and merely gives the sky a milky look; sometimes it more or less dis- 

 tinctly shows a fibrous structure with disordered filaments. 



B. Explanatory Remarks 



A sheet of cirrostratus which is very extensive, though in places 

 it may be interrupted by rifts, nearly always ends by covering the 

 whole sky. The border of the sheet may be straight edged and clear- 

 cut but more often it is ragged or cut up. 



During the day, when the sun is sufficiently high above the horizon, 

 the sheet is never thick enough to prevent shadows of objects on the 

 ground. 



A milky veil of fog (or thin stratus) is distinguished from a veil of 

 cirrostratus of a similar appearance by the halo phenomena which 

 the sun or the moon nearly always produces in a layer of cirrostratus. 



The following are the principal halo phenomena: A circle of 22° 

 radius round the sun or moon; this is roughly the angle subtended by 

 the hand placed at right angles to the arm when the latter is extended ; 

 this halo is sometimes, but rarely, accompanied by one of 46° radius. 

 Parhelia, paraselenae (mock suns or mock moons), luminous patches, 

 often showing prismatic colors, a little over 22° from the sun or moon 

 and at the same elevation. A luminous column, e. g., sun pillar, 

 extending vertically above and below the luminary. 



Often only small fragments of these appearances are visible but 

 they are none the less characteristic of high clouds. 



Wliat has been said above of the transparent character and colors 

 of cirrus is true to a great extent of cirrostratus. 



C. Species 



Cirrostratus has two principal aspects which correspond to the 

 two foUowing species: 



Cirrostratus nebulosus (48, 50). — A very uniform nebulous veil, 

 sometimes very thin and hardly visible, sometimes relatively dense, 

 but always without definite details and usually with halo phenomena. 



Cirrostratus filosus (47). — A white fibrous veil, where the strands 

 are more or less definite, often resembling a sheet of cirrus densus 

 from which indeed it may origmate. 



ALTOCUMULUS 



(28 to 38, inclusive) 

 A. Definition 



A layer (or patches) composed of laminae or rather flattened 

 globular masses, the smallest elements of the regularly arranged 

 layer being fairly small and thin, with or without shading.^ These 

 elements are arranged in groups, in lines, or waves, followmg one or 

 two directions, and are sometimes so close together that their edges 

 join. 



The thin and translucent edges of the elements often show irisations 

 which are rather characteristic of this class of cloud. 



From the definition it follows that altocumulus comprises the sub- 

 genera: 



3 See p. IV for distinction between dense sheets of altocumulus and altostratus. 



