Cloud Base Height Calculator
Enter surface air temperature and dew point to estimate cumulus cloud base height. About 125 m per 1°C difference. Hong Kong 30°C, dew point 24°C → cloud base ≈ 750 m.
Input Data
Results
At a glance:Enter surface air temperature and dew point to estimate cumulus cloud base height. About 125 m per 1°C difference. Hong Kong 30°C, dew point 24°C → cloud base ≈ 750 m.
Formula
Cloud base (m) ≈ (T − Td) × 125.
Cloud base (ft) ≈ (T − Td) × 400.
$$H_{\text{base}} \approx \frac{T - T_d}{\Gamma_d} \times 1000 \approx (T - T_d) \times 125\;\text{m}$$How to Use
- Enter the air temperature T and dew point Td.
- The calculator returns the cloud base in meters and feet.
Cloud base reference (air temperature 30°C)
| Dew point Td (°C) | Gap (°C) | Cloud base (m) | Cloud base (ft) |
|---|---|---|---|
| 27 | 3 | 375 | 1200 |
| 24 | 6 | 750 | 2400 |
| 21 | 9 | 1125 | 3600 |
| 18 | 12 | 1500 | 4800 |
Estimated by the rule of thumb H = (T − Td) × 125 m. Smaller gap → lower cloud base.
FAQ
How is cloud base height estimated?
Using the dry adiabatic lapse rate: air cools about 9.8°C per kilometer of ascent. The larger the gap between surface air temperature and dew point, the farther it must rise to reach the condensation point. Rule of thumb: H ≈ (T − Td) × 125 m, or H ≈ (T − Td) × 400 ft.
Why is the cloud base lower when it is more humid?
High humidity means the dew point is close to the air temperature, so the air needs only a short rise to cool to the dew point and condense. Hence humid weather has a low cloud base and easily brings cloud and rain; dry weather has a high cloud base and more clear sky.
What is the relationship between cloud base and rainfall?
A low cloud base (small gap) means ample atmospheric moisture, and convective clouds easily develop into rain clouds. In Hong Kong summer the gap is often below 6°C, so afternoon thermal convection readily produces sudden showers and thunderstorms.
Does this formula apply to all cloud types?
It mainly applies to convective clouds (cumulus, cumulonimbus). Stratus and other cloud types form by different mechanisms and not necessarily by this rule. Fog can be seen as a ground-level cloud with cloud base height zero.
How does cloud base affect flying?
Cloud base sets visibility and approach minima: below 1000 ft is low-cloud weather requiring instrument flight rules (IFR); above 3000 ft visual flight rules (VFR) are safer.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.