Static Stability and Cloud Development

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• Static Stability and Cloud Development

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Cloud Formation

• What is static stability?
• It describes what would happen to an air parcel
  if it was forced upwards.
• It could keep rising
• It could return back to its original/starting
  height
• How is air is cooled to the saturation point?
• If atmosphere is stable
• If atmosphere is unstable

Generally through vertical lifting
Flat-layered type clouds form (stratus)
Cumulus-type clouds form (puffy clouds)
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Stratus
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Cumulus
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Stability
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Parcel rising and sinking in the atmosphere
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Dry Adiabatic Processes

• Air is unsaturated (RH is below 100)
• The rate at which the temperature decreases with
  increasing altitude dry adiabatic lapse rate
  (DALR.)
• In these conditions, air cools at 5 deg/1000 ft

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Example

• Estimate the temperature at Mt. Hamilton from the
  San Jose temperature. Assume the air is dry.
• Altitude of San Jose ________________
• Altitude of Mt. Hamilton _____________
• Temperature at San Jose _______________
• Temperature at Mt. Hamilton _______________

200 ft
4200 ft
80 degrees
60 degrees
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Moist Adiabatic Processes

• Consider lifting an air parcel. It will cool
  according to the DALR as long as it remains
  unsaturated. At some height, the parcel will
  cool enough so that the air will reach its
  saturation point.

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Moist Adiabatic Processes

• If we were to raise the air above this height,
  then the excess vapor would condense.
• The parcel of air will now cool at a different
  rate than the DALR.

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Moist Adiabatic Processes

• The moist adiabatic lapse rate varies in the
  atmosphere (not constant).
• A typical value may be 3F/1000 ft.
• This will always be less than the dry adiabatic
  lapse rate of 5F/1000 ft
• Air cools LESS quickly when moist because it is
  receiving latent energy from the condensation.

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Example

• Calculate the temperature at Mt. Hamilton from
  the San Jose temperature now assuming the air is
  moist.
• Altitude of San Jose ________________
• Altitude of Mt. Hamilton _____________
• Temperature at San Jose _______________
• Temperature at Mt. Hamilton _______________

200 ft
4200 ft
80 degrees
68 degrees
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Atmospheric Stability
A stable atmosphere
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Atmospheric Stability
An unstable atmosphere
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Conditionally unstable atmosphere
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How does air get forced upward?

• Topography
• Surface heating/convection
• Convergence of surface air
• Lifting associated with weather fronts

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Convection