Popular Science Monthhi
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���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
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