Main differences between Cumulonimbus and similar clouds of other genera

(Section 2.3.10.6)

When Cumulonimbus covers a large expanse of the sky, it can easily be confused with Nimbostratus, especially when identification is based solely on the appearance of the undersurface. The character of the precipitation may help to distinguish Cumulonimbus from Nimbostratus. If the precipitation is of the showery type, or if it is accompanied by lightning, thunder or hail, the cloud is Cumulonimbus.

Certain Cumulonimbus clouds appear nearly identical with Cumulus congestus. The cloud is identified as Cumulonimbus as soon as at least a part of its upper portion loses the sharpness of its outlines or presents a fibrous or striated texture. If it is not possible to decide on the basis of the above criteria whether a cloud is a Cumulonimbus or Cumulus, it is identified as Cumulonimbus if it is accompanied by lightning, thunder or hail.