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. DAY NIGHT Air Stability Adiabatic lapse rate n Environmental tio lapse rate dia e d ing l u t o i r ra t a l Al So IR co 20 30 The ground is warm Temperature [C] The ground is cool 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.