FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesMedium
A pilot obtains an upper air sounding showing the environmental (existing) lapse rate between the surface and 5,000 feet is 4°C per 1,000 feet. The dry adiabatic lapse rate is approximately 3°C per 1,000 feet. How would this layer of the atmosphere be classified?
- AConditionally stable
- BAbsolutely unstable
- CNeutrally stable
- DAbsolutely stable
Show answer & explanationAnswer & explanation
Correct answer: B. Absolutely unstable
When the environmental lapse rate (4°C/1,000 ft) exceeds the dry adiabatic lapse rate (3°C/1,000 ft), a rising parcel of air stays warmer than its surroundings and continues to rise on its own, making the layer absolutely unstable.
Why the other options are wrong
- A. Conditional stability occurs when the environmental rate is between the dry and moist adiabatic rates.
- C. Neutral stability occurs when environmental rate equals the adiabatic rate exactly.
- D. Stability would require the environmental rate to be less than the moist adiabatic rate.
Atmospheric Stability
Stability is determined by comparing the environmental lapse rate to the dry and moist adiabatic lapse rates; a steeper environmental rate promotes instability.
- Dry adiabatic lapse rate ≈ 3°C/1,000 ft
- Moist adiabatic lapse rate ≈ 1.5°C/1,000 ft (varies)
- Environmental rate > dry adiabatic = absolutely unstable
Memory trick: Steeper than dry adiabatic = sky's the limit (unstable).