74 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



appearance, and however uniform it may be, it shows some contrasts 

 and some lighter transparent parts. 



Fractostratus sometimes originates from the breaking up of a layer 

 of stratus; sometimes it forms independently and develops till it 

 forms a layer below nimbostratus, which latter may be seen in the 

 interstices. 



A layer of fractostratus may be distinguished from nimbostratus 

 by its darker appearance, and by being broken up into cloud ele- 

 ments. If these elements have a cumuliform appearance in places, 

 the cloud layer is called fractocumulus and not fractostratus. 



Figure 24. — Nimbostratus (above) with stratocumulus roll along horizon. 

 (John C. Hagau.) 



Nimhostratus. — A low, amorphous, and rainy layer, of a dark grey 

 color and nearly uniform; feebly illuminated, seemingly from inside 

 (fig. 24). When it gives precipitation it is in the form of continuous 

 rain or snow. 



But precipitation alone is not a sufficient criterion to distinguish 

 the cloud which should be called nimbostratus, even when no rain or 

 snow falls from it. 



There is often precipitation which does not reach the ground; in 

 this case the base of the cloud is always diffuse and looks "wet" 

 on account of the general trailing precipitation ("virga"), so that 

 it is not possible to determine the limit of its lower surface. 



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 nimbo- 

 stratus can generally be seen. These very low clouds are called 

 fractocumulus or fractostratus, according as to whether they appear 

 more or less cumuliform or stratiform. 



