Linked from
The 14 pages that link to Cumulonimbus cloud, each with the reason it gives.
Atmospheric convectionBroader topic: Its vertical extent records deep atmospheric convection.
Cumulus mediocrisBroader topic: Further growth from congestus can produce this storm cloud.
Cloud (meteorology)Broader topic: Its great vertical extent makes it a distinctive cloud form.
Vertical draftBroader topic: Its intense updrafts build the cloud and carry water and ice upward.