FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesMedium
A pilot receives a PIREP indicating an outside air temperature of -4°C at 8,000 feet MSL. Using the standard lapse rate, what is the approximate freezing level?
- A4,000 feet MSL
- B6,000 feet MSL
- C10,000 feet MSL
- D8,000 feet MSL
Show answer & explanationAnswer & explanation
Correct answer: B. 6,000 feet MSL
Using the standard lapse rate of 2°C per 1,000 feet, temperature increases by 2°C for every 1,000 feet of descent. Starting at -4°C at 8,000 feet, it takes 2,000 feet of descent (4°C ÷ 2°C per 1,000 ft = 2,000 ft) to reach 0°C, placing the freezing level at 8,000 − 2,000 = 6,000 feet MSL.
Why the other options are wrong
- A. This overestimates the descent needed to reach 0°C.
- C. This would require temperature to decrease further, moving away from freezing, which is incorrect direction.
- D. This is the altitude of the PIREP itself, not the freezing level.
Freezing Level Estimation
The altitude at which the outside air temperature reaches 0°C, often estimated using PIREPs and the standard lapse rate.
- Standard lapse rate: 2°C per 1,000 ft
- PIREPs provide actual observed temperatures aloft
- Freezing level is critical for icing risk assessment
Memory trick: 2 degrees per thousand feet — count down to zero