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Atmospheric Stability and Cloud Formation

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- 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

- 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.

- 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

- 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

- 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.

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]

Cumulus clouds

The surface air temperature is 35 C and the dew point is 25 C

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

STABLE

UNSTABLE