Cloud Base Calculator

What is cloud base height (LCL)?

Cloud base height is the altitude where rising air cools to its dew point and water vapor condenses into visible cloud. In meteorology this level is called the lifting condensation level, or LCL.

What is the formula used by this calculator?

It uses the classic rule of thumb (Bradbury): cloud base ≈ 125 × (air temperature − dew point) in meters per °C of spread, or 400 × spread in feet per °C of spread (≈ 222 × spread in feet per °F). For example, 25 °C air with a 15 °C dew point gives a 10 °C spread, so the cloud base is about 1,250 m (4,101 ft).

Can I work backwards from an observed cloud base?

Yes — use the inverse tab. A cloud base of 1,250 m implies a temperature–dew point spread of about 10 °C; a cloud base of 7,200 ft implies a spread of about 31.6 °F (17.6 °C).

How accurate is the 125 m / 400 ft rule of thumb?

It is a handy approximation, not a measurement. The real cloud base depends on the full temperature and moisture profile of the air, which only a radiosonde sounding shows. Use this tool for VFR flight planning estimates and hiking; for real flight decisions always check an official weather briefing.

What does a very low cloud base mean?

A small temperature–dew point spread (for example under 2 °C) gives a cloud base below about 250 m — that often means fog, mist, or low stratus and poor visibility.

What is the difference between LCL and cloud base?

For fair-weather cumulus they are effectively the same thing: the LCL is where a rising parcel first saturates. For other cloud types, such as stratus formed by lifting a whole air mass, the observed cloud base can differ from the parcel LCL.