C. Species 



Among the principal kinds one may note: 



Altocumulus cumulogenitus (33). — This is an altocumulus cloud 

 formed by the spreading out of the tops of cumulus, the lower parts 

 of the cumulus clouds having melted away ; the layer in the first stages 

 of its growth has the appearance of altocumulus opacus. 



D. Varieties 



An important variety of altocumulus should be noted, namely, 

 altocumulus cumuliformis, which has two different aspects: 



Altocumulus floccus (38). — Tufts resembling small cumulus clouds 

 without a base and more or less ragged. 



Altocumulus castellatus (37). — Cumuliform masses with more or 

 less vertical development, arranged in a line, and resting on 

 a common horizontal base, which gives the cloud a crenellated 

 appearance. 



The caps or hoods which form above a cumulus by the uplift of a 

 damp layer, and which may be pierced by the tops of the cumulus 

 are considered as a detail of cumulus, and denoted by the term pileus 

 attached to the name cumulus ; but in reality they are aberrant forms 

 of altocumulus translucidus. Moreover, similar clouds, independent 

 of cumulus, can be formed by the same process by the effect of a 

 rising current caused by a mountain or any obstacle. They are then 

 named altocumulus, and they are classed, on account of their form, 

 with the variety lenticularis (30). 



ALTOSTRATUS 



(15,25,26,27,34) 

 A. Definition 



Striated or fibrous veil, more or less gray or bluish in color. This 

 cloud is like thick cirrostratus but without halo phenomena; the sun 

 or moon shows vaguely, with a faint gleam, as though through ground 

 glass. Sometimes the sheet is thin, with forms intermediate with 

 cirrostratus. Sometimes it is very thick and dark, sometimes even 

 completely hiding the sun or moon. In this case differences of thick- 

 ness may cause relatively light patches between very dark parts; but 

 the surface never shows real relief,^ and the striated or fibrous struc- 

 ture is always seen in places in the body of the cloud. 



Every form is observed between high altostratus and cirrostratus 

 on the one hand, and low altostratus and nimbostratus on the other. 



Rain or snow may fall from altostratus (altostratus precipitans), but 

 when the rain is heavy the cloud layer will have grown thicker and 

 lower, becoming nimbostratus ; but heavy snow may fall from a layer 

 that is definitely altostratus. 



From the definition of altostratus it follows that there are three 

 subgenera: 



1. Altostratus translucidus (15, 25). — A sheet of altostratus re- 

 sembling thick cirrostratus; the sun and the moon show as through 

 ground glass. 



* See p. IV for distinction between dense sheets of altocumulus and altostratus. 



