STRATUS 



(13, 14) 

 A. Definition 



A low uniform layer of cloud, resembling fog, but not resting on the 

 ground. 



Fractostratus . — When this very low layer is broken up into irregular 

 shreds it is designated fractostratus (13, 25). 



B. Explanatory Remarks 



A veil of true stratus generally gives the sky a hazy appearance 

 which is very characteristic, but which m certain cases may cause 

 confusion with nimbostratus. When there is precipitation the dif- 

 ference is manifest: nimbostratus gives continuous rain (sometimes 

 snow), precipitation composed of drops which may be small and sparse, 

 or else large (at least some of them) and close together, while stratus 

 only gives a drizzle, that is to say, small drops very close together. 



When there is no precipitation a dark and uniform layer of stratus 

 can easily be mistaken for nimbostratus. The lower surface of nimbo- 

 stratus however has always a wet appearance (widespread trailing 

 precipitation, "virga"); it is quite uniform and it is not possible to 

 make out definite detail; stratus on the other hand has a drier appear- 

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

 some lighter transparent parts, that is, places less dark where the cloud 

 is thinner, corresponding to the interstices between the rolls and 

 globular masses of stratocumulus. but considerably larger, while 

 nimbostratus seems only to be feebly illuminated, as though lit up 

 from within. 



Stratus is often a local cloud, and when it breaks up the blue sky 

 is seen. 



Fractostratus sometimes originates from the breaking up of a layer 

 of stratus, sometimes it forms independently and develops till it forms 

 a layer below altostratus or 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 elements. 

 If these elements have a cumuliform appearance in places the cloud 

 layer is called fractocumulus and not fractostratus. 



NIMBOSTRATUS 



(16, 17) 

 A. Definition 



Alow, amorphous, and rainy layer, of a dark gray color, usually nearly 

 uniform; feebly illuminated seemingly from inside. 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 usually diffuse and looks wet on 



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