16 



METEOROIOGY AND CLIMATOLOGY: 



cirrocumulus 



cirrostratus 



^r-30,000 



altocumulus 



<3 c^ cl:d 



altostratus 



cumulonimbus 



stratocumulus 



stratus nimbostratus 



cumulus 



cCj L 



Figure 2.5 Average altitudes (in teet) ond suggested forms of the ten main cloud types. 



15,000 



The two methods of cloud formation produce 

 clouds with a variety of appearances. However, this 

 confusing array of forms is classified by the United 

 States Weather Bureau into ten genera within four 

 families. Although these cloud genera have a num- 

 ber of variations called species, only the genera 

 are emphasized here (Figure 2.5). 



FAMILY A: HIGH CLOUDS (averaging 30,000 feet; 

 mean lower level 20,000 feet) 



Cirrus are detached clouds of delicate and fibrous 

 appearance. They are without shading (except those 

 extending from and composed of the debris of the 

 upper frozen parts of cumulonimbus clouds), gen- 

 erally white in color, often of a silky or feathery 

 appearance, and with ragged and indefinite edges. 

 These are the clouds that are frequently brightly 

 colored at sunset or sunrise. 



Cirrus appear in such forms as isolated tufts, lines 

 drawn across a blue sky, branched featherlike 

 plumes, and curved lines ending in tufts. They are 

 often arranged in bands which cross the sky like 

 meridian lines and, owing to the effect of perspective, 

 converge either to a point on the horizon or to two 

 opposite points on opposite horizons. Cirrocumulus 



and cirrostratus often take part in the formation of 

 these bands. 



Cirrocumulus form either a cirruslike layer or 

 patches. The patches may be either small, unshaded, 

 white flakes, or very small globular masses. Even the 

 masses are generally unshadowed. The flakes or 

 masses are arranged in groups, lines, or, more often, 

 ripples resembling those of beach sand. Often cir- 

 rocumulus arrangement resembles fish scales and is 

 called "mackerel sky." 



Cirrocumulus clouds never appear alone. They are 

 with and represent a degraded state of cirrus and 

 cirrostratus. For this reason, cirrocumulus often 

 retain some scattered point of fibrous structure. 



Real cirrocumulus are uncommon. They must not 

 be confused with small altocumulus on the edges of 

 altocumulus sheets. This confusion is possible be- 

 cause all states of transition occur between 

 cirrocumulus and altocumulus proper. Transition 

 occurs because both are formed by the same process. 

 However, the term "cirrocumulus" is used if any 

 cloud satisfies either of the following criteria: (1) 

 there is definite connection with cirrus and cirro- 

 stratus or (2) the cloud observed results from a 

 change in cirrus or cirrostratus. 



To interpret weather from cirrocumulus clouds. 



