This enormous storm cloud, which is often in the shape of a blacksmith’s anvil, can form individually or co-ordinate with neighbours to form ‘multicell’ and ‘supercell’ storms. It develops from Cumulus congestus. The cloud is defined as a Cumulonimbus once its summit has changed from droplets to ice crystals, which give the top softer edges. Below a Cumulonimbus, you will see just its dark, ragged underside, which (being so low) appears to cover the whole sky.