Weather FIELD NOTES · 4 MIN READ
Cloud Base Explained: Two Thermometer Readings, One Height
The 2.5°C-per-1000-ft spread rule that turns a temperature and a dew point into an estimated cumulus cloud base — plus when it predicts fog instead.
Before Doppler radar and soundings, glider pilots and forecasters estimated the height of the afternoon's cumulus with two numbers read off a sling psychrometer: air temperature and dew point. The arithmetic is a single multiplication — and it still works well enough to plan a hike, a flight or a photo session around where the clouds will sit.
The spread and why it closes
The dew point is the temperature at which air becomes saturated. As a parcel of surface air rises, pressure drops, the parcel expands and cools at the dry adiabatic rate — about 3°C per 1000 ft. Meanwhile its dew point falls too, but slowly: mixing with drier air aloft and slight condensation lower it by roughly 0.5°C per 1000 ft. The gap between temperature and dew point — meteorologists call it the spread — therefore closes at about 2.5°C per 1000 ft of ascent.
The moment the rising parcel cools down to its dew point, the vapor it carries begins to condense. That height is the lifting condensation level — the flat, tidy base of the day's cumulus. Flip the logic around and you get the field rule: divide the surface spread by 2.5°C per 1000 ft, or equivalently multiply it by 400 ft per degree. Ten degrees of spread puts the base near 4,000 ft above the ground; twenty degrees pushes it to 8,000 ft.
The one-line formula
Cloud base (ft) ≈ 400 × (T − Td), with both temperatures in °C. In metric dress it is about 122 m per degree of spread, so 25°C over a 15°C dew point gives (25 − 15) × 122 ≈ 1,220 m. The two scales describe the same rule — 400 ft is 121.92 m — so the calculator reports both and lets you check one against the other.
A tidy corollary falls out of the construction: the air temperature at cloud base equals the surface dew point. The parcel cools until it meets its own dew point, so where they intersect, temperature and dew point are the same number. If you know the surface dew point, you know the temperature the cloud base sits at.
When the sky starts at your boots
Run the formula with a spread of zero — dew point equal to air temperature — and the "cloud base" is the ground. That is not a glitch; it is fog. Saturation at the surface means condensation is already happening where you stand. A negative spread (dew point above air temperature) is rare and usually signals supersaturated data or an instrument error, but it points the same way: moisture at ground level, visibility near zero. A calculator that understands the rule returns a fog note instead of a negative altitude.
How good is the estimate?
Honest answer: good enough for planning, not for instrument flying. Against a full thermodynamic lifting-condensation-level computation the 400-ft rule typically lands within 10–20% when the afternoon boundary layer is well mixed — sunny land, light winds, air mass relatively uniform. It drifts when the real atmosphere refuses to be average: inversions, cold-advection days, terrain-forced mixing, or a humidity field that varies wildly between the tarmac and the next ridge. Aviation weather reports (METAR ceilings) and soundings remain the authority when precision matters.
It also describes convective clouds — the fair-weather cumulus born of surface heating. Layer clouds formed by large-scale frontal lift, or orographic clouds piled against a windward slope, follow different dynamics and will not obey a surface-spread rule.
Making the number useful
Pilots read the estimate against pattern altitude and terrain clearance. Photographers use it to predict whether a ridge will wear a cloud hat by afternoon. Gardeners and sailors use the fog branch as a dawn warning. In every case the inputs are the same two cheap measurements — temperature and dew point from any home weather station or airport METAR — and the arithmetic is small enough to do in your head once you have internalized "400 feet per degree."
Try it yourself
The Cloud Base Calculator runs the spread rule in both metric and imperial, echoes the spread in °C and °F, reports the temperature at cloud base, and switches to an explicit fog note when the spread collapses. To time when the day's heating peaks, the Sunrise & Sunset Calculator gives you the solar window; for the cold-air side of the same "feels like" family, the Wind Chill Calculator is the winter counterpart.