FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesHard
The surface temperature at an airport is 24°C with a dew point of 12°C. Using the standard convective cloud base estimation rule, approximately what height AGL should a pilot expect the base of cumulus clouds to form?
- A3,000 feet AGL
- B2,400 feet AGL
- C4,800 feet AGL
- D6,000 feet AGL
Show answer & explanationAnswer & explanation
Correct answer: C. 4,800 feet AGL
The convective condensation level can be estimated by dividing the temperature/dew point spread by 2.5°C per 1,000 feet. Spread = 24°C − 12°C = 12°C. 12 ÷ 2.5 = 4.8, so the cloud base is approximately 4,800 feet AGL.
Why the other options are wrong
- A. Underestimates by using a rate higher than the standard 2.5°C/1,000 ft convergence rate.
- B. Results from incorrectly halving the spread before dividing by the standard rate.
- D. Overestimates by using an incorrect rate close to 2°C/1,000 ft applied incorrectly.
Convective Cloud Base Estimation
The approximate height of cumuliform cloud bases can be estimated by dividing the surface temperature/dew point spread by 2.5°C (or 4.4°F) per 1,000 feet.
- Temperature decreases ~2°C/1,000 ft (dry adiabatic lapse rate, actually 3°C/1,000ft dry, dew point decreases ~0.5°C/1,000ft)
- Convergence rate of temp and dew point is about 2.5°C per 1,000 ft (4.4°F per 1,000 ft)
- Formula: spread(°C) ÷ 2.5 × 1,000 = cloud base AGL
- Useful for predicting cumulus cloud base height for flight planning
Memory trick: Spread ÷ 2.5, times a thousand — that's where the cotton balls start.