AEROGRAPHER'S MATE 3 & 2 



VARIETIES AND SUPPLEMENTARY FEA- 

 TURES. — Varieties of clouds are established 

 mainly on the basis of the cloud's transparency 

 or its arrangement in the sky. The varieties 

 are nine in number; but since observations of 

 clouds do not ordinarily involve the recording 

 of the specific variety, they are not covered 

 here. A detailed description of the several va- 

 rieties can be found in the International Cloud 

 Atlas. Supplementary features and accessory 

 clouds, like the varieties, aid in the clear 

 identification of clouds. The most common sup- 

 plementary features are mamma, virga, and 

 tuba. They are defined and associated with the 

 parent clouds in the next section on Cloud 

 Types. 



Cloud Types 



HIGH CLOUDS. — High clouds are described 

 as follows: 



1. Cirrus (CI). Cirrus (fig. 15-1(A)) are 

 detached clouds of delicate and fibrous appear- 

 ance, generally white (cirrus are the whitest 

 clouds in the sky) in color, without shading. 

 They appear in the most varied forms, such 

 as isolated tufts, lines drawn across the sky, 

 branching feather-like plumes, and curved lines 

 ending tufts. 



Since cirrus are composed of ice crystals, 

 their transparent character depends upon the 

 degree of separation of the crystals. 



Before sunrise and after sunset, cirrus may 

 still be colored bright yellow or red. Being 

 high altitude clouds, they light up before lower 

 clouds and fade out much later. 



Cirrus often indicate the direction in which 

 a storm may lie. 



2. Cirrocumulus (CC). Cirrocumulus (fig. 

 1 5-1 (B)) , commonly called mackerel sky, look like 

 rippled sand or like cirrus containing globular 

 masses of cotton usually without shadows. Cir- 

 rocumulus are an indication that a storm is prob- 

 ably approaching. The individual globules of 

 cirrocumulus are rarely larger than 1 degree 

 as measured by an observer on the surface of 

 the earth at or near sea level. 



3. Cirrostratus (CS). Cirrostratus (fig. 

 15-2(A)) are a thin, whitish veil, which does not 

 blur the outlines of the sun or the moon, but 



gives rise to halos. A milky veil of fog, thin 

 stratus, and altostratus are distinguished from 

 a veil of cirrostratus of similar appearance by 

 the halo phenomenon, which the sun or moon 

 nearly always produces in a layer of cirro- 

 stratus. 



The appearance of cirrostratus is a good in- 

 dication of rain. In the Tropics, however, cir- 

 rostratus quite often may be observed with no 

 rain following. 



MIDDLE CLOUDS. — Middle clouds are de- 

 scribed as follows: 



1. Altocumulus (AC). Altocumulus (fig. 

 15-2(B)) are a layer (or patches) of clouds com- 

 posed of flattened globular masses, the smallest 

 elements of the regularly arranged layer being 

 fairly small and thin, with or without shading. 

 The balls or patches usually are arranged in 

 groups, in lines, or in waves. This cloud form 

 differs from cirrocumulus by generally having 

 larger masses, by casting shadows, and by 

 having no connection with cirrus forms. A co- 

 rona or irisation is frequently observed on 

 altocumulus. 



2. Altostratus (AS). Altostratus (fig. 15-3(A)) 

 look like thick cirrostratus, but without halo 

 phenomena; altostratus are a fibrous veil or 

 sheet, gray or bluish in color. Sometimes the 

 sun or moon is completely obscured. 



Light rain or heavy snow may fall from a 

 cloud layer that is definitely altostratus. 



Altostratus can sometimes be observed at 

 two different levels in the sky and sometimes 

 in conjunction with altocumulus, which may also 

 exist at two different layers in the sky. 



3. Nimbostratus (NS). Nimbostratus (fig. 15-4) 

 appear as a low, amorphous, and rainly layer 

 of clouds. Of a dark gray color, they are usually 

 nearly uniform and feebly illuminated, seemingly 

 from within. 



When precipitation occurs, it is in the form of 

 continuous rain or snow. However, nimbostratus 

 may occur without rain or snow reaching the 

 ground. In cases In which the precipitation does 

 not reach the ground, the base of the cloud is 

 usually diffuse and looks wet. 



In most cases, nimbostratus evolve from alto- 

 stratus layers which grow thicker and whose 



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