Cloud Base Calculator
Estimate the convective cloud base from the temperature/dewpoint spread.
Inputs
Result
Cloud base (AGL)
4,000ft
Spread 10 °C × 400 ft
Cloud base (MSL)
Includes 500 ft field elevation
4,500ft
Temp / dewpoint spread
10°C
Cloud base ≈ 4,000 ft AGL (4,500 ft MSL), spread 10 °C
Formula
Ref- Spread = temperature − dewpoint [°C]
- Base (ft AGL) ≈ spread × 400
- Base (ft MSL) = base AGL + field elevation
Worked example
RefSurface temp 25 °C, dewpoint 15 °C, field elevation 500 ft.
- 1.Spread = 25 − 15 = 10 °C.
- 2.Base AGL ≈ 10 × 400 = 4,000 ft.
- 3.MSL = 4,000 + 500 = 4,500 ft.
Cloud base ≈ 4,000 ft AGL (4,500 ft MSL).
How it works
RefRising air cools at the dry adiabatic rate while its dewpoint falls more slowly; they converge at about 2.5 °C per 1,000 ft, so every degree of temperature/dewpoint spread lifts the convective cloud base by roughly 400 ft. It estimates cumulus bases on a thermally active day, not layered stratus.
DGCA student tip
ExamThe 400 ft-per-degree rule (from the 2.5 °C/1000 ft convergence) is worth instant recall. A small spread means a low base and possible fog; a large spread means high, well-defined cumulus.
Common mistakes
Watch- Applying the rule to stratus/layer cloud — it is for convective cumulus.
- Mixing temperature in °C with dewpoint in °F.
- Forgetting to add field elevation when an MSL base is needed.