Rising air cools at about 9.8 °C per kilometre while its dew point drops by only about 1.8 °C per kilometre. Where the two meet, water vapour condenses – the flat base of fair-weather cumulus clouds.
How to use the Cloud Base Calculator
- Surface temperature
- Dew point
- Elevation of the station
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the surface temperature and dew point, and the station elevation above sea level.
Inputs explained
| Field | What to enter |
|---|---|
| Surface temperature | Units: °C, °F, K |
| Dew point | Units: °C, °F, K |
| Elevation of the station | Units: m, ft |
What each result means
- Cloud base above ground = ([Surface temperature] – [Dew point])÷8×1000
- Cloud base above sea level = ([Surface temperature] – [Dew point])÷8×1000 + [Elevation of the station]
- Temperature at cloud base = [Surface temperature] – 0.0098×([Surface temperature] – [Dew point])÷8×1000
Example values
How the main result changes when Surface temperature changes and everything else stays at the example values above:
| Surface temperature (°C) | Cloud base above ground (m) |
|---|---|
| 12.5 | -312.5 |
| 18.75 | 468.75 |
| 25 | 1,250 |
| 31.25 | 2,031.25 |
| 37.5 | 2,812.5 |
| 50 | 4,375 |
Formula
h ≈ (T − T_d) / 8 °C per km ≈ 125 m × (T − T_d) · T_base ≈ T − 9.8 °C/km × h
Worked example
Inputs — Surface temperature: 25 °C; Dew point: 15 °C; Elevation of the station: 200 m.
Results — Cloud base above ground = 1,250 m; Cloud base above sea level = 1,450 m; Temperature at cloud base = 12.75 °C.
Key terms
- Lifting condensation level
- Height where rising air becomes saturated.
Frequently asked questions
Why does the spread between T and dew point matter?
It tells how far air must rise to saturate.
How accurate is the estimate?
Good for cumulus on convective days; not for layered or frontal clouds.
Why 8 °C per km?
The dry lapse rate (9.8) minus the dew point lapse (≈ 1.8).
Do pilots use this?
Yes – it is a standard rule of thumb.