Why clouds have a flat base
On a sunny day, bubbles of warm air rise from the ground and cool as they expand. Their temperature falls by about 9.8 °C per kilometer (5.4 °F per 1,000 ft), the dry adiabatic lapse rate. Their dew point falls much more slowly, about 1.8 °C per kilometer. When the two meet, water vapor condenses and a cumulus cloud forms. Because all the bubbles start from similar air, they condense at almost the same height, which gives fair-weather cumulus their flat bases.
The rule of thumb
The gap between temperature and dew point closes by about 8 °C per kilometer, so:
- Cloud base (m) ≈ 125 × (T − Td) with temperatures in °C
- Cloud base (ft) ≈ 400 × (T − Td) with temperatures in °C
- Cloud base (ft) ≈ 228 × (T − Td) with temperatures in °F, often rounded to 222 to 230
The result is the height above the ground. Add the station elevation to get the height above mean sea level.
Worked example
At an airport at sea level, the afternoon METAR reports a temperature of 25 °C (77 °F) and a dew point of 15 °C (59 °F):
- Spread = 25 − 15 = 10 °C (18 °F).
- Cloud base ≈ 10 × 125 = 1,250 m, or about 4,100 ft above the ground.
- At an airport at 1,000 ft, the same spread would put the base near 5,100 ft above sea level.
Typical results
| Spread | Approximate base |
|---|---|
| 2 °C (4 °F) | 250 m (800 ft): low clouds, possible mist |
| 5 °C (9 °F) | 625 m (2,000 ft) |
| 10 °C (18 °F) | 1,250 m (4,100 ft): typical summer cumulus |
| 15 °C (27 °F) | 1,875 m (6,200 ft) |
| 20 °C (36 °F) | 2,500 m (8,200 ft): dry air, high bases or no clouds |
Who uses it
Glider and paraglider pilots estimate how high thermals will carry them. Private pilots check whether a VFR flight can stay below the clouds with the required clearance. Hikers and photographers use it to guess whether summits will be in the clouds.
Limits
The estimate assumes a well-mixed layer near the ground, which is true on sunny afternoons but not in the early morning, at night or over cool water. It does not apply to stratus, fog, high clouds or clouds lifted by fronts and mountains. Always check the official forecast and the reported ceiling before a flight.