Clouds, bodies of vapor in the atmosphere. From the surface of the earth and of the waters aqueous vapor is continually ascending into the atmosphere, where it remains in an invisible state so long as the air is not saturated with moisture. Its capacity to retain this vapor is limited, but varies with its temperature; the greater the warmth, the more the particles of vapor are expanded and carried into higher regions. Cold condenses the particles, their specific gravity is increased, and they appear in visible form. (See Dew.) Saturated to the utmost, the atmosphere cannot contain, it is estimated, more than 6 to 7 inches of water, diffused through it in an invisible state, at one time. But the diffusion of vapor is never uniform, and the temperature also varying in different portions of the atmosphere, there ensues the greatest diversity in the conditions of the moisture contained in these portions. Where it is not too abundant for the existing temperature, it is all dissolved, and the appearance is presented of the clear, blue, invisible ether.

Where the moisture is in excess for the temperature, it is seen in the form of clouds, thin and fleecy if the air is but slightly sursaturated, but dark and lowering if new accessions of moisture or reduction of temperature increase the difference between the moisture present and the capacity of the air to retain it. Thus the conditions are seen that cause frequent changes in the appearance of the clouds; and it is understood how clouds filled with moisture, like those gathered by the Etesian winds as they sweep over the Mediterranean, are dispersed in thin air as they strike the hot rays reflected from the burning sands of Sahara; and how those similarly charged with moisture by the sweeping of the trade winds across the Atlantic shed this in copious showers as they strike the cold summits of the Cordilleras. The formation of a cloud by cold and its dispersion by heat are beautifully exhibited at the Table mountain, Cape of Good Hope. As the wind from the southern ocean strikes the rocky slopes of the mountain and is diverted upward into the colder regions of the atmosphere, a dense white cloud is evolved, which, reaching but little above the mountain, spreads over its summit and is carried down the other side.

It marks the course of the wind, "plunging down with the violence of a cataract," as described by Sir John F. W. Iler-schel, "clinging close to the mural precipices that form a kind of background to Cape Town, which it fills with dust and uproar." But as it reaches the warmer regions below, the moisture soon resumes its invisible state, and the cloud is only seen covering the mountain and hanging down its precipitous sides, suggesting the idea of a huge white cloth, whence its name of "table cloth." There is much difference in the height of clouds. The mean height in winter may be stated as from 4,000 to 4,600 ft., and in summer from 10,000 to 14,000 ft.; but they often have greater altitudes. Gay-Lussac, when in a balloon at a height of 23,000 ft., observed cirrus clouds at a considerable distance above him. In Ethiopia M. d'Abadie saw storm clouds at a height of only 700 ft. above the earth. Dalton asserts that in small fleecy patches they have been seen at full five miles above the surface, sailing even over the highest summits of the Andes. But in cold regions the vapors are soon checked in their ascent by condensation.

Wherever arrested, if the weather be calm, they settle down by their greater relative gravity, undergoing with change of position changes of form and appearance, until they may be at last brought to the warmer airs below, and in these dissolving and disappearing, be again lifted into the colder strata to reassume the form of a cloud. Clouds are thus no permanent collections of the same particles of aqueous vapor, but spaces in the firmament in which successive portions of vapor in the atmosphere are continually presented in visible form, to disappear and be replaced by other portions. To account for the suspension of clouds various theories have been proposed. De Saussure considered the particles of vapor to be minute hollow vesicles filled with air, from 1/4500 to 1/2800 of an inch in diameter, and he has been supported by Halley, Kratzen-stein, and Brevais. One reason offered in favor of the vesicular theory is the fact that rainbows are not produced when the sun shines on clouds or steam, as it was thought they would if the particles were globules of water; but Sir J. Herschel offers the explanation that the globules may be as minute as the lengths of light undulations, and would not therefore, by refraction or reflection, form the solar spectrum.

It appears, moreover, that as long ago as 1847 ("Philosophical Transactions"), Dr. A. Waller disproved the existence of vesicles by projecting steam upon the surface of Canada balsam and examining the particles with a microscope, when it was found that they were not hollow. Assuming this to be a fact, the suspension of clouds in the air is not difficult of explanation when we reflect that, as Prof. Stokes has demonstrated, a globule of water 1/1000 of an inch in diameter falls through the air with a velocity of only "067 of an inch per second, a motion which is inappreciable when compared to the upward movement which the air generally has in large clouds. - The various forms in which clouds present themselves are described after the nomenclature introduced by Mr. Luke Howard in the "Askesian Lectures," 1802 (published in vols. xvi. and xvii. of the "Philosophical Magazine"). He recognizes three primary modifications, the cumulus, stratus, and cirrus; and intermediate between these, into which they blend, three other forms, the cumulo-stratus, cirro-stratus, and cirro-cumulus; and lastly, the nimbus, resulting from the others confusedly intermixed. - The cumulus is the summer day cloud.

About sunrise it is seen to collect by the gathering of the small specks of cloud, which are the scattered night mists, and which, accumulating, present the appearance of distant rounded hills covered with snow. In calm weather it is obviously formed from the columns of vapor that rise irregularly from the surface, invisible below, but brought into view above, as the particles reach the stratum of the atmosphere already at its dew point. There the vapors, arrested in their ascent, form piles of clouds of hemispherical shapes, the apparently flat bases of which mark the level where hygrometric saturation commences. As with the increasing warmth of the day more vapor is carried upward, these piles of clouds increase in height and density. They obscure the rays of the sun, by which, however, they may be dispersed before the decline of day, or they may be gathered and condensed by cool winds, preparatory to returning their moisture to the earth in the form of rain. - The stratus is the cloud of night and of winter, though not limited to these periods any more than the cumulus is to that of the day.

As the day cloud is produced by the ascending vapors, that of the night, called also the fall cloud, is, excepting fogs formed by exhalation, the result of their descent, their settling down in horizontal layers or strata. This form of cloud is at times suddenly produced, overspreading the heavens in a few minutes, by the temperature of the air falling by radiation or by diminished atmospheric pressure. It falls to a lower level than other clouds, creeping along the valleys at night in the form of mists and fog, and vanishing with the return of day. In winter it continues as an overhanging cloud sometimes for several successive days. The smoke fogs are one form of the stratus; at night settling down to a lower level, as if each smoky particle by radiation of heat collected dew, and sunk by increase of weight. - The cirrus is a cloud of feathery form, and in wisps of diverging fibres. It extends in long slender filaments, and again in parallel stripes from one extremity of the heavens to the other. Its appearance often in flexuous fibres has caused the names of curl cloud and mare's tail to be applied to it. No clouds are seen so elevated as the cirri.

From summits where one looks down upon the piles of cumuli, the fleecy cirri are seen floating as far off apparently in the blue ether as when observed from the base of the mountains. Their great elevation and feathery form suggest that the vapor exists in snowy flakes, and this is rendered more probable by the occurrence of those phenomena due to reflection and refraction, as halos, parhelia, etc, most usually in these clouds and the cirro-cumulus. Traces of halos, such as are produced by the refraction of light from frozen particles, may almost always be discovered in them by careful examination with the aid of a blackened mirror. The long filamentous forms of cirrus are evidently connected with the action of currents in the air, maintaining uniform temperature along their lines, and drawing out in these shapes the residues of dissolving clouds. Variations in these currents of temperature or direction impress new forms upon the cirri. In diverging fibres, which is the true cirrus form, the cloud is supposed to indicate changeable weather, in summer rain and wind, in winter frost or snow; if the fibres have for some time continued to point in one direction, a gale of wind is looked for from that quarter. - The cirro-cumulus is often produced by the cirrus descending to a lower level in the atmosphere, and its parallel bands breaking up into the shape of small cumuli.

It also appears independently of the previous existence of cirri. It floats at a high elevation, being often seen in the light of the moon through the scud and drift of the lower clouds, looking not unlike a flock at rest. Its spotted appearance has caused it to be commonly known as the "mackerel sky." - The cirro-stratus is characterized by its arrangement in long parallel lines of cloud in close proximity, straight or waving, and lying in horizontal strata. It is also seen in a long and narrow horizontal sheet, tapering toward its extremities. The name of wane cloud has been given to it, from the fibres of the cirrus waning or subsiding to produce it. Its prevalence indicates wind, rain, or snow. In the form of rows of little clouds curved in a peculiar manner, it is called the cymoid cirro-stratus, and is regarded as a sure indication of approaching storms. When seen as a thin hazy veil spreading over the sky, particularly to-Avard night, and obscuring the sun and moon, it is one of the most certain signs of approaching rain or snow. Virgil, in the Georgics, notices this obscuration of the sun behind a cloud soon after its rise, as sure to be followed by rain.

The refractions of the light of the sun and moon, producing halos and mock suns, often appear in this cloud. - The cumulo-stratus is formed by the passing of the cumulus into the nimbus, or rain cloud, though before the latter is perfected the cloud may be dispersed by evaporation, or turn back to the cumulus. It is also called the twain cloud, from the fact that two or more cumuli are often seen to join together to produce it. A single one is also sometimes observed to spread out laterally at top, till it overhangs the base in irregularly shaped protuberances. Such masses gather together, presenting a most imposing spectacle, as a thunder storm is approaching. The marginal protuberances are then seen, often shining with a strong silvery or golden light, contrasting finely with the darkness and density of the central portions of the cloud. Upon these the cirrus or cirro-stratus is sometimes formed by currents of air, causing the upper portion of the cloud to assume the curly forms of this variety, or spreading its long line horizontally across the summit. - The nimbus is that stage of cloud from which the rain falls.

It usually proceeds from the cumulo-stratus, which grows darker and denser, till its blackness and threatening aspect can no longer be mistaken as a sure sign of impending rain. The blackness gives place to a gray obscurity, an evidence of a change in the disposition of the aqueous particles, and the rain falls from the cloud, which is now a nimbus. The presence of its aqueous drops gives it the distinction among clouds of being the field upon which are displayed the beautiful colors of the rainbow. The nimbus having discharged its moisture, the various forms of cloud are again seen in their several places in the sky; the cirrus in some of its modifications in the upper regions, while the fragments of the nimbus are converted into thin cumuli, which are borne along by the light winds near the surface. But if these gather again into the form of cumulo-strati, rain is likely to return. - The nomenclature of Howard is generally adopted by meteorologists. Prof. Loomis objects to the introduction of the nimbus as a distinct form, considering that it does not differ from some of the other varieties, except in the fall of rain, which is not sufficient to give it a peculiar character; for clouds do not always undergo a decided change as rain begins to fall.

He remarks upon the occurrence, familiar to all who have lived in the vicinity of the great lakes, of a sheet of cloud continuing during a large part of the winter season to overcast the sky, from which snow is frequently emitted for days in succession, without change or apparent motion in the cloud itself. This variety, including all clouds which cover the heavens with a nearly uniform and unbroken sheet, he places in the division stratus, thus supplying a deficiency in the European classification, at least in its adaptation to American meteorology. ("Journal of Science," vol. xli., p. 325.) - Prof. Andre Poey, late director of the observatory at Havana, published a memoir in the report of the Smithsonian institution for 1870, in which he proposes a new classification, which includes only two classes or types in place of the three of Howard, rejecting the stratus on the ground that it is some other form viewed in perspective, or is no proper cloud, but a mist or hoar frost. The following tables show the comparison of the two classifications:

Cumulus.

Fig. 1. - Cumulus.

Stratus.

Fig. 2. - Stratus.

Cirrus.

Fig. 3. - Cirrus.

Nimbus.

Fig. 4. - Nimbus.

NOMENCLATURE OF HOWARD.

First type...

Cirrus.

Derivatives...

Cirro-stratus.

Cirro-cumulus.

Second type...............

Cumulus.

Derivative..

Cumulo-stratus.

Third type................

Stratus.

Derived from other types...

Nimbus.

NEW NOMENCLATURE OF POEY.

First type............

Cirrus.

Derivatives...

Cirro-stratus

Snow clouds.

Cirro-cumulus.

Pallio-cirrus.

Second type..

Cumulus.

Derivatives...

Pallio-cumulus.

Vapor clouds.

Fracto-cumulus.

Prof. Poey's principal reasons for making this classification are stated by himself as follows: "When certain clouds spread out uniformly over the whole face of the heavens and assume a gray or ash color, under which state rain may occur for hours or whole days, what name do we give them? They are not Howard's nimbus, as we conceive them and as they are generally described; they are neither stormy nor electrical; they yield only a fine and continuous rain. Under this stratum we see constantly other clouds of more or less extent, but always isolated, becoming lost in it and increasing its thickness. But just before this stratum begins to break up, and during this operation, we see these same formless fragments detach themselves and fly to other regions. This inferior stratum is not alone; for when its disruption is completed we see through it another stratum of clouds, whiter and less dense, which breaks up in its turn, and ends by disappearing in an opposite direction to that of the inferior stratum. Have we a name for this variety of cloud, so common in time of rain from the intertropical regions to higher latitudes, especially in winter during the fall of snow? Does Howard's term nimbus and his description of it answer for its designation? Certainly not.

We apply the name nimbus to the single storm cloud as well as to this inferior stratum, or to the united strata, and this without electrical manifestations. To this cloud I give the name of pallium. When the superior stratum is formed of cirrus, it constitutes the pallio-cirrus; and when the inferior stratum is formed of cumulus, it constitutes the pallio-cumulus. The fragments of clouds which differ entirely from the cumulus or cumulostratus are the fracto-cumulus." Whether Prof. Poey's classification is well founded time will decide. The propriety of rejecting the third class or type of Howard, the stratus, is rather questionable in the light of the opinions held by Prof. Loomis, some of which have been quoted, and also when we take into consideration the use Prof. Poey himself makes of the word stratum in the explanation of his theory. The name fracto-cumulus appears to be applicable to some forms of cloud not otherwise clearly denominated, such as we often see at sunset; but the placing of the gray vapor cloud, which Prof. Poey calls pallium, in the third division, stratus, by Prof. Loomis, would seem to supply the deficiency in Howard's classification with as near an approach to accuracy as the subject is perhaps capable of admitting. - An interesting feature in the phenomena of clouds is their gathering in the equatorial regions in a vast belt, which encircles the globe, and continues permanent and of nearly uniform breadth.

This belt covers the region of equatorial calms, or the "doldrums," and vibrates with this, as the seasons change, from one side of the equator to the other, its range being from lat. 5° S. to lat. 15° N. It has been observed of various widths, ranging from 60 to several hundred miles. The cloud belt is produced by the vapors gathered up and brought in by the N. E. and S. E. trade winds from each side of the equator, and diverted in the belt of calms, where they meet, into the upper regions of the atmosphere. There, continually fed by new supplies from below of heated air saturated with moisture, the cloud spreads in one vast body over several degrees of latitude, passing even beyond the margin of the calms, seeing that its rains stretch out on each side into the region of the trade winds. Borne upward into the cooler strata of the atmosphere, the heated airs shed their moisture, which falls in frequent torrents of rain. But immense volumes of vapor upon the upper surface of the cloud, exposed in the heat of the day to the direct rays of the sun, are retained in an invisible state, and thus are wafted away to reappear as clouds, and precipitate their waters upon distant portions of the earth.

This cloud belt serves another important purpose in protecting the torrid region it overshadows from the scorching heat of the sun; which, if continued without interruption, would in one season render the equa-torial belt a barren, uninhabitable waste. It is the 1ST. and S. vibration of the cloud ring that causes the fluctuations of the rainy seasons on the parallels of latitude within its range. When it has swung near to its extreme polar limit, the sky about the equator is clear, and the crust of the earth then begins to grow hot. "The dry season continues, the sun is vertical, and finally the earth becomes parched and dry; the heat accumulates faster than the air can carry it away; the plants begin to wither and the animals to perish. Then comes the mitigating cloud ring. The burning rays of the sun are intercepted by it. The place for the absorption and reflection, and the delivery to the atmosphere of the solar heat, is changed; it is transferred from the upper surface of the earth to the upper surface of the clouds." Clouds are thus not merely the gatherers and distributors of rain to all parts of the earth, but they are a curtain spread before the sun to intercept its fierce heat, and check at night the too rapid evaporation from the soil heated during the day by its rays.

Where they are not formed, and the surface is exposed to the sun, there we infer that water cannot be present, and the ground must be a parched desert. Such appears to be the condition of the side of the moon presented to the earth, no clouds ever being seen to obscure its disk. It has been shown by Tyndall that invisible vapor also has the power of absorbing radiant heat, and thus, by intercepting the fierce rays of the sun during the day, or the radiation of heat from the earth at night, of preventing, in atmospheres supplied with moisture, those great changes in temperature which take place in arid tropical regions. (See " Contributions to Molecular Physics in the Domain of Radiant Heat," New York, 1873.) - Little is known of the causes that produce the brilliant and varied colors often assumed by the sky, particularly at sunset. They are unquestionably, however, connected with the aqueous vapor contained in the atmosphere; and the reddish hue, the most common of all, is probably owing to the greater facility with which these rays are transmitted through the watery vesicles.

Reflected from the surface of distant hills, they even give to these a delicate roseate hue. - The electricity of the clouds is a subject for more especial reference in the articles Electricity and Lightning. Aqueous vapor being a better conductor of electricity than the dry air, the heavy clouds gather this force from the atmosphere around, some strata being positively and some negatively electrified. When separated by comparatively dry atmosphere, these clouds, or the clouds and the earth, are in the condition of a Leyden jar, ready to be discharged with sudden report when the non-conductor which separates them is broken through. As the clouds thicken and the watery particles more nearly approach each other, the electricity contained in the whole cloud gathers upon its surface, and at last acquiring sufficient tension to break through the non-conducting medium, the electrical equilibrium is established with the flash of lightning and the roar of thunder. The electrical condition of the clouds has been referred to as the cause of their being often gathered around the summits and sides of mountains, as if they were attracted to these, as light floating substances when electrified rush toward bodies in their vicinity.

But the phenomena explained in the early part of this article show that the low temperature prevailing in these localities is sufficient to account for the continual formation of a cloud, causing it to appear permanent, when it is but a process of condensation of new vapors brought on by the winds replacing those which are swept away and rendered invisible, as they dissolve in the warmer airs at a distance.