thumb|Cumulus mediocris cloud, about to turn into a cumulus congestus With highly unstable atmospheric conditions, large cumulus may continue to grow into even more strongly convective cumulonimbus calvus (essentially a very tall congestus cloud that produces thunder), then ultimately into the species capillatus when supercooled water droplets at the top of the cloud turn into ice crystals giving it a cirriform appearance.
thumb|Cumulus mediocris cloud, about to turn into a cumulus congestus With highly unstable atmospheric conditions large cumulus may continue to grow into even more strongly convective cumulonimbus calvus (essentially a very tall congestus cloud that produces thunder), then ultimately into the species capillatus when supercooled water droplets at the top of the cloud turn into ice crystals giving it a cirriform appearance.