Work out cloud base height instantly with clear inputs, formula shown and shareable results.
Rising air cools at 9.8 C per kilometre while its dew point falls only 1.8 C per kilometre, so the two converge at 8 C per kilometre - which gives a cloud base of 125 m per degree of surface spread. A 10 degree spread puts the cumulus base near 1250 m, or 4100 feet, with a temperature there around 11.8 C.
Cloud base
height(m) = (temperature - dew point) x 125
Convergence rate
dry adiabatic 9.8 C/km minus dew point lapse 1.8 C/km = 8 C/km
Only for convective cumulus formed by surface heating. Layered stratus and frontal cloud form by other mechanisms at other heights.
It is the same rule in imperial units - 125 m per degree Celsius works out at about 227 feet per degree Fahrenheit, often rounded in practice.