CLOUD, CLOUDINESS (AS. dud, inns's of rock, hillock, which a cloud often resemhles). In general, anything that obscures the vision through a clear atmosphere. as clouds of dust, smoke, or moisture. The clouds of smoke over cities and from forest fires and the clouds of dust over the plains of India have an important temporary influence on local climate. In meteorology, the terms denote the moisture of the atmosphere precipitated from an invisible state of vapor into minute glulailar particles that float for a long time in the air. These particles are so small that they cannot descend rapidly through the ordinary atmosphere, even when perfectly still. on account of its viscous resistance or so-called internal fluid-friction. The gentlest ascending current or the slight ver tical component of a nearly horizontal current suffices to keep the cloudy particles from falling to the ground.
The condensation of the invisible moisture of the air into particles of water cannot be ac complished in the free atmosphere without a decided reduction of temperature; and this may occur in three ways: (1) If air comes in contact with a cold solid, the latter may be covered with dew; but if two masses of warm-moist and cold-moist air come together, a slight condensa tion and haze or cloud may be formed where then mix with each other. These clouds by mixture have been extensively studied by Bril louin. (2) If moist air is cooled by radiation of heat, the coolest portions will soon fall to the temperature of the dew-point. and the vapor therein begin to become visible as a fog; these foggy particles radiate rapidly, thereby increas ing the coolness of the air and stimulating the formation of more fog. (3) But the principal method by which cooling is effected in order to form cloud is the process called dynamic cool ing. first expounded by Espy, Kelvin, Ileye, and Peslin, and ddiveloped in detail by Bezold and Bigelow. Air expands when it is brought under lower barometrie pressure; it may be by being pushed up over a mountain, or it may be by rising up because of its own buoyancy. In either case, the expansion tales place against the adjacent air, and presses the latter to one side. This opera don constitutes work done on the resisting air. and work involves the action of some force '•fuel, in the present case, i* almost invariably the expansive force due to the heat that is latent in the atmosphere—viz. in the air and vapor combined, or the so-called thermal content.
The work done in expansion is said to he done at the expense of the internal heat of the air; or, heat is abstracted from the expanding air in order to do work on the air that is being pushed aside. Consequently, the expanding air grows cool in proportion to the work done. When it is thereby cooled to the low-point. the
vapor begins to condense upon dust particles as solid nuclei and forms liquid drops; this in volves the giving up of a large amount of heat known as the latent heat of vaporization, which has to be lost by radiation front the drop, wherefore the cooling of the mass becomes much slower. In this process of condensation, a given amount of cooling requires a much larger amount of expansion, and therefore of work done, than in the previous stage before cloudy condensation began. This stage is illustrated in the formation of the cumulus clouds seen With showers or thun der storms or especially in hail weather. On these occasions, the cumulus clouds grow rapidly up ward to great heights. The upper parts of these clouds can be at such a low temperature as to contain snow or hail in place of water particles. The forms and the quantities of clouds, the direetion and velocity of their movements, the apparent changes they undergo, and many other peculiarities have for a century past formed an item of increasing importance in the study of meteorology.
The first step toward simplifying and harmon izing the old meteorological records was taken by Luke Howard, in 1802, in his proposed classi fication of clouds into three primary forms (cirrus, cumulus, stratus), and three interme diate forms ( cirro-cn alas, ehm-stratus, and rumulo-st rat us) , awl these have been almost universally adopted by modern observers; hut experience has shown that they do not give a sufficient range of terms to enable one easily to classify and describe all the varieties of clouds that are to lie observed. no simple sys tem of nomenclature would suffice to do this, and Cleveland Abbe has proposed for special students a system of symbols based on the methods of formation of the Nylons kinds of clouds. The many other modifications and new terms that have been suggested arc well com pared and discussed in a memoir by Mr. H. II. Clayton, of the Blue Hill \leteoro1ogical Ob servato•y, near Boston, Mass.
The descriptive abbreviations and classifica tion introdueed by the International Aleteorolog ical Congress, held at Munich in ]SIH, is that which is now adopted more or less completely by all the national weather bureaus. Systematic observations upon the heights and movements of all clouds were made at Many stations th•ough out the world in concert front May 1, 1S96, to July I, 1S97. From these observations a general idea of the heights of the respective kinds of clouds may be gathered. as shown in the preced ing list, compiled from Professor Bigelow's Re port on International Clow/quark of the United States Weather Bureau, page 20. The titles and descriptions of the clouds are as used by the International Committee.