Popular Science Monthhi 



241 



Anvil-shaped cumulo nimbus 



the world, is brief and simple, and must 

 serve as the point of departure in our 

 excursion to Cloudland : 



/. Upper Clouds 



1. Cirrus ("Mares' Tails"). Detached 



clouds, delicate and fibrous, tak- 

 ing the form of feathers. 



2. Cirro-stratus. A thin, whitish, oft- 



en web-like sheet of cloud. 



//. Intermediate Clouds 



3. Cirro-cumulus ("Mackerel sky"). 



Small globular masses or wdiite 

 flakes. 



4. Alto-cumulus. Rather large globu- 



lar masses, white or grayish, part- 

 ly shaded. 



5. Alto-stratus. A thick sheet of gray 



or bluish cloud. 



///. Loiuer Clouds 



6. Strato-cumulus. Large globular 



masses or rolls of dark cloud, oft- 

 en covering the whole sky ; espe- 

 cially common in winter. 



7. Nimbus. Dark, shapeless clouds 



attended by rain or snow. 



IV. Clouds Formed by Day in 

 Ascending Air Currents 



8. Cumulus. Thickclouds with more or 



less rounded summitsand flat bases. 



9. Cumulo-nimbus ("Thundercloud"). 



The common cloud of summer 

 thunderstorms ; a mountainous 

 mass, often turret-shaped or an- 

 vil-shaped, generally w i t h a 

 fibrous sheet spreading out above. 



V. High Fog 

 10. Stratum. A uniform layer of cloud 

 resembling fog, but not resting on 

 the ground. 

 The international Classification also 



btxato-cumulus clouds 



recognizes a few minor types : especially 

 fracto-nimbus, or "scud," (shreds of 

 nimbus seen drifting under the rain- 

 cloud) ; fracto-cumulus (small detached 

 fragments of cumulus, undergoing rap- 

 id change in form), and fracto-stratus 

 (formed when a uniform layer of strat- 

 us is broken into irregular patches by 

 wind or by mountains). Mammato-cu- 

 mulus ("sack cloud," or "pocky cloud") 

 is a rare and striking cloud form, seen 

 especially in thundery weather, consist- 

 ing of rounded sack-like clouds, convex 

 downwards. 



The photographs accompanying this 

 article will help the reader to interpret 

 the foregoing descriptions. There are 

 several collections of such pictures, 

 known as "cloud atlases," of which the 

 most important is the International 

 Cloud iVtlas, published in Paris, with de- 

 scriptions in French, English and Ger- 

 man. Equally useful, however, to the 

 American student is the booklet entitled 

 "Classification of Clouds," with beauti- 

 ful illustrations in color, issued by the 

 Weather Bureau and sold at twenty-five 

 cents a copy by the Superintendent of 

 Documents, in \\'ashington. 



The layman who has learned the cloud 

 names given above will sometimes, per- 

 haps, be puzzled to find a variety of 

 other names applied to cloud forms by 

 technical writers. The explanation is 

 that many specialists have sought to in- 

 troduce more elaborate cloud classifica- 

 tions; in which, however, the Interna- 

 tional nomenclature u.sually forms the 

 substructure. None of these systems 

 has ever come into general use. 



Clouds are Composed of Tiny Needles of Ice 



Turning, now, from the ob\iou> to 



