The instrument panel
Cloud Base Estimator
The temperature/dew-point convergence rule — a fair-weather cumulus base estimate from two numbers on the METAR.
ESTIMATED CUMULUS BASE
BASE · MSL
Rule of thumb for convective cloud only — spread ÷ 4.4 °F per 1,000 ft. Verify ceilings with the METAR/TAF.
For training and reference · verify with official sources and your POH before flight · no tool on this site renders a go / no-go judgment
Where the rule comes from
Unsaturated rising air cools at the dry adiabatic rate, about 5.4 °F per 1,000 ft. Its dew point drops much more slowly, about 1 °F per 1,000 ft. The gap closes at roughly 4.4 °F per 1,000 ft, so a 22° spread meets at about 5,000 ft above the field. When the numbers converge, rising thermals condense — that flat cumulus deck gliders and afternoon bumps both come from.
A wide spread also means dry air — which raises density altitude less than a humid day at the same temperature. The two tools read the same METAR line.
Frequently asked questions
How does the temperature/dew-point spread give a cloud base?
Rising air cools at about 5.4 °F per 1,000 ft while its dew point falls about 1 °F per 1,000 ft — the two converge at roughly 4.4 °F per 1,000 ft. Divide the surface spread by 4.4 (or 2.5 in °C) and you get the height, in thousands of feet, where rising air saturates: the base of fair-weather cumulus.
How accurate is the estimate?
It is a rule of thumb for convective (cumulus) cloud, and it assumes the air actually rises. It says nothing about stratus layers, frontal cloud, or bases far from your station — the METAR, TAF, and graphical forecasts are the authorities. Treat this as a cross-check, never a ceiling forecast.
Why does the answer come out AGL?
The convergence starts from surface temperature and dew point, so the estimate is height above the ground where those were measured. Add field elevation for the MSL figure — that is what this calculator's MSL line does.