Altocumulus translucidus (28, 29, 32). — Altocumulus formed of 

 elements whose color — from dazzling white to dark gray — and whose 

 thickness vary much from one example to another, or even in the 

 same layer; the elements are more or less regularly arranged and 

 distinct. In the definition of the elements it is the variation in the 

 transparency of the layer, variable from one point to another, that 

 plays the essential part. There appears in the interstices either the 

 blue of the sky, or at least a marked lightening of the layer of cloud 

 due to a thinning out. 



Altocumulus opacus (35, 36). — An altocumulus sheet which is 

 continuous, at least over the greater part of the layer, and consisting 

 of dark and more or less irregular elements, in the definition of which 

 transparency does not play a great part, owing to the thickness and 

 density of the laye^' ; but the elements show in real relief on the lower 

 surface of the cloud sheet. 



B. Explanatory Remarks 



The limits within which altocumulus is met are very wide. 



At the greatest heights, altocumulus made up of small elements 

 resembles cirrocumulus ; altocumulus, however, is distinguished by 

 not possessing any of the following characters of cirrocumulus: 



1. Connection with cirrus or cirrostratus. 



2. An evolution from cirrus or cirrostratus. 



3. Properties due to physical structure (ice crystals) enumerated 

 under cirrus. 



At lower levels, where altocumulus may be derived from a spreading 

 out of the tops of cumulus clouds, it may easily be mistaken for 

 stratocumulus; the convention is that the cloud is altocumulus if the 

 smallest, well defined, and regularly arranged elements which are 

 observed in the layer (leavmg out the detached elements which are 

 generally seen on the edges) are not greater than 10 solar diameters 

 in their smallest diameters, i. e., approximately the width of three 

 fingers when the arm is held extended. 



Wlien the edge or a thin semitransparent patch of altocumulus 

 passes in front of the sun or moon a corona appears close up to (within 

 a few degrees of) them ; this is a colored ring with red outside and blue 

 inside ; the colors may be repeated more than once. This phenomenon 

 is infrequent in the case of cirrocumulus and only the higher forms of 

 stratocumulus show it. 



Irisation, mentioned above, is a phenomenon of the same type as 

 the corona; it is characteristic of altocumulus as distinguished from 

 cirrocumulus or stratocumulus. 



Altocumulus clouds often appear at different levels at one and the 

 same time. Often, too, they are associated with other types of cloud. 



The atmosphere is often hazy just below altocumulus clouds. 



When the elements of a sheet of altocumulus fuse together and 

 make a continuous layer altostratus or nimbostratus is the result. 

 On the other hand a sheet of altostratus can change into altocumulus. 

 It may happen that these two aspects of a cloud sheet may alternate 

 with each other during the whole course of a day. It is also not rare 

 to have a layer of altocumulus coexisting with a veil resembling alto- 

 stratus at a height very little less than the altocumulus. 



It is interesting to note that one may often observe filiform descend- 

 ing trails to which the name virga has been given. 



6 



