Cloud Base Estimator (Temperature-Dew Point Spread)
The glider pilot's rule made exact: cumulus base ≈ spread × 400 ft — estimate convective cloud base from surface temperature and dew point.
The physics: rising air cools at 3°C/1000 ft (dry adiabatic) while its dew point falls only ~0.5°C/1000 ft — they converge at 2.5°C per thousand feet, and where they meet, cloud. Works for convective (cumulus) bases only; frontal stratus ignores it.
Formula
⚠️ Derived-metric estimates for education and planning — for warnings and operational decisions use official forecasts (NWS/IMD/your national service).
The glider pilot's rule made exact: cumulus base ≈ spread × 400 ft — estimate convective cloud base from surface temperature and dew point.
About Cloud Base Estimator (Temperature-Dew Point Spread)
Every summer cumulus flattens its base at the same altitude across the whole sky — as if drawn with a ruler. The ruler is the lifted condensation level: rising thermals cool at 3°C per thousand feet while their dew point sags at only 0.5, so the two converge at 2.5°C per thousand feet of climb, and the cloud marks the meeting. This calculator runs that convergence from surface temperature and dew point, in AGL and MSL, the way glider pilots, drone operators and skydivers have eyeballed it for decades.
How to use Cloud Base Estimator (Temperature-Dew Point Spread)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula cloud base (ft AGL) ≈ (T − Td in °C) / 2.5 × 1000 — equivalently spread°F × 400; the LCL approximation substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Cloud Base Estimator (Temperature-Dew Point Spread)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula cloud base (ft AGL) ≈ (T − Td in °C) / 2.5 × 1000 — equivalently spread°F × 400; the LCL approximation with sources cited on the page
- ✓The physics: rising air cools at 3°C/1000 ft (dry adiabatic) while its dew point falls only ~0.5°C/1000 ft — they converge at 2.5°C per thousand feet, and where they meet, cloud. Works for convective (cumulus) bases only; frontal stratus ignores it.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why do all the cumulus bases sit at the same height?+
Because every thermal leaving the same airmass carries roughly the same dew point and starts from roughly the same surface temperature — so each one's condensation altitude computes identically. The flat-bottomed cloud street IS the formula made visible. When bases vary across the sky, the surface conditions vary (lake breeze boundaries, irrigated fields lowering local spread) — reading base differences is reading the surface moisture map.
How accurate is the ×400-feet rule?+
Within a few hundred feet for honest convective conditions — the dry-adiabatic and dew-point lapse rates it assumes are physical constants, and the main errors come from unrepresentative surface readings (a METAR sensor over wet grass vs the sun-baked field thermals actually leave from) and entrainment of drier air. It fails entirely for non-convective cloud: stratus from frontal lifting or upslope flow forms wherever the lifting happens, not at the LCL.
Why does cloud base rise through the afternoon?+
Daytime heating raises temperature while dew point stays nearly constant (or drops as mixing brings down drier air): the spread widens, and ×400 turns each added degree into 400 more feet of base. A 7 AM spread of 4°F (1600 ft base) often becomes an 18°F spread (7200 ft) by 4 PM — which is why glider pilots launch midday and why morning fog country becomes afternoon cumulus country. Base height through the day is a live readout of the spread.
What does cloud base mean for drone and VFR flight limits?+
Regulations stack on it: US Part 107 drone rules require staying 500 ft below cloud; basic VFR cloud clearances (500 below/1000 above/2000 horizontal in many airspaces) make a 2000-ft computed base a real ceiling on legal altitudes. Compute base here, subtract the clearance, and the usable airspace shrinks fast on humid days — exactly the planning subtraction this tool's AGL output feeds.
Related Field tools
Sunrise & Sunset Calculator
Exact rise, set, solar noon and day length for any place and date — the NOAA solar equations with the refraction fine print included.
● LiveGolden Hour & Blue Hour Calculator
Tonight's golden hour and blue hour, computed from sun elevation — the photographer's light windows with the angles that define them.
● LiveDay Length Calculator
Hours of daylight for any date and latitude, how fast it's changing, and the swing between your solstices.
● Live