
Sample test questions
• and the temperature at the earth's surface is 25 deg
If the environmental lapse rate is 5 deg C per 1000 m
C, then the air temperature at 2000 m above the
ground is:
♦ a.
25 deg C
♦ b.
35 deg C
♦ c.
20 deg C
♦ d.
15 deg C
♦ e.
10 deg C
Sample test questions
• 1000 m. A radiosonde has measured the temperature
Take the dry adiabatic lapse rate to be 10 deg C per
of the atmosphere to be 30 deg C on the ground and
15 deg C at an altitude of 1000 m. What can you say
about the stability of the atmosphere?
♦ The atmosphere is absolutely unstable
♦ The atmosphere is conditionally unstable
♦ The atmosphere is absolutely stable
♦ The atmosphere is neutrally stable
♦ This question cannot be answered without knowing
the moist adiabatic lapse rate
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How does the Stability of the
Atmosphere Change During the Day?
Daytime:
♦ The sun heats the ground.
♦ The boundary layer is heated from below.
♦ The environmental lapse rate is steep.
♦ The atmosphere can become unstable.
Morning and evening hours:
♦ Radiation cooling results in temperature inversion.
♦ The boundary layer is cooler than the air above.
♦ The environmental lapse rate becomes less steep.
♦ The atmosphere is stable.
Air Stability
DAY
NIGHT
The ground is warm
IR cooling
Environmental
lapse rate
Altitude
Solar radiation
Adiabatic lapse rate
20
30
Temperature [C]
The ground is cool
Cloud Formation
Convection and Clouds
Cumulus Clouds
Cumulus clouds
Formation of Convective Clouds
The surface air temperature is 35 C and the dew point is 25 C
Stability and cloud thickness
• saturated air to keep propagating upwards
A conditionally unstable atmosphere allows for
STABLE
UNSTABLE
Orographic uplift
Wave clouds
Cap cloud
Lenticular cloud
Wave clouds