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 StabilitySolar radiation

The ground is warm

IR cooling

Alti

tude

Temperature [C]

Adiabatic lapse rate

3020

Environmental lapse rate

The ground is cool

DAY NIGHT

Cloud Formation

Convection and Clouds

Cumulus Clouds

Cumulus clouds

Formation of Convective CloudsThe 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