Atmospheric stability and cloud formation
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Atmospheric Stability and Cloud Formation. 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 > Moist always

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Atmospheric Stability and Cloud Formation

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Atmospheric Stability and Cloud Formation


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 > 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 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

  • 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


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


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

Adiabatic lapse rate

Environmental

lapse rate

Solar radiation

Altitude

IR cooling

20

30

The ground is warm

The ground is cool

Temperature [C]


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

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


STABLE

UNSTABLE


Orographic uplift


Wave clouds


Cap cloud


Lenticular cloud


Wave clouds


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