2. Altostratus opacvs (26, 27). — An opaque layer of altostratiis of 

 variable thickness which may entirely hide the sun, at any rate, in 

 parts, but showing a fibrous structure in some parts. 



3. Altostratus precijntans. — A layer of opaque altostratus which 

 has not yet lost its fibrous character, and from which there are light 

 falls of rain or snow, either continuous or intermittent. Tliis precipi- 

 tation may not reach the ground in which case it forms virga. 



B. Explanatory Remarks 



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, nor are the shadows of objects on the gromid 

 visible. 



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 nowhere 

 any whitish gleam or fibrous structure ; one cannot definitely see the 

 limit of its undersurface which has a wet look, due to the rain (or snow), 

 which may not reach the ground. 



The convention is also made that nimbostratus always hides the 

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

 places behmd its darker portions, but they reappear through the lighter 

 parts. 



Careful observation may often detect virga hangmg from altostratus, 

 and these may even reach the ground causing slight precipitation. If 

 the sheet still has the character of altostratus it will then be called 

 altostratus precipitans, but if not it has become nimbostratus. 



A sheet of altostratus, even if it has rifts in places, has a general 

 fibrous (or ground glass) character. A cloud layer, even a contmuous 

 one, which has no fibrous structure, and in which rounded cloud 

 masses may be seen is classed as altocumulus or stratocumulus accord- 

 ing to circumstances. 



Altostratus may result from a transformation of a sheet of alto- 

 cumulus, and on the other hand altostratus may often break up into 

 altocumulus. 



STRATOCUMULUS 



(9,10, 11,12, 18, 19,20,21,28) 



A. Definitions 



A layer (or patches) composed of laminae, globular masses or rolls ; 

 the smallest of the regularly arranged elements are fairly large ; they 

 are soft and gray, with darker parts." 



These elements are arranged in groups, in lines, or in waves, alined 

 in one or in two directions. Very often the rolls are so close that their 

 edges join ; when they cover the whole sky they have a wavy appearance. 



From the definition it follows that stratocumulus comprises two 

 kinds: 



5 Also, a low, continuous sheet, thick or thin, with distinct irregularities of large size. See explanatory 

 remarks under "Stratus." 



