How cloud base calculator works
Rising air cools at roughly 3°C per 1000 ft (the dry adiabatic rate), while the dew point of that air drops more slowly, about 0.5°C per 1000 ft. The gap between temperature and dew point — the spread — therefore closes at about 2.5°C per 1000 ft. When the rising parcel cools to its dew point, water vapor condenses: that height is the cloud base.
The arithmetic is one line: cloud base (ft) ≈ 400 × (temperature − dew point in °C), equivalently 122 m per °C of spread. A 25°C afternoon with a 15°C dew point has a 10-degree spread, putting cumulus bases near 4,000 ft (about 1,220 m) above the ground.
Enter temperature and dew point in °C or °F. The calculator shows the spread in both scales, the estimated base in feet and meters, and the air temperature at cloud base. If the dew point meets or exceeds the temperature, the spread is zero or negative — the air is already saturated and the "cloud base" is the ground: fog.