Cumulus, ball of cotton, day or summer cloud, consists of dense, convex, hemi spherical, or conical heaps of small roundish C., piled or stacked on, each other. The heaps enlarge upwards from a horizontal base, and have crenated tops; they sometimes unite into stupendous white-topped mountains. It forms, says sir J. Herschel, in sum mer calms by the rise of columns of vapor from marshes, lakes, and rivers, into the colder and quickly saturable lower regions of the air; for one liquid traverses another in cylinders. Cumuli begin after sunrise as a few scattered specks in the clear sky; these specks enlarge and unite to form C., which often nearly cover the sky in the afternoon, and generally decrease and vanish about sunset; but rain follows if they increase in number and darkness in the evening. Their tops become Ci. in very dry air. Cu., of pleasing forms, dispositions, and colors, and of moderate size, presage fine dry warm and calm days; but cold, rain, and tempest follow dark, abrupt, dense, shaggy Cu., cover ing the sky, and rolling on each other. Hemispherical silvery white Cu. presage thunder.
Stratus, fall or night-cloud, the lowest of C., is a widely extended, horizontal sheet, of varied thickness, of white mist touching or near the earth. In density it is between Ci. and Cu., audit increases from below. It is common in summer and autumn often from sunset to sunrise, and is densest at or after midnight. It arises in calm clear evenings, after warm days; from the cooling of moist air on damp ground, marshes, lakes, rivers, or from the cooling of moist air mixed with smoke enveloping great cities. From a height, it is seen spreading around like a sea, and creeping up hillsides. After sunrise, it rises from the ground, breaks up into Cu., and vanishes with the increasing heat, to be followed by a serene day; but it may quietly accumulate in layers, and become a Ni. It does not wet objects it touches, and thus differs from a variety of Ci.-st. of like external aspect.
Cirro-cumulus, or sender-cloud, consists of Ci. sinking' in the air, and compressed into dense roundish-white clondlets, or woolly irregular tufts, generally at great heights. It often forms a beautiful sky in beds like flocks of sheep at rest. It is often seen through breaks in lower C., moving differently. It may vanish or pass into Ci. or Ci.-st. Solar and lunar coronet appear in it. It occurs in warm dry weather, and between summer showers, and presages increased heat. Ci,-cu. very dense, round, and close, and with Cu.-st., presages a storm or thunder. In winter, it precedes a thaw and warm wet weather.
Cirro-stratus, or vane-cloud, consists of long, thin, horizontal clouds, with bent, or undulated edges. It often resembles shoals of fish, or has a barred appearance—the mackerel-backed sky. It alone, or with Ci.-cu., forebodes rain, snow, and storm. Waved Ci.-st. generally attends heat and thunder; it often forms nu extended shallow bed or thin veil, through which the sun and moon shine faintly. This variety ofteuest presents the finest solar and lunar halos, parhelias and paraselenes, and it is the surest prognostic of lain and snow.
Cainuto-stratus, or twain-cloud, is a Ci.-st. mixed with Cu. heaps, or a wide flat base surmounted by a bulky Cu., with fleecy protuberances or rocky and mountain masses. It resembles a thick-stemmed fungus, with a flat, anvil-shaped, or cirrose top. It is much denser than Cu., though the air is not dry enough to round off sharply its tops. It often forms vast banks of clond, with pverhanging masses. It is common towards night in dry.windy weather, when tit has.a leaden Ink., It generally arises from Cu. becoming denser, wider, and protruding in large irregular projections over the base. It tends to overspread the sky, and partly or wholly to become Ni., and to fall in showers. Cu.-st. is intermediate between clouds indicating fair, and those indicating rough, rainy weather, and attends sudden atmospheric changes. Distinct Cu.-st. forms before thun der. Cu.-st. increases the grandeur of mountain scenery, and drops on and envelops mountain-tops like a curtain.
Nimbus, or cumulo-cirro-stratus, the black rain-cloud, is a cloud or Mixed system of clouds, ending in showers of rain, snow, or hail. It is a dense, continuous, horizontal black or gray sheet, with fringed edges, a cap of Ci., and Cu. on the sides and below. Before rain, vast towering masses of Cu. often pass into Cu.-st., which, increasing in density, darkness, irregularity, and extent, become Ni. capped by Ci.-st. Thunder storms are always accompanied by ni. in its most perfect form.
The term scud has been applied to loose vapory fragments of C. driven by wind, and cumuli/us to shaggy cumuli.
The formation and height of C. vary with the quantity of vapor in the air, the course and height of air-currents, the climate, season, temperature, disposition, and extent of sea and land, and the height of land. Cloud-strata on the Pvrenees vary in average thickness from 1600 to 3,400 feet. The lower surfaces of considerable masses of clouds in all climates are probably 2,500 to 3,000 ft. above the earth. Remarkable cloud-rings prevail over the calm zones of the equator, and over those of Cancer and Capricorn. Clouds, viewed from above in bright sunshine by the aeronaut or mountaineer, appear as dense volumes of steam or masses of white cotton. Kaeintz regards the usual height of Ci. as 10,000 to 24,000 ft. ; Cu., 3,000 to 10,000; Ni., 1500 to 5,000; but Ci. may descend to 2,000 or 3,000 ft., and Ni. to within hundred ft. of the earth.
C. moderate the sun's rays during day, and the earth's radiation during night. They always exhibit positive or negative electricity, but of greatest tension in thunder-storms. They are the sources of the moisture required by plants; of the water of springs, lakes, and rivers; and of the polar, glacial, and winter snows, which cover temporarily or per manently parts of the earth.
In Britain, six or seven tenths of the sky is on an average daily obscured by clouds. There is most cloud in winter, and about midday, and least in May or June, and during night. Summer and autumn nights are freest of clouds. All the forms of C. may be seen in one day, often very much commingled.