When a cumulonimbus covers nearly all the sky the base alone is 

 visible, and resembles nimbostratus, with or without fractostratus or 

 fractocumulus below. The difference between the base of a cumu- 

 lonimbus and a nimbostratus is often rather difficult to make out. If 

 the cloud mass does not cover all the sky, and if even small portions 

 of the upper parts of the cumulonimbus appear, the difference is 

 evident. If not it can only be made out if the preceding evolution of 

 the clouds has been followed, or if precipitation occurs; its character 

 is violent and intermittent (showers) in the case of cumulonimbus, as 

 opposed to the relatively gentle and continuous precipitation of a 

 nimbostratus. 



The front of a thunder cloud of great extent is sometimes accom- 

 panied by a roll cloud of a dark color in the shape of an arch, of a 

 frayed out appearance, and circumscribing a part of the sky of a 

 lighter gray. This cloud is named arcus and is nothing more or less 

 than a particular case of fractocumulus or fractostratus. 



Fairly often a mammatus structure appears in cumulonimbus, 

 either at the base, or on the lower surface ol the lateral parts of the 

 anvil. 



When a layer of menacing cloud covers the sky and virga and mam- 

 matus structure are both seen it is a sure sign that the cloud is the 

 base of a cumulonimbus, even in the absence of all other signs. 



Cumulonimbus is a real factory of clouds ; it is responsible in great 

 measure for the clouds in the rear of disturbances. By the spreading 

 out of the more or less high parts and the melting away of the under- 

 lying parts, cumulonimbus can produce more or less thick sheets of 

 altocumulus or stratocumulus (spreading out of the cumuliform 

 parts) and dense cirrus (spreading out of the cirriform parts). 



C. Species 



Among the principal species may be noted: 



1. Cumulonimhus calvus (5, 6). — Cumulonimbus characterized by 

 the thunderstorm or the shower that it causes, or by virga, but in 

 which no cirriform parts can be made out. Nevertheless the freezing 

 of the upper parts has already begun; the tops are beginning to lose 

 their cumulus structure, that is to say, their rounded outlines and 

 clear cut contours; the hard and "cauliflower" swellings soon become 

 confused and melt away so that nothing can be seen in the white mass 

 but more or less vertical fibers. The freezing, accompanied by the 

 change into a fibrous structure, often goes on very rapidly. 



2. Cumulonimbus capillatus (7). — Cumulonimbus which displays 

 distinct cirriform parts, having sometimes, but not always, the shape 

 of an anvil. 



PRINCIPAL VARIETIES 



The chief varieties common to different genera are as follows: 

 1. Fumulus. — At all levels, from cirrus to stratus, a very thin veil 

 may form, so delicate that it may be almost invisible. These veils 

 seem to be most frequent on hot days, and in low latitudes. Occa- 

 sionally they may be observed to thicken rapidly, forming clouds 

 easily visible, especially cirrus and cumulus. The clouds thus pro- 

 duced seem unstable however, and usually melt away soon after their 

 formation. 



98863'= 



13 



