Cloud Base Height Calculator (Temperature–Dew Point Spread)
Estimate the height of the cumulus cloud base from the surface temperature and dew point using the dry adiabatic lapse rate spread rule (≈ 400 ft or 125 m per °C), the freezing level, convective condensation height and how the spread changes through the day.
How it works
Base (ft AGL) ≈ (T − Td in °C) ÷ 2.5 × 1,000 ≈ spread × 400 ft; base (m) ≈ spread × 125; (°F: spread ÷ 4.4 × 1,000 ft)
A rising parcel of air cools at the dry adiabatic rate of about 9.8 °C per km while its dew point falls only about 1.8 °C per km, so the two converge at roughly 8 °C per km — one degree of spread per 125 m, or about 400 ft. Where they meet, the parcel is saturated and a cumulus cloud forms; this lifting condensation level is what glider pilots, hikers and VFR pilots estimate from the morning METAR. The estimate is for convective (cumulus) cloud and assumes the surface air is what rises; it does not describe frontal stratus or cloud advected from elsewhere, and dew point can change through the day, but it usually lands within a few hundred feet.
Worked example
With 24 °C and a 12 °C dew point the spread is 12 °C, so cumulus bases should sit about 4,800 ft (1,500 m) above the ground — 5,800 ft MSL at a 300 m station — with a relative humidity near 47%. If the afternoon warms to 29 °C the base lifts to about 7,000 ft, while the freezing level at 3,700 m stays well above it.
Frequently asked questions
Why is the reported ceiling different from this estimate?
The METAR ceiling may be stratus, cloud from a front or an inversion, or measured over a wetter/drier spot than the sensor. The spread rule predicts where convection from the surface will condense, not every cloud layer.
Does the rule work in Fahrenheit?
Yes: divide the spread in °F by 4.4 and multiply by 1,000 ft. A 20 °F spread ≈ 4,500 ft.
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Planning aid using standard flight-training rules of thumb and ISA atmosphere approximations — not a substitute for the aircraft flight manual (AFM/POH) performance charts, ATC instructions or an approved flight computer. Always apply the manufacturer's data and required safety margins.