The cloud base (height) is the lowest altitude of the visible portion of the cloud. It is traditionally expressed either in m or feet above mean sea level (or planetary surface), or as the corresponding pressure level in hectopascal (hPa, equivalent to millibar).

The height of the cloud base can be measured using a ceilometer or it can be estimated from surface measurements of air temperature and humidity. The calculation is based on the assumption that the air temperature drops 9.84°C per 1000 m of altitude and the dewpoint drops 1.82°C per 1000 meters altitude.