Cloud Base Calculator
Estimate the height of fair-weather cumulus clouds from the surface temperature and dew point — the same 125 m/°C rule pilots use before a flight.
Two readings, one altitude
Enter the surface temperature and dew point in °C and the calculator returns the convective cloud base in both metres and feet.
The spread drives the height
The bigger the gap between temperature and dew point, the higher the cloud base. When they are equal the air is already saturated and cloud sits right at the surface — fog.
What is a cloud base calculator?
Temperature and dew point in, cloud height out
A cloud base calculator estimates the height at which rising surface air forms the bottom of a cumulus cloud — the lifted condensation level. As a parcel of warm air rises it cools at roughly 9.8 °C per kilometre, while its dew point falls more slowly. The two meet at the altitude where the air becomes saturated and water vapour condenses into a visible cloud. The simple rule of thumb, Espy's equation, multiplies the temperature-dew-point spread at the surface by about 125 metres per degree. Pilots and glider crews use it to judge where fair-weather cumulus will form and how high they can climb beneath the clouds.
Enter the surface temperature and dew point to get the cloud base in metres and feet instantly.
Espy's equation needs just the surface temperature T and dew point Td, both in °C.
cloud base (m) = 125 · (T − Td); cloud base (ft) = cloud base (m) · 3.28084The temperature-dew-point spread T − Td measures how dry the surface air is. Each degree of spread lifts the condensation level by roughly 125 metres, so a dry day with a wide spread has high cloud bases while a humid day with a narrow spread has low ones. Multiplying the metric result by 3.28084 gives the same height in feet, the unit on most altimeters.
Suppose the surface temperature is 25 °C and the dew point is 15 °C.
Find the spread
T − Td = 25 − 15 = 10 °C — how far the air is from saturation.
Apply 125 m per degree
125 × 10 = 1250 m — the cloud base above the surface.
Convert to feet
1250 × 3.28084 ≈ 4101 ft — the same height for an altimeter.
The cloud base tells you how much clear air sits below the clouds. A result of 1250 m means cumulus clouds will start forming about 1.25 km above the ground — comfortable room for a glider or a low-flying aircraft. The single most useful insight is that the spread controls the height: keep the temperature fixed and a higher dew point (more humid air) lowers the cloud base, while a drier day raises it. A very small spread, under a degree or two, signals a low ceiling or even fog, which matters for visual flight. Use the result to anticipate the day's cloud ceiling, judge soaring conditions, or simply understand why some afternoons have towering cumulus and others have flat, low cloud.
Espy's equation is a fast field estimate, but a couple of caveats are worth keeping in mind.
A rule of thumb, not a sounding
The 125 m/°C factor assumes a well-mixed boundary layer and convective (cumulus) cloud rising from the surface; it does not predict layered stratus, frontal cloud, or orographic cloud forced up a mountain. The real lapse rates vary with conditions, so the true cloud base can differ by a few hundred feet from the estimate. The height is above the surface where the readings are taken, not above sea level — add your station elevation for an absolute altitude. When the dew point equals the temperature the spread is zero and the cloud base is at the surface (fog); the calculator therefore requires the dew point to be at or below the temperature.