The lifting condensation level (LCL) is the height at which rising air cools to its dew point and condensation begins. It is calculated from surface temperature and humidity: LCL ≈ 125 × (T − Td) metres, where Td is the dew point.
The LCL marks the base of cumulus clouds: this is why all cumulus on a given day have bases at the same altitude (they are the visual manifestation of the LCL). A low LCL (500–1,000 m) indicates high moisture; a high one (3,000+ m) indicates dry air.