C. Species 



Among the principal forms one may note: 



Cirrus filosus (42). — More or less straight or irregularly curved 

 filaments (neither tufts nor little hooks and without any of the parts 

 being fused together). 



Cirrus uncinus (45, 52).- — Cirrus in the shape of a comma, the 

 upper part either ends in a little tuft or is pomted. 



Cirrus densus (43). — Cirrus clouds with such thickness that without 

 care an observer might mistake them for middle or low clouds. 



Cirrus nothus (44). — Cirrus proceeding from a cumulonimbus and 

 composed of the debris of the upper frozen parts of these clouds. 



D. Varieties 



Ordinary cirrus may appear in many very different forms. One 

 may particularly note the forms floccus and vertebratus which are 

 really aspects of the varieties cumuliformis and undulatus radiatus, 

 respectively. 



CIRROCUMULUS 



(49, 51, 52) 



A. Definition 



A cirriform layer or patch composed of small white flakes or of very 

 small globular masses, usually without shadows, which are arranged in 

 groups or lines, or more often in ripples resembling those of the sand 

 on the seashore. 



B. Explanatory Remarks 



In general cirrocumulus represents a degraded state of cirrus and 

 cirrostratus both of which may change into it. In this case the 

 changing patches often retain some fibrous structure in places. 



Real cirrocumulus is uncommon. It must not be confused with 

 small altocumulus on the edges of altocumulus sheets. There are in 

 fact all states of transition between cirrocumulus and altocumulus 

 proper; this is only to be expected as the process of formation is the 

 same. In the absence of any other criterion the term cirrocumulus 

 should only be used when — 



1. There is evident connection with cirrus or cirrostratus. 



2. The cloud observed results from a change in cirrus or cirro- 

 stratus. 



3. The cloud observed shows some of the characteristics of ice 

 crystal clouds which will be found enumerated under cirrus. 



Clear rifts are often seen in a sheet of cirrocumulus. 



CIRROSTRATUS 



(46, 47, 48, 49, 50) 



A. Definition 



A thin whitish veil which does not blur the outlines of the sun or 

 moon, but usually gives rise to halos. Sometmies it is quite diffuse 



