Cloud Base Calculator — Lifted Condensation Level
Estimate the altitude of a cumulus cloud base (the lifted condensation level) from surface air temperature and dew point temperature.
°C
°C
≈ 1,220 m • spread: 10°C
- 1
Temperature – dew point spread
T − T_d = 20 − 10 = 10 - 2
Cloud base altitude
spread × 400 = 10 × 400 = 4,000The 400-ft rule: for every 1 °C of spread, the cloud base rises ~400 ft (122 m).
How does this calculator work?
Cloud base altitude ≈ (air temperature − dew point) × 400 ft, or × 122 m, where both temperatures are in Celsius. This "400-foot rule" estimates the lifted condensation level — where rising air cools to its dew point and cumulus cloud begins to form. Works best for convective thermals over flat terrain.
Formula
How this is calculated
When a parcel of unsaturated air rises, it cools at the dry adiabatic lapse rate — approximately 9.8 °C per 1000 m. Its dew point decreases at a slower rate of about 1.8 °C per 1000 m. These two temperatures converge at the altitude where the air becomes saturated: the lifted condensation level (LCL). This is where condensation begins and a cumulus cloud base forms.
The standard "400-foot rule" is a linear approximation that follows from the ratio of those lapse rates. For every 1 °C of temperature–dew-point spread (T − T_d) at the surface, the cloud base is approximately 400 ft (122 m) higher. A spread of 10 °C gives roughly 4000 ft / 1220 m; a spread of 20 °C gives about 8000 ft / 2440 m.
The formula works best for convective conditions — afternoon thermals over relatively flat terrain with a well-mixed boundary layer. It underestimates cloud base in very dry or elevated locations, and it does not apply to stratus, fog, or frontal cloud, which form through different mechanisms such as radiative cooling, advection, or lifting along a front. Temperature and dew point should be the standard 2-metre surface values, and the result is height above ground level (AGL), not above sea level.
Frequently asked questions
It is simply the surface air temperature minus the surface dew point, both in the same unit. A spread of 0 °C means the air is already saturated (fog or cloud at ground level). The larger the spread, the higher the cloud base and the less likely cloud formation.
The dew point is the temperature at which air becomes saturated. When rising air cools to its dew point, water vapour condenses into cloud droplets. The altitude where this happens — the LCL — is the cloud base, and it depends on how far the air temperature starts from the dew point.
This calculator uses Celsius. If your values are in °F, convert first: °C = (°F − 32) ÷ 1.8. Alternatively, the Fahrenheit equivalent of the rule is cloud base (ft) = (T_F − T_d_F) × 222, which gives the same result.
TG we-Calculate Editorial Team. (2026). Cloud Base Calculator — Lifted Condensation Level [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/cloud-base-calculator
TG we-Calculate Editorial Team. "Cloud Base Calculator — Lifted Condensation Level." TG we-Calculate. 2026. https://we-calculate.com/calculator/cloud-base-calculator.
TG we-Calculate Editorial Team, "Cloud Base Calculator — Lifted Condensation Level," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/cloud-base-calculator
@misc{wecalculate_cloud_base_calculator, title = {Cloud Base Calculator — Lifted Condensation Level}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/cloud-base-calculator}}, year = {2026}, note = {TG we-Calculate} }
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