FAA Aircraft Dispatcher (ADX)MeteorologyHard
A pilot report (PIREP) indicates an outside air temperature of +4°C at 4,000 feet MSL and −2°C at 8,000 feet MSL, with a linear temperature change between the two altitudes. Assuming a standard lapse rate is not used and interpolating directly from the PIREP data, at approximately what altitude should the dispatcher expect the freezing level?
- AApproximately 5,700 feet MSL
- BApproximately 4,700 feet MSL
- CApproximately 7,700 feet MSL
- DApproximately 6,700 feet MSL
Show answer & explanationAnswer & explanation
Correct answer: D. Approximately 6,700 feet MSL
The temperature drops from +4°C to −2°C, a 6°C decrease over 4,000 feet (1.5°C per 1,000 ft). To go from +4°C to 0°C requires a 4°C drop, which takes 4 ÷ 1.5 × 1,000 ≈ 2,667 feet. Adding this to the 4,000-foot base gives a freezing level of approximately 6,667 feet, or about 6,700 feet MSL.
Why the other options are wrong
- A. This value is too low; it does not account for the full 4°C temperature drop needed.
- B. This underestimates the altitude needed to reach 0°C given the calculated lapse rate.
- C. This overestimates the freezing level, exceeding the interpolated calculation.
Freezing Level Interpolation from PIREPs
The freezing level can be determined by interpolating between two reported temperatures at different altitudes using the actual observed lapse rate.
- Actual lapse rate may differ from the standard 2°C per 1,000 ft
- Interpolation: find rate of change, then solve for altitude where temp = 0°C
- PIREPs provide real-time temperature data more accurate than forecasts
Memory trick: Find the actual rate, then ride it down to zero degrees.