DEFINITIONS AND DESCRIPTIONS OF THE 

 FORMS OF CLOUDS 



CIRRUS 



(41, 42, 43, 44, 45, 46, 51, 52)^ 

 A. Definition 



Detached clouds of delicate and fibrous appearance, without shad- 

 ing,^ generally white in color, often of a silky appearance. 



Cirrus appears in the most varied forms, such as isolated tufts, 

 lines drawn across a blue sky, branching feather-like plumes, curved 

 lines ending in tufts, etc.; they are often arranged in bands which 

 cross the sky like meridian lines, and which, owing to the effect of 

 perspective, converge to a point on the horizon, or to two opposite 

 points (cirrostratus and cirrocumulus often take part in the formation 

 of these bands). 



B. Explanatory Remarks 



Cirrus clouds are always composed of ice crystals, and their trans- 

 parent character depends upon the degree of separation of the 

 crystals. 



As a rule when these clouds cross the sun's disk they hardly di- 

 minish its brightness. But when they are exceptionally thick they 

 may veil its light and obliterate its contour. This would also be the 

 case with patches of altostratus, but cirrus is distinguished by the 

 dazzling and silky whiteness of its edges. 



Halos are rather rare in cirrus. 



Sometimes isolated wisps of snow are seen against the blue sky, 

 and resemble cirrus; they are of a less pure white and less silky than 

 cirrus; wisps of rain are definitely gray, and a rainbow, should one be 

 visible, shows their nature at once, for this cannot be produced in 

 cirrus. 



Before sunrise and after sunset, cirrus is often colored bright yellow 

 or red. These clouds are lit up long before other clouds and fade out 

 much later; some time after sunset they become gray. At all hours 

 of the day cirrus near the horizon is often of a yellowish color; this is 

 due to distance and to the great thickness of air traversed by the rays 

 of light. 



Cirrus, being in general more or less inclined to the horizontal, 

 tends less than other clouds to become parallel to the horizon, under 

 the effect of perspective, as the horizon is approached; often on the 

 contrary it seems to converge to a point on the horizon. 



1 Numbers inserted in parentheses in connection with a cloud name refer to the pictures in which that 

 cloud is illustrated. 



2 Dense cirrus derived from an anvil (cimis noihus) may have shading. 



