CLOTILDA. of her in the Luxembouri;, iuul a fine church in leer honor was built in I'aris between lS-10 and I80O. CLOUD, The. One of Percy Bysshe Shelley's best-known pociiis (1820). CLOUD, CLOUDINESS (AS. clinl, mass of rock, hillock, which a cloud often resembles). 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 iircs and the clouds of dust over the plains of India have an important temporary inlluence on local climate. In meteorology, the terms denote the moisture of the atmosphere precipitated from an invisible state of vapor into minute globular particles that float for a long time in the air. These particles are so small that they cannot descend rapidly through the ordinaiy atmosphere, even when perfectly still, on account of its vi.scous resistance or so-called internal lluid-friction. The gentlest ascending current or the slight ver- tical component of a nearly horizontal current surtices to keep the cloiuly 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 they 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 tlie 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 cloiul is the process called dynamic cool- ing, tirst expounded bv Espy. Kelvin. Rcye. aiul Peslin. and developed in detail by Bezold and Bigelow. Air expands when it is brought under lower barometric pressiire; it may be by beiiig pushed up over a mountain, or it may be by rising up because of its own buoyancy. In either case, the expansion takes place against the adjacent air, and presses the latter to one side. This opera- tion constitutes work done on the resisting air, and work involves the action of some force 51 CLOUD. which, in the i)resent case, is almost invariably the expansive force due to the heat that is latent in tlu' atnuisphen — viz. in the air and vaj)or combined, or the so-called thermal content. The work done in exjiansion is said to be d(me at thi^ expense of the internal heat of the air; (jr, heal is abstracted from the expanding air in order to do work on the air that is being pushed aside. Consecpiently, the expanding air grows cool in i)roportion to the work done. When it is thereby cooled to the dew-point, the vapor begins to condense ujjon dust particles as solid nuclei and forms liquid drops; this in- volves the giving up of a large amount of heat kuown as the latent heat of vaporization, which has to be lost by radiation from the di'op, 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 tipper parts of these clouds can be at such a low temperature as to contain snow or hail in place of water particles. Tile forms and the quantities of clouds, the direction 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 studj' 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 jirimary forms (cirrus, cumulus, stratus), and three interme- diate forms {cirro-cumulus, cirro-stratus, and cuiniilo-strntus) , and these have been almost universally adopted by modern ob.servers; but 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 be ob.served. Probably 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 various kinds of clouds. The many other modificntions and new terms that have been suggested are well com- pared and discussed in a memoir by Mr. H. H. Clayton, of the Blue Hill Meteorological Ob- servatorj', near Boston, Mass. Mean Heights and VELOaTiEB of Clouds at Washington. O.C., April. 1896, to March. 1897.. Cirrus Cirro-stratus CiiTO-cumuluB Alto-stratus .lto-fuimilus Strato-cumulus Nimbus Cumulus, dome top base Cumulo-Dimbus, top... " •• base.. Stratus MEAN HEIGHTS April to October to September March Meters 10.368 10,620 8,820 5,772 .■i.OSO 2,870 1.926 3.068 1.819 1.182 1.965 1.760 838 Meters 9..'ill 9,626 7.413 4.801 3,822 2.399 1,804 2,866 1.694 1.198 3,730 1,132 MEAN VELOCITIES EXTREME HEIGHTS Se^timtr ° M^a'ci.*" Maximum Minimum M. P. S. 30.3 26,9 23.4 17.6 16,5 10.5 8.6 7.0 i's'.'s M. P. S. 34.9 30.4 .33.4 21,3 21.1 15,1 11,9 10,9 2i','i Meters 17.182 16,144 15.411 16,562 10.167 7.285 4.022 5,242 15,903 Meters 5.354 5-.142 3.067 1.613 1.624 1,375 634 646 'i,a49
