Enter temperature and dew point from any METAR and see the cumulus cloud base in feet AGL and MSL instantly.
Educational only — always fly your POH & current weatherTemp/dew point are in every METAR, e.g. 25/13. Add field elevation for MSL.
Estimated cumulus base above the ground for your temperature/dew point spread.
Educational estimates only — not for flight planning or dispatch.
Rising air cools at about 2.5 °C per 1,000 ft faster than its dew point falls — so the gap between temperature and dew point tells you where cloud forms. Enter the surface temperature and dew point (straight off any METAR) to estimate the cumulus cloud base in feet AGL and MSL. Pairs naturally with the density altitude calculator for a same-day performance picture.
Temperature and dew point are in every METAR (e.g. 25/13 = 25 °C over a 13 °C dew point). Add your field elevation to get the base in MSL too. This rule estimates fair-weather cumulus bases; verify ceilings with METARs, TAFs, and the Graphical Forecasts for Aviation (GFA).
Cloud base (ft AGL) ≈ (temperature − dew point in °C) ÷ 2.5 × 1,000 — or, working in °F, (temperature − dew point in °F) ÷ 4.4 × 1,000. Both are approximations of the same rule, so they differ by about 2%. It works because unsaturated rising air cools at the dry adiabatic lapse rate (~3 °C per 1,000 ft) while its dew point falls only ~0.5 °C per 1,000 ft; the two converge at roughly 2.4–2.5 °C (4.4 °F) per 1,000 ft, and where they meet, water vapor condenses into cloud.
Worked example: METAR reads 25/13. Spread = 12 °C. Base ≈ 12 ÷ 2.5 × 1,000 = 4,800 ft AGL. Over a 1,200 ft field elevation, that's a base near 6,000 ft MSL — matching how ATIS and METARs report ceilings AGL while your altimeter reads MSL.
The rule only fits convective (cumulus) cloud in well-mixed daytime air. Stratus, frontal lifting, upslope cloud, and nighttime inversions don't obey it — that's why a summer afternoon forecast works beautifully and a marine-layer morning doesn't.
The same OAT that sets your cloud base feeds your takeoff numbers on a hot day. See how to calculate density altitude.
Divide the temperature/dew point spread in °C by 2.5 (or the spread in °F by 4.4) and multiply by 1,000 to get the estimated cumulus base in feet above ground — both are approximations of the same ~2.4 °C (4.4 °F) per 1,000 ft convergence. A 10 °C spread puts the base near 4,000 ft AGL. Add field elevation for the MSL value.
The formula gives height above the ground (AGL), because the spread is measured at the surface. Add the field elevation to convert to MSL — the number that matches your altimeter. METAR and ATIS ceilings are reported AGL.
The rule assumes convectively rising surface air, so it predicts fair-weather cumulus bases. If cloud formed by frontal lifting, stratus, or an inversion sits overhead, the real ceiling can be far from the estimate. Treat it as a planning cross-check, never a substitute for reported ceilings.
Temperature at the dew point means saturated surface air — fog or a ceiling at or near the surface. Watch converging temperature/dew point pairs at dusk: a closing spread is a classic fog setup.
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