1. Stratocum.ulus translucidus (12). — A not very thick layer; in the 

 interstices between its elements either the blue sky appears, or at any 

 rate there are much lighter parts of the cloud sheet, which here is 

 thinned out on its upper surface. 



2. Stratocumulus opacus (21). — A very thick layer made up of a con- 

 tinuous sheet of large dark rolls or rounded masses; their shape is 

 seen not by a difference in transparency, but they stand out in real 

 relief from the under surface of the cloud layer. 



There are transitional forms between stratocumulus and alto- 

 cumulus on the one hand and between stratocumulus and stratus 

 on the other. 



B. Explanatory Remarks 



The difference between stratocumulus and altocumulus is given 

 under the latter. 



It should also be noted that the cloud sheet called altocumulus by 

 an observer at a small height might appear as stratocumulus to an 

 observer at a greater height (12). 



It often happens that stratocumulus is not associated with any 

 clouds of the second or third families ; but it fairly often coexists with 

 clouds of the fourth family. 



The elements of thick stratocumulus (stratocumulus opacus) ofteii 

 tend to fuse together completely, and the layer can, in certain cases, 

 change into nimbostratus. The cloud is called nimbostratus when 

 the cloud elements of stratocumulus have completely disappeared 

 and when, owing to the trails of falling precipitation, the lower surface 

 has no longer a clear cut boundary. 



Stratocumulus can change into stratus, and vice versa. The 

 stratus being lower, the elements appear very large and very soft, 

 so that the structure of regularly arranged globular masses and waves 

 disappears as far as the observer can see. The cloud will be called 

 stratocumulus as long as the structure remains visible. 



C. Species 



Among the principal species may be mentioned: 



1. Stratocumulus vesperalis (9). — This name is given to flat elon- 

 gated clouds which are often seen to form about sunset as the final 

 product of the diurnal changes of cumulus. 



2. Stratocumulus cumulogenitus (10). — Stratocumulus formed by 

 the spreading out of the tops of cumulus clouds, which latter have 

 disappeared; the layer in the early stages of its formation looks like 

 stratocumulus opacus. 



D. Varieties 



The cloud called roll cumulus in England and Germany is desig- 

 nated stratocumulus undulatus (11) ; its wave system is in one direction 

 only. It must not be confused with flat cumulus clouds ranged in 

 line. Stratocumulus often has a mammatus (festooned) character; 

 that is to say there is a high relief on the lower surface where pendant 

 rounded masses or corrugations are observed, and at times these look 

 as though they would become detached from the cloud. Care must 

 be taken not confuse this cloud with some kinds of altostratus opacus 

 whose under surface may appear to be slightly corrugated or mam- 

 millated; altostratus opacus is distinguished by its fibrous structure. 



9 



