FAA Aircraft Dispatcher (ADX)MeteorologyHard
A temperature sounding shows the environmental lapse rate through a saturated cloud layer is 2.5°C per 1,000 feet. Given that the average moist adiabatic lapse rate is about 1.5°C per 1,000 feet and the dry adiabatic lapse rate is 3°C per 1,000 feet, this atmospheric layer is best classified as:
- AAbsolutely unstable, because the ELR exceeds both the moist and dry adiabatic lapse rates
- BNeutrally stable, because the ELR equals the standard lapse rate
- CConditionally unstable, because the ELR lies between the moist and dry adiabatic lapse rates
- DAbsolutely stable, because the ELR is less than the dry adiabatic lapse rate
Show answer & explanationAnswer & explanation
Correct answer: C. Conditionally unstable, because the ELR lies between the moist and dry adiabatic lapse rates
Stability is determined by comparing the environmental lapse rate (ELR) to the dry and moist adiabatic lapse rates. When the ELR (2.5°C/1,000 ft) falls between the moist (1.5°C/1,000 ft) and dry (3°C/1,000 ft) adiabatic rates, the layer is conditionally unstable: stable for an unsaturated parcel but unstable if the parcel becomes saturated and cooling continues at the slower moist rate.
Why the other options are wrong
- A. Absolute instability requires ELR to exceed the dry adiabatic rate, which it does not here.
- B. Neutral stability requires ELR to exactly equal one of the adiabatic rates, not the standard lapse rate.
- D. Absolute stability requires ELR to be less than the moist adiabatic rate, not just less than the dry rate.
Atmospheric Stability Classification
Stability is determined by comparing the environmental lapse rate (ELR) to the dry adiabatic lapse rate (DALR, ~3°C/1,000ft) and moist adiabatic lapse rate (MALR, ~1.5-2°C/1,000ft).
- ELR < MALR: absolutely stable
- ELR > DALR: absolutely unstable
- MALR < ELR < DALR: conditionally unstable
Memory trick: Between moist and dry, conditional lies