Cumulus cloud, with sun rays. Cumulus clouds are generally deep and dark in appearance from the ground, although not rising as high as cumulonimbus clouds. Cumulus may be formed at a frontal system boundary or as a result of thermal cell activity, with a cloud base at between 300 and 1,000 metres (1,000 and 3,000 feet). The depth varies generally between 500 and 1,500 metres (1,500 feet and 5,000 feet). Cumulus clouds are normally associated with rain.