How is cloud base estimated?

It uses the lifted condensation level rule of thumb: divide the temperature minus dew point spread in degrees Celsius by 2.5, then multiply by 1,000 ft. A 10-degree spread gives a base near 4,000 ft above the ground. Adding field elevation gives the height above sea level.

Why 2.5 degrees per 1,000 ft?

As a parcel of air rises it cools at about 3 degrees Celsius per 1,000 ft, while its dew point falls more slowly at about 0.5 degrees per 1,000 ft. The spread therefore closes at roughly 2.5 degrees per 1,000 ft, and cloud forms where the two meet.

Which clouds does this predict?

Only convective cumulus cloud, the puffy fair-weather and shower clouds formed by surface heating and lifting. Stratus, frontal, and orographic cloud form by different processes such as advection or large-scale ascent and will not match this estimate.

Is AGL or MSL more useful?

Pilots usually plan ceilings in feet above ground level, which is what this rule produces directly. The above-sea-level figure helps when comparing to charted obstacle and terrain elevations. Enter the field elevation to get both.

It is a quick estimate, typically within a few hundred to a thousand feet for well-mixed afternoon convection. Strong inversions, advection, and moisture layers aloft can shift the real base. Always cross-check with an actual METAR ceiling and a current forecast.

Cloud Base Calculator

Before a VFR flight or a soaring task, a fast estimate of cloud base from simple surface readings is invaluable. This calculator applies the lifted condensation level rule to turn a surface temperature and dew point into an estimated convective cloud base in feet above the ground and above sea level.

How it works

A rising air parcel cools at about 3 degrees Celsius per 1,000 ft while its dew point falls only about 0.5 degrees per 1,000 ft, so the temperature and dew point spread closes at roughly 2.5 degrees per 1,000 ft. Cloud forms where they meet — the lifted condensation level:

cloud base AGL (ft) = (temperature - dew point in C) / 2.5 x 1000

Adding the field elevation converts the above-ground figure to an above-sea-level height for comparison with charted terrain.

Worked example

With a surface temperature of 20 degrees Celsius and a dew point of 12, the spread is 8 degrees. Dividing by 2.5 gives 3.2, so the estimated convective base is about 3,200 ft above the ground — a typical fair-weather cumulus day.

Notes

This rule predicts convective cumulus only. Stratus, frontal, and orographic cloud form by other mechanisms and will not match. If the dew point equals or exceeds the temperature the air is already saturated, meaning fog or surface cloud. Always confirm with a real METAR ceiling and the current forecast.