A pilot's weather briefing indicates a surface temperature of 20°C and a temperature of 5°C at 6,000 feet MSL. Using a dry adiabatic lapse rate of 3°C per 1,000 feet, what does this tell the pilot about the stability of the atmosphere?
- AStable, because the actual (environmental) lapse rate of 2.5°C per 1,000 feet is less than the dry adiabatic rate
- BUnstable, because the actual lapse rate exceeds the dry adiabatic rate
- CStable, because the actual lapse rate exceeds the dry adiabatic rate
- DNeutral, because the two lapse rates are equal
Show answer & explanationAnswer & explanation
Correct answer: A. Stable, because the actual (environmental) lapse rate of 2.5°C per 1,000 feet is less than the dry adiabatic rate
The environmental (actual) lapse rate is calculated as (20°C - 5°C) / 6,000 ft = 15°C / 6 (thousands of feet) = 2.5°C per 1,000 feet. Since this is less than the dry adiabatic lapse rate of 3°C per 1,000 feet, a rising parcel cools faster than its surroundings and becomes cooler (denser) than the environment, causing it to sink back — indicating a stable atmosphere.
Why the other options are wrong
- B. This would be true only if the environmental lapse rate exceeded 3°C/1,000 ft, which it does not here.
- C. The math shows the actual rate is less than, not greater than, the dry adiabatic rate.
- D. The two rates (2.5 vs 3) are not equal, ruling out a neutral classification.
Lapse Rate Comparison for Stability
Comparing the environmental (actual) lapse rate to the dry adiabatic lapse rate (3°C/1,000 ft) determines atmospheric stability: a lower environmental rate indicates stability, a higher one indicates instability.
- Environmental lapse rate = temperature difference divided by altitude difference (in thousands of feet)
- Dry adiabatic lapse rate is a constant 3°C per 1,000 feet
- If environmental rate < adiabatic rate, atmosphere is stable; if greater, unstable
Memory trick: Slower cooling with height than 3°C/1,000ft means the air 'holds itself down' — stable