finally be pierced by them. Tlus cloud which does not constitute a 

 species is given the name of pileus, a cap or hood. 



The clouds which form below altostratus or nimbostratus and which 

 can develop into a complete layer, through whose interstices the alto- 

 stratus or nimbostratus is generally seen, are usually fractostratus ; but 

 if they have a cumuliform appearance they should be classed as 

 fractocumulus. They rarely have this appearance during or soon 

 after rain; on the other hand it is frequent at the beginning of the 

 formation of the low cloud, and w^hen it breaks up. 



C. Species 



Among the principal species one may note: 



1. Cumulus humilis (1). — Cumulus with little vertical development, 

 and seemingly flattened. These clouds are generally seen in fine 

 weather. 



2. Cumulus congestus (3, 4). — Very distended and sprouting 

 cumulus, whose domes have a cauliflower appearance. 



CUMULONIMBUS 



(5,6,7,8,21,22,24) 

 A. Definition 



Heavy masses of cloud, with great vertical development, whose 

 cumuhform summits rise in the form of mountains or towers, the 

 upper parts ha\dng a fibrous texture and often spreading out in the 

 shape of an an^^l. 



The base resembles nimbostratus, and one generally notices virga. 

 This base has often a layer of very low ragged clouds below it (frac- 

 tostratus, fractocumulus) . 



Cumulonimbus clouds generally produce showers of rain or snow 

 and sometimes of hail, and often thunderstorms as well. 



If the wdiole of the cloud cannot be seen the fall of a real shower is 

 enough to characterize the cloud as a cumulonimbus. 



B. Explanatory Remarks 



Even if a cumulonimbus were not distinguished by its shape from a 

 strongly developed cumulus its essential character is evident in the 

 difference of structure of its upper parts, when these are visible (fibrous 

 structure and cumuliform structure). Masses of cumulus, however 

 heavy they may be, and however great their vertical development, 

 should never be classed as cumulonimbus unless the whole or a part of 

 their tops is transformed or is in process of transformation into a cirrus 

 mass. 



Although the upper cirriform parts of a cumulonimbus may take on 

 very varied shapes, yet in certain cases they spread out into the form 

 of an anvil. To this interesting feature the name incus is given. 



In certain types of cumulonimbus, which are especially common in 

 spring in moderately high latitudes, the fibrous structure extends to 

 nearly the whole cloud mass, so that the cumuliform parts almost 

 wholly disappeai-; the cloud is reduced to a mass of cirrus and of virga. 



The veil cloud jpileus is seen with cumulonimbus clouds as with 

 cumulus. 



12 



