CLOUDS are masses of fog, consisting of minute particles of water, often in a frozen state, floating in the atmosphere. When air saturated, or nearly so, with vapor, has its temperature lowered, either by ascending and becoming rarer, or by meeting a colder current, a portion of the vapor loses its gaseous form, and becomes condensed into minute specks of water. See EVAPORATION, DEW, RAIN, SNOW-LINE. A cloud, therefore, does not consist of vapor, in the proper sense of the word, but of water in the form of dust, as it were. How this water-dust is suspended in the atmos phere--why the particles do not descend as soon as formed, has never been satisfac torily explained. It has been assumed that the watery particles are hollow, like blown bubbles. Bht there is no proof of this; nor would the hollowness of the particles account for their floating, unless it could be shown that they must be filled with a gas lighter than the surrounding air. Prof. G. G. Stokes holds that they are prevented from falling mainly by the friction and drag of the air-particles, just as fine powders remain suspended in liquids of much less specific gravity than themselves. But, as sir J. Herschel says, rising and horizontal air-currents must also oppose the fall of C.; for at night, in the absence of rising currents, they often descend to and dissolve in lower and warmer levels. The conditions under which C. are formed, and afterwards deposited in rain, are more fully considered under EVAPOBATICN, DEW, RAIN, SNOW-LINE. The present article is confined to a description of the various kinds of C., and of the weather they indicate.
A general haze of precipitated vapor covering the sky; and coining down to the earth is termed a fog or mist; and the term cloud is usually confined to inasses of fog flott,ting in the higher regions, and not descending to the ground. They are mostly within a mile of the earth's surface; and none are more than 5 or 0 m. above it. They rise higher in the equatorial regions than towards the poles. C. spread and move with the wind in
varied, often grand forms; they arc generally disposed in beds parallel to the earth's sur face; vertical C. occur rarely, if at all.
Mr. Luke Howard's classification of C., proposed in 1802, into three primary forms —cirrus (Ci.), cumulus (Cu.), and stratus (St.); three intermediate—cirro-cumulus (Ci.-en.), cirrostratus (Ci.-st.), and cumulo-stratus (Cu.-st.); and one compound form, nimbus (Ni.) —has been universally adopted, and holds good in all climates and atmospheric con ditions.
Cirrus, or curl cloud, consists of parallel, curling, flexuous, diverging, and partly straight fibers, increasing in any or in all directions by elongation, branching, or addi tion of new fibers. It is the highest and least dense of C.; forms at least 3 m. above the earth; varies most in extent, direction, and shape; retains longest its varied outlines; and is the longest illuminated after sunset and before sunrise. It. resembles a mare's or cat's tail, a leek of hair, fine trellis-work, or thin silvery streaks, and it may cover all the sky. Cirri seem to arise from the mixing of parallel air-eurrents, or are the relics of dissolving clouds drawn out in fibers by wind. Threads and groups of Ci., during gentle wind after severe weather, presage serene settled weather. But after a long tract of fair days, whitish filaments or parallel bands of Ci. crossing the sky, with the ends converg ing by perspective in each horizon, and traveling longitudinally, though seemingly stationary, foretell a change to wet. Ci., being so high, must consist of minute snow crystals, whose refractions and reflections produce the halos, coronae, and mock suns and moons almost restricted to this cloud and its derivatives the Ci.-st. and Ci.-cu. The fibers often wave back and fore, or to and from each other. Ci., especially with fine tails, varying much in a few hours, presage rain or snow, and windy variable weather.