account of the general trailing precipitation, virga, so that it is not 

 possible to determine the limit of its lower surface. 



B. Explanatory Remarks 



The usual evolution is as follows: A layer of altostratus grows thicker 

 and lower until it becomes a layer of nimbostratus. Beneath the latter 

 there is generally a progressive development of very low ragged clouds, 

 isolated at first, then fusing together into an almost continuous layer, 

 in the interstices of which, however, the nimbostratus can generally 

 be seen. These very low clouds are called fractocumulus or fracto- 

 stratus according as to whether they appear more or less cumuhform 

 or stratiform. 



Generally the rain only falls after the formation of these very low 

 clouds, which are then hidden by the precipitation or may even melt 

 away under its action. The vertical visibility then becomes very bad. 



In certain cases the precipitation may precede the formation of 

 fractocumulus or fractostratus, or it may happen that these clouds do 

 not form at all. 



Rather rarely a sheet of nimbostratus may form by an evolution 

 from a stratocumulus. 



CUMULUS 



(1, 2, 3, 4, 18, 19, 20, 23, 33, 37, 44) 

 A. Definition 



Dense clouds with vertical development; the upper surface is dome 

 shaped and exhibits rounded protuberances, while the base is nearly 

 horizontal. 



When the cloud is opposite the sun the surfaces normal to the 

 observer are brighter than the edges of the protuberances. When the 

 light comes from the side, the clouds exhibit strong contrasts of light 

 and shade; against the sun, on the other hand, they look dark with a 

 bright edge. 



Fractocumulus. — True cumidus is definitely limited above and 

 below, its surface often appears hard and clear cut. But one may also 

 observe a cloud resembling ragged cumulus in which the different 

 parts show constant change. This cloud is designated fractocumulus 

 (2). 



B. Explanatory Remarks 



Typical cumulus, over land areas, develops on days of clear skies, 

 and is due to the currents of diurnal convection; it appears in the 

 morning, grows, and then more or less dissolves again toward the 

 evening. 



Cumulus, whose base is generally of a gray color, has a uniform 

 structure, that is to say it is composed of rounded parts right up to its 

 summit, with no fibrous structure. Even when highly developed, 

 cumulus can only produce light precipitation. 



Cumulus, when it reaches the altocumulus level, is sometimes capped 

 with a light, diffuse, and white veil of more or less lenticular shape, 

 with a delicate striated or flaky structure on its edges ; it is generally 

 shaped like a bow which may cover several domes of the cumulus, and 



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