3. Cumulo-nimbus (Plates [49] to [52]). Storm cloud.
Characterized by the expanded, anvil-shaped, or disc-shaped top, cirrifying at its edges.
General Terms
Nimbus, a term applied to a cloud from which rain is falling. When the form of the cloud is visible, the term should be attached to that belonging to the cloud. It may, however, be used as a substantive alone when there is nothing to show from what sort of cloud, or combination of clouds, the rain is falling (Plates [35] and [36]).
Nimbus is either heavy stratus, massive strato-cumulus, or a combination of these with stratiform clouds above, and possibly ragged masses of fracto-cumulus below. N. either alone or after the sign of the cloud.
Fracto- is a term placed as a prefix before the name of a cloud to indicate that the cloud has ragged irregular margins, as if it had been more or less torn to pieces. It is sometimes less awkward to append the word fractus after the name of the cloud.
A convenient abbreviation would be to write F. after the name of the cloud.
Undatus, or waved, should always be added to the name of any cloud which shows the arrangement so described.