Cloud Base Calculator
Cumulus cloud base is roughly 400 ft above ground for every 1 °C between temperature and dew point, so a 10 °C spread gives a base near 4,000 ft AGL.
For general informational use. Verify critical aviation calculations and weather with an authoritative source.
Temperature–dew point spread and cloud base
Rising air cools at about 3 °C per 1,000 ft until it saturates, while the dew point falls only about 0.5 °C per 1,000 ft. The gap between them closes by about 2.5 °C per 1,000 ft, so each 1 °C of spread at the surface is worth roughly 400 ft (about 122 m) of cloud base.
| Surface spread | Estimated base above ground | In metres |
|---|---|---|
| 1 °C | 400 ft | 120 m |
| 2 °C | 800 ft | 240 m |
| 3 °C | 1,200 ft | 370 m |
| 5 °C | 2,000 ft | 610 m |
| 8 °C | 3,200 ft | 980 m |
| 10 °C | 4,000 ft | 1,220 m |
When the estimate works, and when it does not
- Works best for cumulus forming from surface heating on a sunny day, with air mixed from the ground up.
- Unreliable for layer cloud brought in by a front, for sea fog moving over the coast, at night under an inversion, or when the report is from a station far from where you are flying.
- The result is above the ground at the reporting station. Add the station elevation to compare it with your cruising altitude.
A small spread is a warning
A spread of 2 °C or less, especially in the evening as the air cools, often means mist or fog is on the way. Pilots watch the trend in successive METARs: a spread that is shrinking hour by hour matters more than any single value. Decode the latest reports with the METAR and TAF Decoder.
Always prefer the reported ceiling
If a METAR reports cloud, use those heights. The spread method is for estimating where no observation exists, or for checking whether the forecast cloud base looks plausible.
Why the dew point spread predicts cloud base
Rising air cools at about 3 °C per 1,000 ft while its dew point falls more slowly, at about 0.5 °C per 1,000 ft. They meet at roughly 2.5 °C per 1,000 ft, and that is where the water vapour condenses into cloud.
How to use it
- Enter the surface temperature and dew point from a METAR or weather station.
- Choose °C or °F.
- Optionally enter field elevation to see the base above sea level too.
- Read the estimated base above ground level.
Method and calculation
Cloud base (ft AGL) ≈ (temperature − dew point) ÷ 2.5 × 1,000 in °C, or ÷ 4.4 × 1,000 in °F. Adding field elevation gives an approximate base above mean sea level.
Worked example and practical notes
Temperature 24 °C and dew point 12 °C give a 12 °C spread: 12 ÷ 2.5 × 1,000 = about 4,800 ft above ground. At a field elevation of 600 ft the base is about 5,400 ft above sea level.
Accuracy and practical tips
The rule works for cumulus formed by surface heating on a well-mixed day. It does not predict stratus, fog, frontal cloud or cloud that has drifted in from elsewhere. Always compare with the reported ceiling in the METAR.
Standard meteorology rule of thumb taught in pilot training. Use official observations and forecasts for flight decisions.
Last reviewed: October 2026
Read the guide: How to Read a METAR and TAF: A Line-by-Line Guide
Frequently asked questions
How accurate is the estimate?
Usually within a few hundred feet for fair-weather cumulus, but it can be far off for other cloud types.
Why 2.5 °C per 1,000 ft?
It is the difference between the dry adiabatic lapse rate (about 3 °C) and the dew point lapse rate (about 0.5 °C).
Does it give height above sea level?
It gives height above the ground. Add field elevation for an approximate MSL height.
What does a small spread mean?
A spread of 2–3 °C or less warns of low cloud, mist or fog.