The true cumulus has clear superior and inferior limits. It is often broken up by strong winds, and the detached portions undergo continual changes. These altered forms may be distinguished by the name of Fracto-cumulus.
9. Cumulo-nimbus (Cu.-N.); The Thunder-cloud; Shower-cloud.—Heavy masses of clouds, rising in the form of mountains, turrets or anvils, generally having a sheet or screen of fibrous appearance above (false cirrus) and underneath, a mass of cloud similar to nimbus. From the base there generally fall local showers of rain or snow (occasionally hail or soft hail). Sometimes the upper edges have the compact form of cumulus, rising into massive peaks round which the delicate false cirrus floats, and sometimes the edges themselves separate into a fringe of filaments similar to that of cirrus. This last form is particularly common in spring showers. See fig. 10.
The front of thunderclouds of wide extent frequently presents the form of a large bow spread over a portion of the sky which is uniformly brighter in colour.
10. Stratus (S.).—A horizontal sheet of lifted fog. When this sheet is broken up into irregular shreds by the wind, or by the summits of mountains, it may be distinguished by the name of Fracto-stratus. See fig. 8.
The scheme also provides that where a stratus or nimbus takes a lumpy form, this fact shall be described by the adjective cumuliformis, and if its base shows downward projecting bosses the word mammato is prefixed.
Issued as it has been with the authority of an international congress of specialists, this scheme has been generally accepted, and must be regarded as the orthodox system, and for the great majority of observations it is quite detailed enough. But it does not give universal satisfaction. Cirrus clouds, for instance, exhibit many forms, and these so diverse that they must be due to very different causes. Hence for the minuter study of cloud forms a more elaborate scheme is still needed.
Hence in 1896 H. H. Clayton of the Blue Hill observatory, Massachusetts, published in the Annals of the astronomical observatory of Harvard College a highly detailed scheme in which the International types and a number of subdivisions were grouped under four classes—stratiforms or sheet clouds; cumuliforms or woolpack clouds; flocciforms, including strato-cumulus, alto-cumulus and cirro-cumulus; and cirriforms or hairy clouds. The International terms are embodied and the special varieties are distinguished by the use of prefixes such as tracto-cirrus or cirrus bands, grano-cirro-cumulus or granular cirrus, &c.
Again in 1904 F. L. Obenbach of the Cleveland observatory devised a different system, published in the annual report, in which the International types are preserved, but each is subdivided into a number of species. In the absence of any atlas to define the precise meaning of the descriptions given, neither of these American schemes has come into general use.
Further proposals were put forward by A. W. Clayden in Cloud Studies (1905). His scheme accepts the whole of the International names which he regards as the cloud genera, and suggests specific Latin names for the chief varieties, accompanying the descriptions by photographs. The proposed scheme is as follows.
| Genus. | Species. | |
| Cirrus | Cirro-nebula | Cirrus haze. |
| Cirro-filum | Thread cirrus. | |
| Cirrus Excelsus | High Cirrus | |
| Cirrus Ventosus | Windy Cirrus | |
| Cirrus Nebulosus | Hazy Cirrus | |
| Cirrus Caudatus | Tailed Cirrus | |
| Cirrus Vittatus | Ribbon Cirrus | |
| Cirrus Inconstans | Change Cirrus | |
| Cirrus Communis | Common Cirrus | |
| Cirro-stratus | Communis | Common Ci. S. |
| Nebulosus | Hazy Ci. S. | |
| Vittatus | Ribbon Ci. S. | |
| Cumulosus | Flocculent Ci.-S. | |
| Cirro-cumulus | Cirro-macula | Speckle cloud. |
| Nebulosus | Hazy Ci. cu. | |
| Alto-clouds | Alto-stratus | |
| Alto-stratus maculosus | Mackerel sky. | |
| Alto-stratus fractus | ||
| Alto-strato-cumulus | ||
| Alto-cumulus informis | ||
| Alto-cumulus nebulosus | ||
| Alto-clouds | Alto-cumulus castellatus | Turret cloud. |
| Alto-cumulus glomeratus | High ball cumulus. | |
| Alto-cumulus communis | ||
| Alto-cumulus stratiformis | Flat alto-cum. | |
| Stratus | Stratus maculosus | |
| Stratus maculosus radius | Roll cloud. | |
| Stratus maculosus lenticularis | Fall cloud. | |
| Strato-cumulus | ||
| Cumulus | Cumulus minor | Small cumulus. |
| Cumulus major | Large cumulus. | |
| Cumulo-nimbus | Storm cloud. |