FAA Instrument Flight Instructor (FII)WeatherHard
A morning sounding shows a surface temperature of 22°C and a temperature of -2°C at 6,000 feet. Using the dry adiabatic lapse rate of 3°C per 1,000 feet as a reference, how should a CFII characterize the stability of this air mass?
- AAbsolutely unstable, because the environmental lapse rate exceeds the dry adiabatic rate
- BAbsolutely stable, because the environmental lapse rate is less than the dry adiabatic rate
- CNeutrally stable, because the lapse rates are equal
- DConditionally unstable, because saturation has occurred
Show answer & explanationAnswer & explanation
Correct answer: A. Absolutely unstable, because the environmental lapse rate exceeds the dry adiabatic rate
The environmental lapse rate is calculated as (22°C − (−2°C)) / 6,000 ft = 24°C / 6 (thousands of feet) = 4°C per 1,000 feet. Since this exceeds the dry adiabatic lapse rate of 3°C per 1,000 feet, a rising parcel of unsaturated air will remain warmer (and less dense) than its surroundings at every level, making the air mass absolutely unstable.
Why the other options are wrong
- B. This would apply only if the environmental lapse rate were less than 3°C/1,000 ft, which is not the case here.
- C. Neutral stability requires the environmental lapse rate to equal the dry adiabatic rate exactly (3°C/1,000 ft), not 4°C/1,000 ft.
- D. Conditional instability applies when the environmental lapse rate is between the dry and moist adiabatic rates, not exceeding both.
Absolute Instability (Lapse Rate Comparison)
A condition where the environmental (actual) lapse rate exceeds the dry adiabatic lapse rate, causing rising unsaturated air to remain warmer than its surroundings at every altitude.
- Dry adiabatic lapse rate is approximately 3°C per 1,000 feet
- If environmental lapse rate > DALR, the air mass is absolutely unstable
- Environmental lapse rate is calculated from actual temperature soundings
Memory trick: Steeper than 3-per-1000 means the air is racing upward—absolutely unstable.