Atmospheric Stability and Cloud Formation

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Atmospheric Stability and Cloud Formation
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RECAP

• Mechanical equilibrium stable, unstable,
  neutral.
• Adiabatic expansion/compression no heat
  exchange.
• Adiabatic lapse rate
• Dry adiabatic lapse rate 10 K/km
• Moist adiabatic lapse rate 6 K/km
• Remember Dry gt Moist always
• Environmental lapse rate.
• Atmospheric stability
• Absolutely stable atmosphere
• Absolutely unstable atmosphere
• Neutrally stable atmosphere
• Conditionally unstable atmosphere

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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 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.

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Stable atmosphere
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Unstable atmosphere
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Sample test questions

• If the environmental lapse rate is 5 deg C per
  1000 m and the temperature at the earth's surface
  is 25 deg 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

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Sample test questions

• Take the dry adiabatic lapse rate to be 10 deg C
  per 1000 m. A radiosonde has measured the
  temperature 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.

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Air Stability
DAY
NIGHT
Adiabatic lapse rate
Environmental lapse rate
Solar radiation
Altitude
IR cooling
30
20
The ground is warm
The ground is cool
Temperature C
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Cloud Formation
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Convection and Clouds
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Cumulus Clouds
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Cumulus clouds
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Formation of Convective Clouds
The surface air temperature is 35 C and the dew
point is 25 C
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Stability and cloud thickness

• A conditionally unstable atmosphere allows for
  saturated air to keep propagating upwards

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STABLE
UNSTABLE
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Orographic uplift
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Wave clouds
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Cap cloud
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Lenticular cloud
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Wave clouds