About the Cloud Base Calculator
This cloud base calculator estimates how high convective (cumulus) clouds will form above you. Enter the surface air temperature and dew point in °C or °F, plus your field or station elevation if you want the height above sea level, and it returns the estimated cloud base in feet and metres, the temperature at cloud base, the surface relative humidity and the common pilot rule-of-thumb figure for comparison.
It is used by glider and paraglider pilots planning thermal flying, by private pilots checking whether a VFR flight will stay clear of cloud, by drone operators, and by weather enthusiasts and students learning how the lifting condensation level works. The chart shows how quickly the base rises as the air dries out.
The estimate assumes a well-mixed lower atmosphere, where rising air parcels cool at the dry adiabatic rate until they reach saturation. It works best for daytime cumulus on sunny days with surface readings taken away from local moisture sources; it does not predict stratus, frontal cloud or fog layers, and it is no substitute for an official forecast or observation.
How to use the cloud base calculator
- 1Read the current surface temperature and dew point (a METAR or weather station works).
- 2Enter both values and choose °C or °F.
- 3Optionally enter your station or airfield elevation for the MSL height.
- 4Read the estimated cloud base above ground in feet and metres.
- 5Compare with the pilot rule of thumb and official observations before flying.
Formula and method
A rising parcel of unsaturated air cools at the dry adiabatic lapse rate of about 9.8 °C per kilometre, while its dew point falls much more slowly, about 1.8 °C per kilometre. The temperature and dew point therefore converge by roughly 8 °C per kilometre, and the parcel saturates — forming cloud — at a height of about 1,000 ÷ 8 = 125 m for every degree Celsius of surface spread. This is Espy’s approximation to the lifting condensation level.
Pilots often use the rounder rule “spread in °C ÷ 2.5 × 1,000 ft” (400 ft per °C), shown alongside for comparison. Height above sea level adds the station elevation. Surface relative humidity is computed from temperature and dew point with the Magnus formula.
- h
- Cloud base height above ground
- T
- Surface air temperature
- Td
- Surface dew point
- T_base
- Temperature at cloud base (°C)
Worked examples
Summer day: 25 °C with a 15 °C dew point
The spread is 10 °C, so the base is about 10 × 125 = 1,250 m or 4,101 ft above ground. The air cools to roughly 12.8 °C by then. The pilot rule (10 ÷ 2.5 × 1,000) gives a similar 4,000 ft.
Airfield at 1,600 ft: 80 °F, dew point 62 °F
An 18 °F spread is 10 °C, so the base is about 4,100 ft above the field. Adding the 1,600 ft elevation puts it near 5,700 ft MSL — the altitude to use against your cruise plan.
Moist morning: 10 °C with a 9 °C dew point
A 1 °C spread puts saturation only about 125 m (410 ft) up, and relative humidity is about 93.5% — low cloud, mist or fog is likely.
Frequently asked questions
How do you calculate cloud base from temperature and dew point?+
Subtract the dew point from the temperature and multiply by about 125 m (410 ft) per °C, or about 228 ft per °F. A 10 °C spread gives a cloud base near 1,250 m or 4,100 ft above ground.
What is the pilot rule of thumb for cloud base?+
A common rule is: temperature–dew point spread in °C divided by 2.5, times 1,000 ft (or the spread in °F divided by 4.4, times 1,000 ft). It gives about 400 ft per °C, slightly lower than the 410 ft physical estimate.
Why does a smaller spread mean lower clouds?+
When the temperature and dew point are close, the air is near saturation, so rising air only needs to cool a little before water vapour condenses. With a spread of zero the air is saturated at the surface, which means fog.
Does this work for all types of cloud?+
No. The method estimates the base of convective cumulus formed by surface heating in well-mixed air. Stratus, frontal cloud, cloud over mountains and layers advected from elsewhere can be at very different heights.
Is the result above ground or above sea level?+
The basic result is above ground level (AGL), because it uses surface readings. Add the station or airfield elevation to get the height above mean sea level (MSL); this calculator does that when you enter the elevation.