FAA Private Pilot Airplane (PAR)Weather Theory and ServicesHard
At an airport with a field elevation of 1,000 feet MSL, the surface temperature is 24°C and the dew point is 14°C. Using the standard rule of thumb relating temperature-dew point spread to convective cloud base height, approximately what height MSL should the pilot expect the base of any developing cumulus clouds to be?
- A6,000 feet MSL
- B5,000 feet MSL
- C3,000 feet MSL
- D4,000 feet MSL
Show answer & explanationAnswer & explanation
Correct answer: B. 5,000 feet MSL
Using the rule of thumb of dividing the temperature-dew point spread (in °C) by 2.5 and multiplying by 1,000 to estimate cloud base AGL: (24-14)/2.5 x 1,000 = 4,000 feet AGL. Adding the field elevation of 1,000 feet MSL gives an estimated cloud base of 5,000 feet MSL.
Why the other options are wrong
- A. This overestimates the cloud base by using an incorrect conversion factor.
- C. This would result from using an incorrect spread or omitting the field elevation addition.
- D. 4,000 feet is the AGL cloud base but the question asks for MSL height.
Convective Cloud Base Estimation
The height of cumulus cloud bases can be estimated using the temperature-dew point spread: divide the spread (°C) by 2.5 and multiply by 1,000 to get the height in feet AGL.
- Formula: Cloud base (ft AGL) = (Temp − Dew Point in °C) ÷ 2.5 × 1,000
- Add field elevation to convert AGL height to MSL height
- This estimate applies to convective (cumulus) cloud formation, not stratiform layers
Memory trick: 'Spread by 2.5, Cloud Base Comes Alive' — divide the spread by 2.5, times 1,000 for AGL height.