cumulus and altocumulus (C,„ = 6). None of these cumulus clouds 

 should show ice crystal clouds at their tops; this would mean that 

 they had reached the cumulonimbus stage which would entail code 



3. Cumulonimbus (5, 6, 7, 8). 



Cumulus clouds of great vertical development, with the tops com- 

 posed of ice crystal clouds. Sometimes the nascent ice crystal cloud 

 is merely mingled with the "cauliflower" tops, where a fibrous struc- 

 ture appears and the clear-cut outlines fray out; sometimes the 

 completely formed ice crystal clouds crown the cumulus with a definite 

 plume of cirrus of a shape more or less like that of an anvil. Some- 

 times, especially in the spring and in high latitudes, the ice crystal 

 formation involves nearly the whole cloud even to the base. At the 

 end of the growth of a cumulonimbus the lower cumuliform part of 

 the cloud often tends to disappear leaving only the upper or cirrus 

 part. 



When the anvil, reaching nearly to the zenith, begins to overshadow 

 the observer, a mammatus structure will often be seen on the lower 

 surface of the anvil projection. 



Lilce the heavy and swelling cumulus, cumulonimbus is formed either 

 in calms, especially on hot thundery days, or in a strong wind in the 

 rear of disturbances. 



Cumulonimbus is a regular factory of clouds. By extension at 

 various levels it often produces either cirrus masses by an extension 

 of the ice crystal parts, or masses of altocumulus or stratocumulus by 

 an extension of the cumuliform parts, and these may end by becoming 

 detached from the parent cloud. Thus cumulonimbus, Cl = 3, may 

 coexist with cloud sheets that should be coded (7^=3 or 0^^ = 6. 



At the end of the evolution of cumulonimbus, Cl = S should only 

 be coded when the cumuliform parts are still visible. When the 

 cumuliform clouds have degenerated, the anvil top or the clouds 

 derived therefrom should be coded C„=3. 



When cumulonimbus nears the zenith, and its base, with low dark 

 clouds under it, often in the form of a roller or an arch, has covered all 

 or nearly all the sky, code Oi,=3 should be replaced by (7^=9. 



4. Stratocumulus Formed by the Flattening of Cumulus Clouds (9, 10) . 



Cumulus tops may settle down and the bases may spread out ; this is 

 a frequent end to the progressive changes of the cumulus of fine 

 weather. On the other hand, the bases may melt away and the tops 

 may spread out; this is a common phenomenon in the rear of a dis- 

 turbance after squalls or showers. Very opaque sheets or a layer of 

 stratocumulus may be formed in this wa}^, often showing a festooned 

 formation (mammatus) in places. At the end of the process the clouds 

 thus formed may tliin out. 



The first case is stratocumulus vesperalis, the second stratocumulus 

 cumulogenitus. 



5. Layer of Stratus or Stratocumulus (11, 12, 13, 14). 



Clouds usually forming a single layer, fairly regular and not very 

 dark or menacing; they have a certain stability. The stratocumulus 

 has often semitransparent parts, or even clear spaces between the 

 elements of the cloud. 



23 



