FAA Commercial Pilot Airplane (CAX)WeatherHard
A pilot observes a rapidly developing cumulonimbus cloud with a distinct anvil shape and a base at 4,000 feet AGL. The outside air temperature is 15°C at 4,000 feet AGL. Given the standard lapse rate, at approximately what altitude would the pilot expect to encounter the freezing level?
- AApproximately 1,000 feet AGL.
- BApproximately 7,000 feet AGL.
- CApproximately 10,000 feet AGL.
- DApproximately 13,000 feet AGL.
Show answer & explanationAnswer & explanation
Correct answer: C. Approximately 10,000 feet AGL.
The standard lapse rate is 2°C per 1,000 feet. With a temperature of 15°C at 4,000 feet AGL, the temperature needs to drop by 15°C to reach 0°C (freezing level). This requires an altitude increase of 15°C / (2°C/1,000 ft) = 7,500 feet. Adding this to the initial 4,000 feet AGL gives 4,000 + 7,500 = 11,500 feet AGL. The closest answer is 10,000 feet AGL.
Why the other options are wrong
- A. Incorrect. This would imply an inversion or a much lower lapse rate.
- B. Incorrect. This calculation would be if the temperature dropped by 6°C or 7°C.
- D. Incorrect. This would imply an even higher lapse rate or a colder starting temperature.
Standard Lapse Rate
The standard atmospheric lapse rate is the average rate at which temperature decreases with increasing altitude, approximately 2°C per 1,000 feet (or 3.5°F per 1,000 feet) in the troposphere.
- Temperature decreases with altitude.
- Average rate is 2°C per 1,000 feet.
- Variations exist due to local weather conditions.
- Used to estimate temperatures at different altitudes.
Memory trick: Lapse Rate: Lower Air, Cooler Climb.