Cloud Base Calculator

How it works

Rising air cools at about 3 °C per 1,000 ft while its dewpoint falls at about 0.5 °C per 1,000 ft, so the gap closes at roughly 2.5 °C per 1,000 ft. Divide the surface spread by 2.5 and multiply by 1,000 to get the height where they meet — the convective condensation level, which is the base of fair-weather cumulus.

What it applies to

Convective cloud formed by surface heating. It says nothing about stratus rolling in off the sea, frontal cloud, or an overcast that was already there — those have nothing to do with today's surface spread.

The other half of the number

A small spread means the air is close to saturation. Under 2 °C with light wind and clearing skies overnight is a strong fog signal, and it is worth checking on any morning departure. See reading a METAR for where to find both values.