How this instrument works
Cloud base height, for the ordinary fair-weather cumulus that grows over sun-warmed ground, is set by a race between two cooling rates. A rising bubble of unsaturated air cools at close to the dry adiabatic lapse rate, about 3°C per 1,000 feet, because it does work expanding into lower pressure as it climbs. Its dew point falls too, but far more gently — roughly 0.5°C per 1,000 feet — since the parcel's actual water-vapor content barely changes on the way up. The gap between temperature and dew point closes at about 2.5°C for every 1,000 feet of climb, and the cloud forms exactly where that gap reaches zero, where the still-rising air finally saturates.
That 2.5°C-per-1,000-ft convergence rate is why the formula reads height = (T − Td) ⁄ 2.5 × 1000: divide the surface spread by the rate at which it closes, and the quotient scaled by 1,000 is the altitude, in feet, where it reaches zero. The relationship traces back to the nineteenth-century convective-cloud work of James Espy and survives today as flight-school shorthand — 'about 400 feet of clear air for every degree Celsius of spread,' the same arithmetic written the other way round, so a pilot can size up a cumulus ceiling from a METAR without touching a calculator.
The estimate only holds for cloud built by surface heating rising into unsaturated air below; it says nothing about frontal stratus, orographic cloud pushed up a slope, or fog that has already settled on the ground. A temperature inversion aloft can also cap the rising air and stop convection long before the spread ever closes. It further assumes the textbook lapse rates hold steadily with height, which a real morning sounding rarely does exactly — read the output as a forecaster's pencil estimate for planning, not a figure good to the foot.
- Enter the air temperature at the ground in the Surface temperature field; switch its unit menu to °F if that is what your station reports.
- Enter the dew point in the Dew point field — any unit is fine, since the instrument converts both readings to Celsius before subtracting.
- Keep temperature at or above dew point; a value below it trips the built-in check, because dew point physically cannot exceed the air temperature.
- Read Estimated cloud base, ft AGL — the height above the ground itself, not above sea level, where the rising air is expected to saturate.
Worked example — 25°C surface, 15°C dew point
Take a warm afternoon with a surface temperature of 25°C and a dew point of 15°C, a comfortable 10-degree spread. The instrument computes (25 − 15) ⁄ 2.5 × 1000 = 4,000 ft AGL, the altitude at which a thermal rising off sun-warmed ground should saturate into the flat gray bottom of a fair-weather cumulus.
Narrow that spread to zero — temperature equal to dew point — and the same arithmetic returns 0 ft, correctly flagging fog sitting right on the ground rather than a cloud with any base overhead. Widen it to 20 degrees instead, common on a dry inland afternoon, and the base climbs to 8,000 ft; the relationship is linear, so every extra degree of spread buys roughly 400 more feet of clear air beneath the deck.
Questions
Why divide the spread by 2.5 instead of some other number?
Because 2.5°C is how much the temperature/dew-point spread closes for every 1,000 feet a parcel rises: dry air cools near 3°C per 1,000 ft while its dew point drops only about 0.5°C per 1,000 ft, and the difference between those two rates is 2.5°C per 1,000 ft. Dividing the surface spread by that convergence rate, then scaling by 1,000, finds the altitude where the gap closes to zero.
Does this rule work for any kind of cloud?
No — it estimates the base of fair-weather cumulus formed by surface heating rising into unsaturated air. Frontal stratus, orographic cloud forced up a mountainside, and fog that has already settled on the ground follow different physics, and a temperature inversion aloft can cap convection long before the calculated spread ever closes.
What does a result of 0 feet mean?
That the surface temperature and dew point are equal — the air is already saturated at ground level, which describes fog rather than a cloud with a base overhead. Entering 20°C for both fields returns exactly 0 ft AGL, correctly describing a foggy morning instead of a low cumulus deck.
Can I enter Fahrenheit instead of Celsius?
Yes. Both the Surface temperature and Dew point fields carry a unit menu offering °F alongside °C; pick whichever your station or METAR reports and the instrument converts internally before subtracting, so the readout is always in feet AGL regardless of which unit you typed.
How accurate is this compared to a real weather balloon sounding?
It is a rule of thumb, not a substitute for a radiosonde. It assumes idealized, uniform lapse rates for temperature and dew point, which real atmospheres only approximate; an inversion, strong subsidence, or a moist layer aloft can shift the true base well away from the pencil-and-paper answer. Treat it as a planning estimate, typically good to within a few hundred feet on an ordinary convective day.
Why is the answer given in feet above the ground rather than sea level?
Because the formula measures how far the surface air itself must rise before it saturates, and that climb starts at the ground where the temperature and dew point were measured, not at sea level. To compare against airport or sectional-chart altitudes given above mean sea level, add the field elevation to the Estimated cloud base, ft AGL reading.