Cloud Base Height
Calculator
Results
- Cloud base height (m)
- 1,625
- Cloud base height (ft)
- 5,331.364829
- Temperature-dew point spread (°C)
- 13
Weather results
| Cloud base height (m) | 1,625 |
| Cloud base height (ft) | 5,331.364829 |
| Temperature-dew point spread (°C) | 13 |
formula-map diagram
- Cloud base height (m)
- 1,625
- Cloud base height (ft)
- 5,331.364829
- Temperature-dew point spread (°C)
- 13
Weather relationship
Formula
Cloud base (m) = (T − Td) × 125 (Espy's rule, dry adiabatic 9.8 °C/km vs dew point 1.8 °C/km)= 1625
Note
This is a simplified model: it applies the published standard formula to the numbers you entered and ignores local terrain, radiation, precipitation type and instrument error. Wind chill is the 2001 NWS/Environment Canada regression, valid at or below 10 °C with wind at or above 4.8 km/h; the heat index is the Rothfusz regression, reliable above about 27 °C; dew point uses the Magnus approximation (a = 17.27, b = 237.7 °C) over liquid water; wet bulb uses Stull's approximation at standard sea-level pressure; pressure and density altitude assume the International Standard Atmosphere and dry air. Do not use these results for aviation, structural or safety decisions without an official forecast or a qualified professional.
More in Weather and climate
See all →Frequently asked questions
How is cloud base height estimated from surface weather readings?+
It uses the difference between surface air temperature and dew point, multiplied by a standard lapse rate constant, since air rises, cools, and eventually reaches its dew point (saturation) at a predictable rate, forming the visible cloud base at that altitude.
What is the standard lapse rate used and why is it roughly constant?+
Rising unsaturated air cools at approximately 9.8°C per kilometre (the dry adiabatic lapse rate), while its dew point drops more slowly, around 1.8°C per kilometre, as the air expands. The commonly used shortcut is that the temperature-dew point spread in °C divided by roughly 8 (or in °F divided by 4.4) gives cloud base height in kilometres (or thousands of feet).
Why does a smaller temperature-dew point spread mean lower clouds?+
A small spread means the air is already close to saturation at the surface, so it only needs to rise a short distance and cool a little before reaching its dew point, producing low cloud bases or even fog. A large spread means the air must rise much farther before condensation begins.
Does this method predict all cloud types accurately?+
It reliably predicts the base of cumulus clouds formed by simple surface heating and convection, since those clouds form exactly where rising air reaches saturation. It's much less reliable for cloud layers formed by frontal lifting, temperature inversions, or advection fog, which don't follow the same simple thermodynamic rise.
Why do pilots and forecasters care about this calculation specifically?+
Cloud base height determines the ceiling for visual flight rules and affects takeoff and landing planning, so a quick, reliable field estimate from readily available surface temperature and dew point readings is operationally valuable even when a precise instrument reading isn't available.