FAA Private Pilot Helicopter (PRH)WeatherHard
At a departure airport, the surface temperature is 24°C and the dew point is 12°C. Using the standard formula (dividing the temperature/dew point spread by 2.5 and multiplying by 1,000), what is the approximate height of the cumuliform cloud bases above the surface?
- A2,880 feet
- B3,600 feet
- C6,000 feet
- D4,800 feet
Show answer & explanationAnswer & explanation
Correct answer: D. 4,800 feet
The spread is 24°C − 12°C = 12°C. Dividing by 2.5 gives 4.8, then multiplying by 1,000 gives an estimated cloud base of 4,800 feet AGL.
Why the other options are wrong
- A. This would result from an incorrect divisor or spread calculation.
- B. This assumes a spread of 9°C, not the actual 12°C spread.
- C. This assumes a spread of 15°C, which is not the given value.
Convective Cloud Base Estimation
The approximate height of cumuliform cloud bases can be estimated by dividing the temperature/dew point spread (°C) by 2.5 and multiplying by 1,000 feet.
- Formula: (Temp − Dew Point) ÷ 2.5 × 1,000 = cloud base AGL
- Based on typical adiabatic cooling and moisture lapse rates
- Useful for estimating convective cloud bases, not stratiform layers
Memory trick: Spread divided by 2.5, times a thousand, tells you where clouds are dumped