Cloud Base Calculator
Calculate cloud base instantly in your browser — the published formula, worked locally with no data sent anywhere.
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What this tool does
Calculate cloud base instantly in your browser — the published formula, worked locally with no data sent anywhere. The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
How to use the Cloud Base Calculator
- Enter or select air temperature (°c).
- Enter or select dew point (°c).
- Enter or select field / ground elevation in ft.
- Read the calculated result; change any measurement to compare alternatives.
Formula
Aviation spread rule: cloud base (ft AGL) ≈ (temperature − dew point) × 400 ft per °C of spread — rising air cools about 3 °C per 1000 ft while its dew point falls about 0.5 °C, so the two meet roughly 400 ft higher for every degree of spread.
- temp
- Air temperature (°C)
- dew
- Dew point (°C)
- elev
- Field / ground elevation (ft, before metric conversion)
The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
Worked example
For cloud base calculator, the following measurements illustrate the exact method: Air temperature (°C): 25; Dew point (°C): 13; Field / ground elevation: 0.
Inputs
- Air temperature (°C)25
- Dew point (°C)13
- Field / ground elevation0 ft
Result
- Estimated cloud base (ft AGL)4,800
- Estimated cloud base (ft MSL)4,800
- Estimated cloud base (m AGL)1,463.04
- Temperature–dew point spread (°C)12
- ConditionsScattered-to-broken cumulus bases near this height
Results explained
- Estimated cloud base (ft AGL)
- Estimated cloud base (ft AGL) from the formula above. The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
- Estimated cloud base (ft MSL)
- Estimated cloud base (ft MSL) from the formula above. The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
- Estimated cloud base (m AGL)
- Estimated cloud base (m AGL) from the formula above. The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
- Temperature–dew point spread (°C)
- Temperature–dew point spread (°C) from the formula above. The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
- Conditions
- Conditions from the formula above. The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
Frequently asked questions
Aviation spread rule: cloud base (ft AGL) ≈ (temperature − dew point) × 400 ft per °C of spread — rising air cools about 3 °C per 1000 ft while its dew point falls about 0.5 °C, so the two meet roughly 400 ft higher for every degree of spread.
The 400 ft/°C rule estimates the convective cloud base (lifting condensation level) for cumulus formed by surface heating. Layer clouds from fronts or advection can sit far from this estimate.
When the spread closes to about 1 °C or less at the surface, the air is near saturation and fog or a very low ceiling is likely — a key go/no-go check in preflight weather briefings.
Every field is labelled with its unit — enter values in exactly the labelled unit and read the result in the labelled output unit. Fields with a unit symbol also support the site-wide imperial/metric switch, which converts before the formula runs.
It uses double-precision arithmetic and the published constants and formulas named on this page. The last displayed digit may be rounded, and real conditions can differ from the idealized model described in the assumptions.
Yes. Every calculation runs entirely in your browser; nothing you enter is uploaded, stored or shared.