Cirrus fumulus must not be confused with cirrostratus nebulosus. 

 The latter is much more stable and does not show the phenomenon of 

 the formation and subsequent rapid disappearance of cirrus clouds. 



2. Lenticularis (30). — Clouds of an ovoid shape, with clean-cut 

 edges, and sometimes irisations, especially common on days of strong, 

 dry winds in rough country. Tliis form exists at all levels from cir- 

 rostratus to stratus. 



3. Cumuliformis. — The rounded form resembling cumulus which 

 the upper parts of other clouds may sometimes assume. This may be 

 seen at all levels from cirrus to stratus. 



4. Mammatus. — Tliis description is given to all clouds whose lower 

 surfaces form pouches or breasts. This form is found especially in 

 stratocumulus and in cumulonimbus, either at the base, or even more 

 often on the lower surface of anvil projections. It is also found, 

 though rarely, in cirrus clouds, probably when they have originated 

 in the anvil of a dispersing cumulonimbus. 



5. Undulatus. — This term is applied to clouds composed of elongated 

 and parallel elements, like waves of the sea. There is sometimes an 

 appearance of two distinct systems, as when the cloud is divided into 

 rounded masses by undulations in two directions. 



6. Radiatus. — This term is applied to clouds in parallel bands (polar 

 bands), which owing to perspective seem to converge to a point on the 

 horizon, or to two opposite points if the bands cross the whole sky. The 

 point is called the radiant point, or vanishing point. 



CHIEF CASUAL DETAILS 



The chief casual varieties are the following: 



1. Virga: Wisps or falling trails of precipitation; applied princi- 

 pally to altocumulus and altostratus. 



2. Pileus: A cap or hood; applied principally to cumulus or cumu- 

 lonimbus. 



3. Incus: Anvil; upper part of cumulonimbus. 



4. Arcus: Arch cloud; usually associated with cunmlonimbus. 



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