various forms op clouds—the cirrus, or curl-cloud—the cumulus, or stacken-cloud—the stratus, or fall-cloud—the cirro-cumulus, or sonder-cloud—the cirro-stratus, or wane-cloud—the cumulo-stratus, or twain-cloud—the nimbus, or rain-cloud—arrangement of rain-clouds—appearances of a distant shower—scud—cause of rain—formation of clouds—mists—heights of clouds—appearance of the sky above the clouds.

Many persons are apt to suppose that the clouds are among the most fitful and irregular appearances in the world; fleeting and unstable in their nature, uncertain in their forms, apparently subject to no fixed laws, and obedient neither to times nor seasons. Attentive observers, however, have proved that the beauty and harmony which are everywhere found to prevail in nature when rightly understood, can also be traced, even in the clouds. Although very much still remains to be discovered respecting them, yet it is found that, like all the other natural productions, they admit of being arranged and classified. So obvious was this to persons whose interest it is to observe the weather, that, long before scientific men had studied the subject, country people had noticed the different forms of clouds, and had learned to distinguish them by different names.

The first scientific man who made the clouds the object of his particular study, was Luke Howard, who, from an attentive consideration of their forms and appearances, found that they might all be arranged under three simple or primary forms, namely:—

1. The Cirrus—so called from its resemblance to a curled lock of hair. (Figures, 1, 2; page 77.)

2. The Cumulus, from the heaped appearance presented by the convex masses which form this cloud. (Figure 7.)

3. The Stratus, from its spreading out horizontally in a continuous layer, and increasing from below. (Figure 10.)

These three primary forms are subject to four modifications:—

The first is the Cirro-cumulus, consisting of small roundish and well-defined masses, in close horizontal arrangement. (Figure 3.)

Various forms of clouds

The second is the Cirro-Stratus, and the masses which compose it are small and rounded, but thinned off towards a part, or towards the whole of their circumference. They are sometimes separate, and sometimes in groups. (Figures 4, 5, 6.)

The third is the Cumulo-Stratus, which is made up of the cirro-stratus blended with the cumulus. (Figure 8.)

The fourth is the Cumulo-Cirro-Stratus, or Nimbus. This is the true rain-cloud, or system of clouds from which rain is falling. (Figure 9.)

The term modification applies to the structure or manner in which a given mass of cloud is made up, and not to its precise form or size, which in most clouds varies every instant. Mr. Howard remarks, that it may be at first difficult to distinguish one modification from another, or to trace the narrow limits which sometimes separate the different modifications; but a moderate acquaintance with the subject will soon enable any one to point out the various forms, and to a great extent to judge of the state of the weather by them. In order, therefore, to assist the reader in gaining a certain amount of knowledge on this interesting subject, it may be useful to state more fully the various phenomena of the different forms of clouds already enumerated.

The Cirrus, or curl-cloud

The Cirrus occurs in very great variety, and in some states of the air is constantly changing. It is the first cloud that appears in serene weather, and is always at a great height. The first traces of the cirrus are some fine whitish threads, delicately-pencilled on a clear blue sky; and as they increase in length others frequently appear at the sides, until numerous branches are formed, extending in all directions. Sometimes these lines cross each other and form a sort of delicate net-work. In dry weather the cirrus is sharp, defined, and fibrous in texture, the lines vanishing off in fine points. When the air is damp this cloud may be seen in the intervals of rain, but is not well defined, and the lines are much less fibrous. Such cirri as these often grow into other varieties of cloud, and are frequently followed by rain.

The cirrus may last a few minutes only, or continue for hours. Its duration is shortest when near other clouds. Although it appears to be stationary, it has some connexion with the motions of the atmosphere; for whenever, in fair weather, light variable breezes prevail, cirri are generally present. When they appear in wet weather, they quickly pass into the cirro-stratus.

According to Dalton, these clouds are from three to five miles above the earth’s surface. When viewed from the summits of the highest mountains they appear as distant as from the plains. Another proof of their great height is, their continuing to be tinged by the sun’s rays in the evening twilight with the most vivid colours, while the denser clouds are in the deepest shade.

The cirrus appears to be stationary; but, on comparison with a fixed object, it will sometimes be found to make considerable progress.

THE CUMULUS, OR STACKEN-CLOUD.

“And now the mists from earth are clouds in heaven:
Clouds, slowly castellating in a calm
Sublimer than a storm; while brighter breathes
O’er the whole firmament the breadth of blue,
Because of that excessive purity
Of all those hanging snow-white palaces,
A gentle contrast, but with power divine.”

The Cumulus is a day cloud; it usually has a dense, compact appearance, and moves with the wind. In the latter part of a clear morning a small irregular spot appears suddenly at a moderate elevation. This is the nucleus or commencement of the cloud, the upper part of which soon becomes rounded and well defined, while the lower forms an irregular straight line. The cloud evidently increases in size on the convex surface, one heap succeeding another, until a pile of cloud is raised or stacked into one large and elevated mass, or stacken-cloud, of stupendous magnitude and beauty, disclosing mountain summits tipped with the brightest silver; the whole floating along with its point to the sky, while the lower surface continues parallel with the horizon.

The Cumulus, or stacken-cloud

When several cumuli are present, they are separated by distances proportioned to their size: the smaller cumuli crowding the sky, while the larger ones are further apart. But the bases always range in the same line; and the increase of each cloud keeps pace with that of its neighbour, the intervening spaces remaining clear.

The cumulus often attains its greatest size early in the afternoon, when the heat of the day is most felt. As the sun declines, this cloud gradually decreases, retaining, however, its characteristic form till towards sunset, when it is, more or less, hastily broken up and disappears, leaving the sky clear as in the early part of the morning. Its tints are often vivid, and pass one into the other in a most pleasing manner, during this last hour of its existence.

This cloud accompanies and foretells fine weather. In changeable weather it sometimes evaporates almost as soon as it is formed; or it appears suddenly, and then soon passes off to some other modification.

In fair weather this cloud has a moderate elevation and extent, and a well-defined rounded surface. Before rain it increases more rapidly than at other times, and appears lower in the atmosphere, with its surface full of loose fleeces.

The formation of large cumuli to leeward, in a strong wind, indicates the approach of a calm with rain. When they do not disappear or subside about sun-set, but continue to rise, thunder is to be expected in the night.

Independently of the beauty and magnificence which this description of cloud adds to the face of nature, it serves to screen the earth from the direct rays of the sun; by its multiplied reflections to diffuse and, as it were, economise the light; and also to convey immense stores of vapour from the place of its origin to a region in which moisture may be wanted.

THE STRATUS, OR FALL-CLOUD.

As the Cumulus belongs to the day, so does the Stratus to the night. It is the lowest of all the clouds, and actually rests upon the earth, or the surface of water. It is of variable extent and thickness, and is called Stratus, a bed or covering. It is generally formed by the sinking of vapour in the atmosphere, and on this account has been called Fall-cloud. It comprehends all those level, creeping mists, which, in calm evenings, spread like an inundation from the valleys, lakes, and rivers, to the higher ground. [85] But on the return of the sun the beautiful level surface of this cloud begins to put on the appearance of cumulus, the whole, at the same time, rising from the ground like a magnificent curtain. As the cloud ascends, it is broken up and evaporates or passes off with the morning breeze. The stratus has long been regarded as the harbinger of fine weather; and, indeed, there are few days in the year more serene than those whose morning breaks out through a stratus.

The Stratus, or fall-cloud

THE CIRRO-CUMULUS, OR SONDER-CLOUD.

The cirrus having continued for some time increasing or stationary, usually passes either to the cirro-cumulus or to the cirro-stratus, at the same time descending to a lower station in the atmosphere.

The Cirro-cumulus is formed from a cirrus, or a number of small separate cirri, passing into roundish masses, in which the extent of the cirrus is no longer to be seen. This change takes place either throughout the whole mass at once, or progressively from one extremity to the other. In either case the same effect is produced on a number of neighbouring cirri at the same time, and in the same order. It appears, in some instances, to be hastened by the approach of other clouds.

The Cirro-Cumulus, or sonder-cloud

The cirro-cumulus forms a very beautiful sky, exhibiting sometimes numerous distinct beds of small connected clouds floating at different heights. It is frequent in summer, and accompanies warm, dry weather. On a fine summer’s evening the small masses which compose this cloud, are often well defined, and lying quite asunder, or separate from one another; and on this account the term sonder-cloud has been applied to this modification. The whole sky is sometimes covered with these small masses. They are occasionally, and more sparingly, seen in the intervals of showers, and in winter.

Bloomfield, in the following beautiful lines, has noticed the appearance of the sonder-cloud:—

“For yet above these wafted clouds are seen
(In a remoter sky still more serene)
Others, detach’d in ranges through the air,
Spotless as snow, and countless as they’re fair;
Scatter’d immensely wide from east to west,
The beauteous semblance of a flock at rest:
These, to the raptur’d mind, aloud proclaim
The mighty Shepherd’s everlasting name.”

This cloud may either evaporate or disappear, or it may pass to the cirrus, or sink lower and become a cirro-stratus. In stormy weather, before thunder, a cirro-cumulus often appears, composed of very dense and compact round bodies, in very close arrangement. When accompanied by the cumulo-stratus, it is a sure indication of a coming storm.

THE CIRRO-STRATUS, OR WANE-CLOUD.

This cloud appears to be formed from the fibres of the cirrus sinking into a horizontal position, at the same time that they approach each other sideways. This cloud is to be distinguished by its flatness and great horizontal extension, in proportion to its height; a character which it always retains, under all its various forms. As this cloud is generally changing its figure, and slowly sinking, it has been called the wane-cloud. A collection of these clouds, when seen in the distance, frequently give the idea of shoals of fish. Sometimes the whole sky is so mottled with them, as to obtain for it the name of the mackerel-back sky, from its great resemblance to the back of that fish. Sometimes they assume an arrangement like discs piled obliquely on each other. But in this, as in other instances, the structure must be attended to rather than the form, for this varies much, presenting, at times, the appearance of parallel bars or interwoven streaks, like the grain of polished wood. It is thick in the middle and thinned off towards the edge.

The Cirro-Stratus, or wane-cloud

These clouds precede wind and rain. The near or distant approach of a storm may often be judged of from their greater or less abundance and duration. They are almost always to be seen in the intervals of storms. Sometimes the cirro-stratus, and the cirro-cumulus, appear together in the sky, and even alternate with each other in the same cloud, presenting many curious changes; and a judgment may be formed of the weather likely to ensue, by observing which prevails at last.

The cirro-stratus most frequently forms the solar and lunar halo. Hence the reason of the prognostics of bad weather commonly drawn from the appearance of halos.

THE CUMULO-STRATUS, OR TWAIN-CLOUD.

The Cumulo-Stratus, or twain-cloud

This is a blending of two kinds of cloud (hence the name of twain-cloud,) and it often presents a grand and beautiful appearance, being a collection of large fleecy clouds overhanging a flat stratum or base. When a cumulus increases rapidly a cumulo-stratus frequently forms around its summit, resting thereon as on a mountain, while the former cloud continues to be seen, in some degree, through it. This state of things does not continue long. The cumulo-stratus speedily becomes denser and spreads, while the upper part of the cumulus extends likewise, and passes into it, the base continuing as it was. A large, lofty, dense cloud is thus formed which may be compared to a mushroom with a very thick, short stem. The cumulo-stratus, when well formed and seen singly, and in profile, is quite as beautiful an object as the cumulus. Mr. Howard has occasionally seen specimens constructed almost as finely as a Corinthian capital; the summit throwing a well-defined shadow upon the parts beneath. It is sometimes built up to a great height. The finest examples occur between the first appearance of the fleecy cumuli and the commencement of rain, while the lower atmosphere is comparatively dry, and during the approach of thunder storms. The appearance of the cumulo-stratus, among ranges of hills, presents some interesting phenomena. It appears like a curtain dropping among them and enveloping their summits; the hills reminding the spectator of the massy Egyptian columns which support the flat-roofed temples of Thebes. But when a whole sky is crowded with these clouds, and the cumulus rises behind them, and is seen through the interstices, the whole, as it passes off in the distant horizon, presents to the fancy mountains covered with snow, intersected with darker ridges, lakes of water, rocks and towers. Shakspeare seems to have referred to this modification in the well-known lines:—

“Sometimes we see a cloud that’s dragonish;
A vapour, sometimes, like a bear or lion,
A towered citadel, a pendent rock,
A forked mountain, a blue promontory,
With trees upon ’t that nod unto the world,
And mock our eyes with air.—
That which is now a horse, even with a thought
The rack dislimns, and makes it indistinct
As water is in water.

The distinct cumulo-stratus is formed in the interval between the first appearance of the fleecy cumulus and the commencement of rain, while the lower atmosphere is yet dry; also during the approach of thunder storms when it has frequently a reddish appearance. Its indistinct appearance is chiefly in the longer or shorter intervals of showers of rain, snow, or hail.

THE CUMULO-CIRRO-STRATUS; NIMBUS OR RAIN-CLOUD.

Clouds, in any one of the preceding forms, at the same degree of elevation, or two or more of these forms at different elevations, may increase and become so dense as completely to obscure the sky; this, to an inexperienced observer, would seem to indicate the speedy commencement of rain. But Mr. Howard is of opinion that clouds, while in any of the states above described, never let fall rain.

Before rain the clouds always undergo a change of appearance, sufficiently remarkable to give them a distinct character. This appearance, when the rain happens overhead, is but imperfectly seen; but from the observations of aËronauts, it appears that whenever a fall of rain occurs, and the sky is at the same time entirely overcast with clouds, there will be found to exist another stratum of clouds at a certain elevation above the former. So, also, when the sky is entirely overcast and rain is altogether or generally absent, the aËronaut, upon traversing the canopy immediately above him, is sure to enter upon an upper hemisphere either perfectly cloudless or nearly so. These remarks were, we believe, first made by Mr. M. Mason, and he states that they have been verified during many hundred ascents.

In October, 1837, two ascents were made by Mr. Mason, which well illustrate what has been said. On the 12th, “the sky was completely overspread with clouds, and torrents of rain fell incessantly during the whole of the day. Upon quitting the earth, the balloon was almost immediately enveloped in the clouds, through which it continued to work its way upwards for a few seconds. Upon emerging at the other side of this dense canopy, a vacant space, of some thousand feet in breadth, intervened, above which lay another stratum of a similar form and observing a similar character. As the rain, however, still continued to pour from this second layer of clouds, to preserve the correctness of the observation, a third layer should, by right, have existed at a still further elevation; which, accordingly, proved to be the case. On the subsequent occasion of the ascent of the same balloon, (October 17th,) an exactly similar condition of the atmosphere, with respect to clouds, prevailed; unaccompanied, however, with the slightest appearance of rain. No sooner had the balloon passed the layer of clouds immediately above the surface of the earth, than, as was anticipated, not a single cloud was to be found in the firmament beyond; an unbroken expanse of clear blue sky everywhere embracing the frothy plain that completely intercepted all view of the world beneath.”

Mr. Howard had not the advantages of a balloon to assist his observations. He has noticed that during rain and before the arrival of the denser and lower clouds, or through their interstices, there exists, at a greater height, a thin light veil or a hazy appearance. When this has considerably increased, the lower clouds are seen to spread till they unite in all points and form one uniform sheet. The rain then commences, and the lower clouds arriving from the windward, move under this sheet and are successively lost in it. When the latter cease to arrive, or when the sheet breaks, letting through the sun-beams, every one’s experience teaches him to expect that the rain will abate or leave off.

But there often follows an immediate and great addition to the quantity of cloud. At the same time the darkness becomes less, because the arrangement, which now returns, gives free passage to the rays of light; the lower broken clouds rise into cumuli, and the upper sheets put on the various forms of the cumulo-stratus, sometimes passing to the cirro-cumulus.

The various phenomena of the rain-cloud are best seen in a distant shower. If the cumulus be the only cloud at first visible, its upper part is seen to become tufted with cirri. Several adjacent clouds also approach and unite at its side. The cirri increase, extending upwards and sideways, after which the shower is seen to commence. At other times, the cirro-stratus is first formed above the cumulus, and their sudden union is attended with the production of cirri and rain. In either case the cirri spring up in proportion to the quantity of rain falling, and give the cloud a character by which it is easily known at great distances, and which has long been called by the name of nimbus.

When one of these arrives hastily with the wind, it brings but little rain, and frequently some hail or driven snow.

Since rain may be produced and continue to fall from the slightest obscuration of the sky by the nimbus, while a cumulus or a cumulo-stratus, of a very dark and threatening aspect, passes on without discharging any until some change of state takes place; it would seem as if nature had destined the latter as reservoirs, in which water is collected from extensive regions of the air for occasionally irrigating particular spots in dry seasons; and by means of which it is arrested, at times, in its descent in wet ones.

Although the nimbus is one of the least beautiful of clouds, it is, nevertheless, now and then adorned by the splendid colouring of the rainbow, which can only be seen in perfection when the dark surface of this cloud forms for it a background.

The small ragged clouds which are sometimes seen sailing rapidly through the air, are called scud. They consist of portions of a rain-cloud, probably broken up by the wind, and are dark or light according as the sun shines upon them. They are the usual harbingers of rain, and, as such, are called by various names, such as messengers, carriers, and water-waggons.

In attempting to explain the production of clouds and rain, it is necessary to observe that the subject is beset with difficulties—the discussion of which does not belong to this little volume; but the following notice of Dr. Hutton’s theory may not be out of place.

It has been already stated, that the air supplies itself with moisture from the surface of the waters of the earth. This it continues to do at all temperatures, until it is so charged with vapour that it cannot contain any more. The air is then said to be saturated. Now, the quantity of moisture which a given bulk of air can contain, depends entirely upon the temperature of the air for the time being. The higher the temperature of the air the greater will be the quantity of vapour contained in it; and, although it may be perfectly invisible to the eye, on account of the elasticity which the heat imparts to it, yet it can easily be made visible by subtracting a portion of the heat. If, for example, a glass of cold water be suddenly brought into a warm room, moisture from the air will be condensed upon the outside of the glass in the form of dew. A similar change is supposed to take place when two currents of air having different temperatures, but both saturated with vapour, are mingled together; an excess of vapour is set free, which forms a cloud or falls down as rain. If the currents continue to mingle uniformly, “the clouds soon spread in all directions, so as to occupy the whole horizon; while the additional moisture, incessantly brought by the warmer current, keeps up a constant supply for condensation, and produces a great and continued deposition of moisture in the form of rain. By degrees, the currents completely intermingle, and acquire a uniform temperature; condensation then ceases; the clouds are re-dissolved; and the whole face of nature, after being cooled and refreshed by the necessary rain, is again enlivened by the sunshine, thus rendered still more agreeable by its contrast with the previous gloom.”

If the cloud, produced by the mingling of two differently heated currents of moist air, happen to form in the upper regions of the sky, it may be heavier than its own bulk of air, and will consequently begin to sink. Should the atmosphere near the earth be less dense than the cloud, the latter will continue to descend till it touches the ground, where it forms a mist. If the vapour has been condensed rapidly and abundantly, the watery particles will form rain, hail, or snow, according to the temperature of the air through which they pass. But it may happen that the cloud, in descending, arrives in a warmer region than that in which it was formed: in this case, the condensed moisture may again become vapour, and ascend again to a region where condensation may again take place.

Mr. Daniell’s explanation of the formation of rain differs from the above in some of its particulars, which are not sufficiently elementary to be given here; but it may be instructive to give a few of Mr. Howard’s illustrations respecting the formation of the various clouds. If hot water be exposed to cool air, it steams—that is, the vapour given off from the surface is condensed in mixing with the air; and the water thus produced appears in visible particles, the heat of the vapour passing into the air. This effect may be seen about sunrise, in summer, on the surface of ponds warmed by the sun of the previous day, and also with water newly pumped from a well. But the small cloud formed in these instances usually disappears almost as soon as formed, the air being too dry to allow it to remain. But in the wide regions of the atmosphere the case is different, on account of the vast supply of vapour, and the ascent and descent of the cloud to regions which allow it to remain tolerably permanent. In the fine evenings of autumn, and occasionally at other seasons, mists appear suddenly in the valleys, gradually filling these low places, and even rising to a certain height, forming a foggy atmosphere for the following day. These collections of visible vapour resting on the earth, and often cut off so as to form a level surface above, so nearly resemble a sheet of water, as to have been occasionally mistaken for an inundation, the occurrence of the previous night. Such is the origin and appearance of the stratus: it constitutes the fog of the morning, and sometimes, as at the approach of a long frost, occupies the lower atmosphere for several days. But the sun, we will suppose, has broken through and dissipated this obscurity, and cleared the lower air. On looking up to the blue sky, we see some few spots showing the first formation of a cloud there: these little collections increase in number, and become clouds, heaped, as it were, on a level base, and presenting their rounded forms upwards; in which state they are carried along in the breeze, remaining distinct from each other in the sky. This is the cumulus, or heap.

By and by, if the clouds continue to form, and enough vapour is supplied from above, these heaps are seen to grow over their base like a mushroom or cauliflower. Perhaps a flat top is seen forming separately, and this afterwards joins the simple heap of cloud; or the flat forms and the heaps become mixed irregularly among each other, occupying the spaces everywhere, till the sky becomes overcast, and presents the usual appearance of dense clouds. This is the cumulo-stratus, or heaped and flat cloud. It is not productive of rain, and it forms, both in summer and in winter, the common scenery of a full sky.

On examining minutely the higher regions of the air, especially after the sky has been clear for some time, the spectator will probably see the cirrus descending from above in the form of threads or locks and feathers, which go on increasing until they fill the sky. They are more commonly seen above the two former kinds, which float upon the clear air below. On continuing to watch the cirri, they will be seen to pass to the intermediate form of cirro-cumulus, consisting of smaller rounded clouds attached to each other, or simply collected together in a flat aggregate, and forming the mottled or dappled sky.

The cumulo-stratus is more dense and continuous in its structure; thick in the middle, and thinned off towards the edges. Over-head it is a mere bed of haze, more or less dense. In the horizon, when seen sideways, it often resembles shoals of fish, as already noticed; but it is liable to put on the most ragged and patchy appearances, making a very ugly sky.

The nimbus, or rain cloud, is seen to the greatest advantage in profile, in the horizon, and at a great distance, when it often resembles a lofty tower raised by its greater height to a conspicuous place among the dark threatening clouds, and catching the sun’s last rays upon its broad summit and sides. In its nearer approach, it may always be known by being connected below with an obscurity caused by the rain it lets fall, and which reaches down to the horizon.

In ascending from the lower valleys to the tops of lofty mountains, clouds may be traced through six modifications, the cirrus being seen from the loftiest summits, while the other forms are only skirting the sides of the mountains. Mr. Mason remarks, that clouds occasionally lie so low, that before the balloon seems to have entirely quitted the earth, it has been received between their limits, and entirely enveloped within their watery folds. Clouds, on the contrary, are sometimes at such a height, that the balloon either never comes into contact with them at all, or, if it passes through one layer, the aËronaut continues to behold another occupying a still remoter region of the skies above.

As a general rule, it is stated that the natural region of clouds is a stratum of the atmosphere lying between the level of the first thousand feet, and that of one removed about ten thousand feet above it. Of course it is not supposed but that clouds are occasionally found on both sides of the bounds here assigned to them; the mist occupies the lowest valleys, while, on the other hand, long after the aËronaut has attained the height of ten thousand feet, some faint indications of clouds may still be seen partially obscuring the dark blue vault above him. As he continues to ascend, the blue of the sky increases in intensity; and should a layer of clouds shut out all view of the earth, “above and all around him extends a firmament dyed in purple of the intensest hue; and from the apparent regularity of the horizontal plane on which it rests, bearing the resemblance of a large inverted bowl of dark blue porcelain standing upon a rich Mosaic floor or tesselated pavement. Ascending still higher, the colour of the sky, especially about the zenith, is to be compared with the deepest shade of Prussian blue.”

Various forms of hail-stones