THE ATMOSPHERE, WEATHER, AND CLIMATE 



17 



one must note what happens to them. If they change 

 into cumulus, the weather is likely to be fair. If they 

 change into cirrostratus, which in turn change to 

 lower, thicker, clouds, it is likely to rain. 



Cirrostratus form a thin, whitish veil which does not 

 blur the outlines of the sun or moon, but the light of 

 these objects usually gives rise to rings or halos. 

 Sometimes the cloud is quite diffuse and only gives 

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

 tinctly shows a fibrous structure with disordered 

 threads. This latter characteristic resembles alto- 

 stratus; however, cirrostratus do not prevent sun- 

 light from casting shadows upon the grourid. 



Weather forecasting necessitates observing the fate 

 of cirrostratus. If they are followed by lower clouds, 

 rain and higher temperature are likely. If they de- 

 generate into cirrocumulus, fair weather is most 

 probable. Cirrostratus in the early evening is sup- 

 posed to indicate clearing by morning. 



FAMILY B: MIDDLE CLOUDS (mostly 13,000 to 22,000 

 feet; mean upper level, 20,000 feet; lowest level to 6500 

 feet) 



Altocumulus occur either as advancing, small, iso- 

 lated patches, parallel bands, or a layer of flattened 

 globular masses. In a regularly arranged layer, the 

 smallest masses are fairly small and thin, shaded or 

 not, and are often at different levels. Also, the globu- 

 lar masses are arranged in groups, lines, or waves that 

 in turn are organized in one or two directions and are 

 sometimes so close together that their edges join. 

 When they interrupt moonlight or sunlight, a corona 

 of small, colored, rainbowlike rings (red outside and 

 blue inside) is formed. Cirrostratus also can have a 

 halo, but the ring is blue outside and red inside. 

 Altocumulus frequently change into altostratus and 

 forecast storm conditions. 



Altostratus form a lined to fibrous sheet or veil that 

 is more or less gray or bluish in color. Such a cloud 

 resembles thick cirrostratus, but it displays no halo 

 phenomenon; rather, the sun or moon shows vaguely, 

 with a faint gleam, as though through ground glass. 

 When an altostratus sheet is thin, it is most like 

 cirrostratus; when very thick and dark, it sometimes 

 completely hides the sun or moon. Also, when it is 

 thick, differences in thickness may cause relatively 

 light patches between very dark parts, but the sur- 

 face never shows real relief and the lined or fibrous 

 structure is always seen in parts of the cloud. 



Rain or snow may fall from altostratus, but when 

 the rain is heavy, the cloud layer will have grown 

 thicker and lower, becoming nimbostratus. On the 

 other hand, heavy snow may fall from a layer that is 

 definitely altostratus. 



FAMILY C: LOW CLOUDS (generally under 10,000 feet; 

 mean upper level, 6500 feet; mean lower level, close to 

 the surface) 



Stratocumulus form a layer (or patches) composed 

 of rounded patches, globular masses, or rolls. The 

 smallest of the regularly arranged individual parts 

 are fairly large, soft, and gray, but have darker areas. 

 Arrangement of parts is in groups, lines, or waves 

 that are aligned in one or two directions. Very often 

 the parts are so close together that their edges join. 

 When these clouds cover the sky they have a wavy ap- 

 pearance. 



Stratocumulus may indicate changing weather. 

 They are frequently found near thunderheads and 

 before and behind storms. 



Stratus is a low, uniform layer of cloud. The layer 

 may be partly broken up into irregular shreds, a 

 variety called fractostratus. Fog is a variation that 

 reaches the ground. Stratus clouds often indicate fair 

 weather. 



Nimbostratus is much like stratus, but is darker and 

 much thicker. This low cloud type is usually of ho- 

 mogeneous but formless structure. Also, the cloud 

 has a dark gray color and appears to be feebly lighted 

 from within. 



Nimbostratus are called rain clouds and frequently 

 have a rainy or snowy lower layer; however, they may 

 occur without rain or snow. Even if precipitation 

 is released, rain or snow might not reach the ground. 

 In this case, the base of the cloud is usually diffuse 

 and looks wet on account of the trailing precipitation. 

 When this occurs, it is not possible to determine the 

 cloud's lower surface. 



These clouds frequently have ragged fragments 

 that are detached but closely associated with the 

 main cloud mass. Such fragments are generally quite 

 dark and are called scuds. Scuds are also produced 

 by cumulonimbus clouds. 



FAMILY D: CLOUDS WITH VERTICAL DEVELOPMENT 



(.'urmilus is a dense, low-altitude cloud with vertical 

 development; however, its height is insignificant in 



