Case 1
- Surface temperature
- 25°C
- Surface dew point
- 15°C
1,250m
Open with these valuesCloud Base Calculator
1,250m
Result: 1,250 mThe flat bottom of a fair-weather cumulus sits roughly 125 metres above ground for every degree between the surface temperature and the dew point. At 25 °C with a 15 °C dew point that is 1250 m, or about 4101 ft. It is a rule of thumb for convective cloud, good to a few hundred feet.
Worked examples
Case 2
- Surface temperature
- 30°C
- Surface dew point
- 12°C
2,250m
Open with these valuesCase 3
- Surface temperature
- 22°C
- Surface dew point
- 16°C
750m
Open with these valuesHow it's calculated
cloud base (m) = 125 × (T − Td)
- StepEnter the air temperature at the surface in degrees Celsius.
- StepEnter the dew point at the surface — it may be negative.
- StepThe spread T − Td is the whole input; each degree adds 125 m.
- ResultFor feet, multiply the metres by 3.280839895.
Reference table
| Temperature, dew point | In feet | Cloud base |
|---|---|---|
| 20, 20 | 0 ft — saturated, cloud on the ground | 0 |
| 22, 16 | 2461 ft | 750 |
| -3, -10 | 2871 ft | 875 |
| 25, 15 | 4101 ft | 1250 |
| 15, -2 | 6972 ft | 2125 |
| 30, 12 | 7382 ft | 2250 |
| 28, 4 | 9843 ft | 3000 |
Questions
What is the cloud base?
The cloud base is the height at which rising surface air cools to its dew point and condenses — the flat bottom of fair-weather cumulus, also called the lifted condensation level. At 25 °C with a 15 °C dew point it sits about 1250 m above the surface.
Where does the figure of 125 metres per degree come from?
Rising air cools by about 9.8 K per kilometre while its dew point falls by only about 1.8 K per kilometre. The two curves close at roughly 8 K per kilometre, and inverting that gives 0.125 km per kelvin. James Espy pointed this out in the early nineteenth century, and the rule still carries his name.
Why does the temperature-dew-point spread matter?
The spread measures how dry the surface air is. A wide spread means the rising parcel has to climb high before it reaches saturation, giving a high cloud base; a narrow spread gives a low one. Each degree of spread is worth roughly 125 m of height.
Can I use negative temperatures?
Yes — the formula only needs the difference, so both values may be negative or zero. A surface temperature of 15 °C with a -2 °C dew point gives a 17 degree spread and a base near 2125 m. A dew point above the air temperature cannot occur, so the result is held at zero there.
How accurate is this?
It is a fast rule of thumb for convective cloud rising out of a well-mixed surface layer. Real lapse rates vary, so the true base can differ by a few hundred feet, and the method says nothing about layered stratus or frontal cloud. Treat it as a first estimate, not a substitute for a sounding.
Sources and last check
Information, not professional advice.