Cloud Base from Dew Point Spread
Estimated cumulus base from surface temperature and dew point at the convergence rate you enter, above ground and sea level, with the base temperature.
The height at which a rising parcel reaches saturation — the cumulus base — from the dew-point spread, in feet or metres AGL and MSL, the temperature there, and the freezing level at a lapse rate you choose.
Example: 25 °C with a 15 °C dew point at a 1,200 ft field: a 10 °C spread at 2.5 °C per 1,000 ft puts the base 4,000 ft above the ground, 5,200 ft above sea level, at about 13 °C.
Two lines
that meet.
Why the spread predicts a cumulus base, where the 2.5 °C per 1,000 ft figure comes from, what the base temperature and freezing level are, and what kind of cloud this is not.
Two lapse rates
A parcel of air lifted from the surface cools at the dry adiabatic rate, about 3 °C per 1,000 ft (9.8 °C per km), while its dew point falls only about 0.5 °C per 1,000 ft as it expands. The gap between them closes at about 2.5 °C per 1,000 ft (4.4 °F), so the parcel saturates — and a cumulus base forms — at spread ÷ 2.5 thousand feet above the ground. That rate is a rule of thumb, not a constant; the page takes it as an input.
Base temperature and freezing level
The temperature at the base is the surface temperature less the dry adiabatic cooling over the height climbed — useful for judging icing in the cloud. The freezing level uses an environmental lapse rate you enter (the standard atmosphere’s is about 2 °C per 1,000 ft): the height at which the surface temperature has fallen to 0 °C.
What kind of cloud
This is the base of convective cloud formed by surface heating on a day when the air is lifted from the ground. Stratus under an inversion, frontal cloud and cloud advected from elsewhere do not follow the surface spread, and a METAR ceiling or a forecast is the authority for any of them.
What the number does not settle
An estimate for one moment from one surface reading; it is not a forecast and not a ceiling. A planning aid, not a substitute for the pilot’s operating handbook, the aircraft’s limitations or the rules that apply to the flight. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.