Cloud base calculator
Estimate the base of convective cloud from the temperature–dewpoint spread.
How it works
Rising air cools at about 3 °C per 1,000 ft while its dewpoint falls at about 0.5 °C per 1,000 ft, so the gap closes at roughly 2.5 °C per 1,000 ft. Divide the surface spread by 2.5 and multiply by 1,000 to get the height where they meet — the convective condensation level, which is the base of fair-weather cumulus.
What it applies to
Convective cloud formed by surface heating. It says nothing about stratus rolling in off the sea, frontal cloud, or an overcast that was already there — those have nothing to do with today's surface spread.
The other half of the number
A small spread means the air is close to saturation. Under 2 °C with light wind and clearing skies overnight is a strong fog signal, and it is worth checking on any morning departure. See reading a METAR for where to find both values.
Live: the lowest estimated cloud bases right now
Estimated from each station's temperature-dewpoint spread using the same 2.5 °C per 1,000 ft relationship as the calculator. A spread near zero means fog, not cloud.
Other calculators
- Density altitude calculator
- Pressure altitude calculator
- True airspeed calculator
- Crosswind component calculator
- Wind correction angle and groundspeed calculator
- Top of descent calculator
- Climb gradient calculator
- Time, speed and distance calculator
- Glide distance calculator
- Aviation unit converter
For planning and study, not for operational use. Aircraft performance comes from your POH, not from a generic formula.