FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesHard
A PIREP reports moderate rime icing at 9,000 feet MSL. The surface temperature is 15°C. Assuming a standard atmosphere lapse rate, at approximately what altitude would the freezing level be located, helping confirm the icing report is consistent with conditions aloft?
- A7,500 feet MSL
- B9,000 feet MSL
- C5,000 feet MSL
- D6,000 feet MSL
Show answer & explanationAnswer & explanation
Correct answer: A. 7,500 feet MSL
Using the standard temperature lapse rate of 2°C per 1,000 feet, the freezing level is found by dividing the surface temperature by 2: 15°C ÷ 2 = 7.5, giving a freezing level of approximately 7,500 feet MSL. Since the PIREP altitude of 9,000 feet is above this level, the reported icing is consistent with expected conditions aloft.
Why the other options are wrong
- B. This is the PIREP altitude, not the calculated freezing level.
- C. Underestimates the freezing level by using too high a lapse rate.
- D. Close but results from an incorrect rounding of the calculation.
Freezing Level Estimation
The approximate freezing level can be estimated by dividing the surface temperature (°C) by the standard lapse rate of 2°C per 1,000 feet.
- Standard atmosphere lapse rate: 2°C per 1,000 ft (approx., using 3.5F/1000ft or 2C/1000ft rule of thumb)
- Freezing level (ft) = surface temp(°C) ÷ 2 × 1,000
- Icing potential generally increases above the freezing level in visible moisture
- PIREPs help verify forecast icing/freezing levels in actual conditions
Memory trick: Divide surface temp by two, multiply by a thousand — that's where ice starts to grow.