Cloud Base Height Calculator
Estimate the height of fair-weather cumulus cloud bases from the gap between the surface air temperature and the dew point. A wider temperature/dew-point spread means drier air and higher cloud bases; a narrow spread means humid air and low cloud.
Enter Values
How to use this calculator
- Enter the current surface air temperature in °C (it can be negative on a cold day).
- Enter the dew point temperature in °C — it must be below the air temperature.
- Read the estimated cloud base in both metres and feet above ground level (AGL), plus the spread the estimate is based on.
How it works
Rising air cools until it reaches saturation and cloud forms — the lifting condensation level. A well-known convective rule of thumb places that level about 125 m above ground for every 1 °C of difference between the air temperature and dew point, so cloud base (m AGL) = 125 × (T − Td). Dividing the metres by 0.3048 converts to feet, which works out to roughly 410 ft of cloud base per °C of spread.
Worked example
Worked example. On a fair afternoon the air temperature is 25 °C and the dew point is 15 °C. The spread is 10 °C, so the estimated cloud base is 125 × 10 = 1,250 m AGL, or 1,250 / 0.3048 ≈ 4,101 ft AGL.
Common mistakes
- Entering a dew point equal to or above the air temperature — the spread must be positive, and a zero spread means saturation at the surface (fog), not a lifted cloud base.
- Applying it to stratus, fog, frontal or orographic cloud — the 125 m/°C rule is only for convective (thermal-driven) cumulus.
- Treating the result as an exact or aviation-grade figure — real cloud bases shift with the actual lapse rate, humidity and terrain.
Frequently asked questions
Why 125 metres per degree?
It comes from the difference between the dry adiabatic lapse rate (how fast rising unsaturated air cools) and the rate at which the dew point falls with height. The two converge at roughly 125 m per °C of surface spread, which is where saturation and cloud formation occur.
Is this accurate enough for flying?
No. It is a rough educational estimate for convective cumulus only. For aviation always use official aerodrome forecasts (TAF), METARs and observed ceilings rather than a rule-of-thumb calculation.
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Tip: Enter any known values to calculate the remaining results.
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