From Glossary of Meteorology
A formula for the calculation of the approximate height of the lifting condensation level.
Therefore, it is employed to estimate the height of the base of convective clouds, under suitable atmospheric and topographic conditions. In a practical but simplified form, it may be stated
where H is the height above the surface in km and T and Td are the temperature and dewpoint in degrees Celsius. This is derived from assuming that the temperature decreases 9.8°C and the dewpoint decreases 1.8°C for each km increase in altitude, values which are approximately correct for a well-mixed boundary layer. The convective-cloud height diagram is a graphic form of this relationship.