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Static Stability and Cloud Static Stability and Cloud DevelopmentDevelopment

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Cloud FormationCloud Formation What is static stability?What is static stability?

It describes what would happen to an air parcel if it It describes what would happen to an air parcel if it was forced upwards.was forced upwards.

It could keep rising It could keep rising It could return back to it’s original/starting heightIt could return back to it’s original/starting height

How is air is cooled to the saturation point?How is air is cooled to the saturation point?

If atmosphere is stable:If atmosphere is stable:

If atmosphere is unstable:If atmosphere is unstable:

Generally through vertical liftingGenerally through vertical lifting

Flat-layered type clouds form (stratus)Flat-layered type clouds form (stratus)

Cumulus-type clouds form (puffy clouds)Cumulus-type clouds form (puffy clouds)

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StratusStratus

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CumulusCumulus

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StabilityStability

Parcel rising and sinking in Parcel rising and sinking in the atmospherethe atmosphere

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Dry Adiabatic Dry Adiabatic ProcessesProcesses

Air is unsaturated (RH Air is unsaturated (RH is below 100%)is below 100%)

The rate at which the The rate at which the temperature temperature decreases with decreases with increasing altitude: increasing altitude: dry adiabatic lapse dry adiabatic lapse raterate ( (DALRDALR.).)

In these conditions, In these conditions, air cools at 5 air cools at 5 deg/1000 ftdeg/1000 ft

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ExampleExampleEstimate the temperature at Mt. Hamilton from the Estimate the temperature at Mt. Hamilton from the

San Jose temperature. Assume the air is dry.San Jose temperature. Assume the air is dry.

Altitude of San Jose: ________________Altitude of San Jose: ________________

Altitude of Mt. Hamilton: _____________Altitude of Mt. Hamilton: _____________

Temperature at San Jose: _______________Temperature at San Jose: _______________

Temperature at Mt. Hamilton _______________Temperature at Mt. Hamilton _______________

4200 ft4200 ft

~ 200 ft~ 200 ft

60 degrees 60 degrees

80 degrees80 degrees

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Moist Adiabatic Moist Adiabatic ProcessesProcesses

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

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Moist Adiabatic Moist Adiabatic ProcessesProcesses

If we were to raise the air above this If we were to raise the air above this height, then the excess vapor would height, then the excess vapor would condense. condense.

The parcel of air will now cool at a The parcel of air will now cool at a different rate than the DALR. different rate than the DALR.

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Moist Adiabatic Moist Adiabatic ProcessesProcesses

The moist adiabatic lapse rate varies in The moist adiabatic lapse rate varies in the atmosphere (not constant).the atmosphere (not constant).

A typical value may be ~3°F/1000 ft. A typical value may be ~3°F/1000 ft.

This will always be less than the dry This will always be less than the dry adiabatic lapse rate of 5°F/1000 ftadiabatic lapse rate of 5°F/1000 ft

* * Air cools LESS quickly when moist because it is Air cools LESS quickly when moist because it is receiving latent energy from the condensation.receiving latent energy from the condensation.

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ExampleExampleCalculate the temperature at Mt. Hamilton from the Calculate the temperature at Mt. Hamilton from the

San Jose temperature now assuming the air is San Jose temperature now assuming the air is moist. moist.

Altitude of San Jose: ________________Altitude of San Jose: ________________

Altitude of Mt. Hamilton: _____________Altitude of Mt. Hamilton: _____________

Temperature at San Jose: _______________Temperature at San Jose: _______________

Temperature at Mt. Hamilton _______________Temperature at Mt. Hamilton _______________

4200 ft4200 ft

~ 200 ft~ 200 ft

80 degrees80 degrees

68 degrees68 degrees

Atmospheric StabilityAtmospheric Stability

A A stablestable atmosphere atmosphere

Atmospheric StabilityAtmospheric Stability

An An unstableunstable atmosphere atmosphere

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Conditionally unstable atmosphere

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How does air get How does air get forced upward?forced upward?

TopographyTopography

Surface heating/convectionSurface heating/convection

Convergence of surface airConvergence of surface air

Lifting associated with weather frontsLifting associated with weather fronts

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ConvectionConvection