Enter the surface temperature and dew point, or paste a METAR. Get the estimated cumulus cloud base in AGL and MSL, plus an approximate freezing level.
Reads the temperature/dew point group (22/09, M02/M05).
Can't exceed the temperature
Only needed for the MSL and freezing-level numbers
Estimated cumulus base (ft AGL) = temperature/dew point spread in °C ÷ 2.5 × 1000. A 22 °C afternoon with a 9 °C dew point has a 13 °C spread, so cumulus bases form near 5,200 ft AGL. The rule works for convective cloud built by surface heating; it says nothing about stratus, frontal cloud, or what the METAR is reporting right now.
A parcel of air rising from the surface cools at the dry adiabatic rate, about 3 °C per 1,000 ft, while its dew point falls much more slowly, about 0.5 °C per 1,000 ft. The two converge at roughly 2.5 °C per 1,000 ft, and where they meet, the parcel saturates and cloud forms. That gives the rule:
Worked example: temperature 22 °C, dew point 9 °C, field elevation 500 ft. Spread 13 °C, so the base is 13 × 400 = 5,200 ft AGL, or about 5,700 ft MSL.
The formula assumes the cloud you care about is being built by thermals rising off the surface into well-mixed air. That's a fair-weather cumulus afternoon. It is not:
Use the estimate to anticipate where cumulus will pop on a cross-country afternoon and whether you'll fit underneath. For the actual ceiling at an airport, read the METAR and the TAF; our TAF guide covers the forecast groups that matter.
The spread formula produces a height above the ground under the rising air. Reported ceilings in a METAR are AGL at that station; cloud bases you plan a cruise altitude around are MSL. This calculator prints both so the conversion mistake never happens at 8,500 ft over rising terrain, where a 5,200 ft AGL base over a 500 ft valley floor becomes a much closer number over a 3,000 ft plateau.
The same lapse-rate arithmetic gives a quick freezing-level estimate: the standard atmosphere cools about 2 °C per 1,000 ft, so the 0 °C level sits near surface temperature ÷ 2 × 1,000 ft above the field. A 10 °C surface puts it near 5,000 ft above field elevation. If your estimated cloud base is at or above the estimated freezing level, the cloud tops you'd climb through hold structural icing risk. Confirm against the winds and temperatures aloft forecast; real lapse rates wander a long way from standard.
Watch the spread trend, not just its value. A spread narrowing through 3 °C as the sun goes down is the classic setup for radiation fog: the temperature keeps falling after the dew point stops. If the destination METARs show a shrinking spread and light wind near dusk, plan the alternate before you need it, and give the fuel and endurance numbers a second look.
Common QuestionsNo. It estimates the base of convective cumulus formed by surface heating. Stratus, frontal cloud, marine layers, and cloud above an inversion don't follow the spread, and no formula replaces the ceiling in the METAR.
On the well-mixed cumulus afternoons it's built for, usually within several hundred feet. Treat it as planning-grade situational awareness and check it against reported ceilings, PIREPs, and the TAF.
A spread of 2 °C or less, particularly with falling evening temperatures and light wind, is the classic fog setup. Many pilots treat a narrowing spread at the destination as the trigger to line up an alternate and extra fuel.
The other thing a hot afternoon does to your flight.
Decode the forecast groups that decide whether your window is flyable.
Set your own ceiling and visibility numbers before the pressure is on.
FlightDecide reads the actual METARs and TAFs along your route for your specific flight window, scores ceilings and visibility against your personal minimums, and shows the raw reports behind every call.
Get FlightDecide on the App StoreEducational tool for flight-planning practice. It is advisory only and not a substitute for your POH, an official weather briefing, or your own judgment as pilot in command (14 CFR 91.3). Sources: FAA Aviation Weather Handbook (FAA-H-8083-28), Ch. 13; FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Ch. 12. Last reviewed: August 26, 2026.