This type of altocumulus is distinguished from that coded as 

 0^ = 3 by the progressive deterioration of the sky, and by the irregular 

 thickness of the layer. Again the proportions of these sheets and 

 their regular structure differentiate them from those coded as Cm = 4. 



6. Altocumulus Formed by a Spreading Out of the Tops of Cumulus 



(33). 



Cumulus clouds of sufficiently great vertical development may 

 undergo an extension of their summits while their bases may gradu- 

 ally melt away. These sheets of altocumulus are generally fairly 

 thick and opaque at first with rather large elements, dark and soft; 

 later they may thin out and finally have rifts in them, or at any rate 

 semitransparent interstices. 



The phenomenon of the extension of cumuliform masses is common 

 in the rear of disturbances, after squalls and showers. 



These extensions from the summits of cumulus clouds must not be 

 confused with the ice crystal extensions of a cumidonimbus from 

 which the anvil and hybrid cirrus is formed. The anvil, and even the 

 cirrus, detached from the cumulus which produced it, may sometimes 

 have a festooned appearance on the lower surfaces, and thus may 

 have a certain likeness to a sheet of altocumulus. But the alto- 

 cumulus does not have the whiteness, the silky appearance, nor the 

 fibrous structure of a cirrus anvil. 



To discriminate between the altocumulus coded (7^=6 on the one 

 hand, and the altocumulus coded 0^ = 3, Cm = 4, and (7^=5 on the 

 other, see the explanations of (7,^ = 3, Cm = 4, and Cm = 5. 



The observer may in certain cases be in doubt between 0^=6 and 

 Ci=5. At the end of the evolution of sheets of altocumulus cumulo- 

 genitus when they may have thinned out and become semitransparent 

 one would not of course think of coding 0^ = 5; but at the beginning 

 of the change when the cloud elements are rather large, dark, and soft 

 there are all the transitions between stratocumulus and altocumulus. 

 It is a question of the apparent size of the elements; the convention 

 is that the cloud in question is altocumulus when the smallest well- 

 defined cloud elements observed over the sheet as a whole (and 

 ignoring the separated portions which may exist beyond the edges 

 of the sheet) are not greater than 10 diameters of the sun, i. e., approxi- 

 mately the width of three fingers when the arm is held extended; 

 in case of doubt it is best to code Cm=Q, which allows one to note 

 (Cl = 2 or Cz,=3) cumulus clouds which are visible at the same time. 



7. Altocumulus Associated With Altostratus, or Altostratus With a 



Partially Altocumulus Character (34, 35, 36). 

 Different clouds are comprised in this section: 



A. Typical altostratus can lie above sheets of altocumulus wliich 

 are at a definitely lower level. This type including typical alto- 

 stratus can only exist in the central part of a disturbance. 



B. A more or less continuous layer of altocumulus can have below 

 it a gray veil of cloud, often hardly visible, lying at a level very little 

 lower; in places, and for short times it hides the cloudlets of the alto- 

 cumulus sheet sufficiently to^ give it in places the appearance of alto- 

 stratus. This type is found in the central part of a weak disturbance, 

 or on the lateral edge of the central part of a typical disturbance. 



C. In a layer of altocumulus which is growing thicker ((7,^ = 5) 

 the cloudlets may fuse together and the layer may become alto- 



27 



