Formula and Method for the Cloud Base Calculator
Cumulus cloud base height can be estimated from two numbers you can read off almost any weather station: the surface air temperature and the dew point. As an unsaturated air parcel rises from the surface, it cools at the dry adiabatic lapse rate while its dew point falls much more slowly, so the two values converge with altitude. The point where they meet — where the rising air becomes saturated — is the lifting condensation level (LCL), which for convective clouds is a good estimate of the visible cloud base. This calculator applies the standard rule of thumb, h = 125 × (T − Td), to estimate that height above ground and, if you provide a station elevation, above sea level.
How the 125 m/°C rule is derived
Rising unsaturated air cools at the dry adiabatic lapse rate of about 9.8°C per kilometer. Its dew point, meanwhile, falls at only about 1.8°C per kilometer, because expansion cools the parcel much faster than it dilutes its moisture content. The temperature and dew point lines therefore converge at a rate of roughly 9.8 − 1.8 ≈ 8°C per kilometer of ascent. Solving for the height where they meet gives h (km) = (T − Td) / 8, or equivalently h (meters) ≈ 125 × (T − Td) with T and Td in °C — the formula this calculator uses. In Fahrenheit and feet, the same physics works out to roughly 1,000 feet of cloud base per 4.4°F of spread.
Reading the AGL and MSL results
The AGL (above ground level) result is the height of the cloud base measured straight up from your location. Pilots, air traffic controllers, and weather reports also care about MSL (above mean sea level), which is the AGL height plus your station's elevation above sea level — enter that elevation and the calculator adds it automatically. Two nearby stations at different elevations can report very different AGL ceilings for clouds that actually sit at the same altitude MSL.
Where the estimate breaks down
This rule of thumb assumes a well-mixed, unsaturated surface layer and clouds that form by daytime convective heating — classic fair-weather cumulus. It is far less reliable for layered stratus decks, clouds forming along a front or over rising terrain, temperature inversions that trap moisture near the ground, or air that is already saturated (spread near 0°C), which produces fog or a cloud base essentially at the surface rather than the formula's predicted height.