Cloud base calculator
Enter the surface air temperature and dew point to estimate how high cumulus clouds will form. Add the station elevation to get the height above sea level as well.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Choose the units.
- Enter the current air temperature and dew point near the ground, for example from a weather report.
- Optionally enter the elevation of the place to get the cloud base above sea level, the figure used on aviation charts.
cloud base (ft above ground) = (temperature − dew point) in °C × 400
- 400 ft per °C = 1000 ft per 2.5 °C ≈ 1000 ft per 4.5 °F;
- metres = feet × 0.3048, about 122 m per °C;
- above sea level = above ground + station elevation.
Worked example
68 °F and a dew point of 54 °F (the default values): the spread is 14 °F, which is 7.778 °C. 7.778 × 400 = 3,111 ft above the ground, or 948 m.
At an airfield 1,000 ft above sea level with 75 °F and a dew point of 60 °F, the base is 3,333 ft above the ground and 4,333 ft above sea level.
Tips
- Use temperature and dew point measured at the same time and place.
- The estimate works best in the afternoon, once the sun has mixed the lowest layer of air.
- A small spread (2–3 °C, 4–5 °F) means a low base; when it is near zero, expect mist, fog or low cloud instead of cumulus.
Frequently asked questions
How does the calculator estimate the cloud base?
A rising parcel of air cools faster than its dew point falls, so the gap between them closes by about 2.5 °C every 1000 ft. The cloud forms where the gap reaches zero: base = spread ÷ 2.5 × 1000 ft, or 400 ft per °C of spread (about 122 m per °C).
What is the rule in Fahrenheit?
The same rule is about 1000 ft for every 4.5 °F of spread. Many pilot handbooks round it to 4.4 °F per 1000 ft, which gives almost the same height.
Why does a metric source say 125 m per °C?
400 ft is 121.92 m, and many metric textbooks round the rule to 125 m per degree. The difference is about 2.5 %, well inside the accuracy of the rule itself.
Does it work for every type of cloud?
It estimates the base of cumulus clouds formed by air rising from the surface on a mixed, sunny day. It does not predict stratus, fog, frontal or high clouds, which form for other reasons.