INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS QQ 



through ground glass (fig. 20). Sometimes the sheet is thin, with 

 forms intermediate with cirrostratus. Sometimes it is very thick and 

 dark, sometimes even completely hiding the sun or moon. In this case 

 differences of thickness may cause relatively light patches between 

 very dark parts ; but the surface never shows relief, and the striated 

 or fibrous structure is always seen in places in the body of the 

 cloud. 



Figure 19. — ^Altocumulus, undui.ucil lorm. 



Every form is observed between high altostratus and cirrostratus 

 on the one hand and low altostratus and nimbostratus on the other. 



Rain or snow may fall from altostratus, but when the rain is heavy 

 the cloud layer will have grown thicker and lower, becoming nimbo- 

 stratus; but heavy snow may fall from a layer that is definitely 

 altostratus. 



The limits between which altostratus may be met with are fairly 

 wide (about 5,000 to 2,000 meters) . 



A sheet of high altostratus is distinguished from a rather similar 

 sheet of cirrostratus by the convention that halo phenomena are not 

 seen in altostratus. 



A sheet of low altostratus may be distinguished from a somewhat 

 similar sheet of nimbostratus by the following characters: Nimbo- 

 stratus is of a much darker and more uniform gray and shows no- 

 where any whitish gleam or fibrous structure; one cannot definitely 

 see the limit of its undersurface, which has a "soft" look, due to the 

 rain, which may not reach the ground. 



The convention is also made that nimbostratus always hides the 

 sun and moon in every part of it, while altostratus only hides them in 

 places behind its darker portions, but they reappear through the 

 lighter parts. 



