The term _modification_ applies to the structure or manner in which a given ma.s.s 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 a.s.sist 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.
[Picture: 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 pa.s.s 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 c.u.mULUS, 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 c.u.mulus 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 ma.s.s, 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.
[Picture: The c.u.mulus, or stacken-cloud]
When several c.u.muli are present, they are separated by distances proportioned to their size: the smaller c.u.muli 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 s.p.a.ces remaining clear.
The c.u.mulus 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 pa.s.s 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 pa.s.ses 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 c.u.muli 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 c.u.mulus 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 c.u.mulus, 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 pa.s.ses 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.
[Picture: The Stratus, or fall-cloud]
THE CIRRO-c.u.mULUS, OR SONDER-CLOUD.
The cirrus having continued for some time increasing or stationary, usually pa.s.ses either to the cirro-c.u.mulus or to the cirro-stratus, at the same time descending to a lower station in the atmosphere.
The Cirro-c.u.mulus is formed from a cirrus, or a number of small separate cirri, pa.s.sing into roundish ma.s.ses, in which the extent of the cirrus is no longer to be seen. This change takes place either throughout the whole ma.s.s 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.
[Picture: The Cirro-c.u.mulus, or sonder-cloud]
The cirro-c.u.mulus 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 ma.s.ses 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 ma.s.ses. 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 pa.s.s to the cirrus, or sink lower and become a cirro-stratus. In stormy weather, before thunder, a cirro-c.u.mulus often appears, composed of very dense and compact round bodies, in very close arrangement. When accompanied by the c.u.mulo-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 a.s.sume 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.
[Picture: 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-c.u.mulus, 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 c.u.mULO-STRATUS, OR TWAIN-CLOUD.
[Picture: The c.u.mulo-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 c.u.mulus increases rapidly a c.u.mulo-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 c.u.mulo-stratus speedily becomes denser and spreads, while the upper part of the c.u.mulus extends likewise, and pa.s.ses 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 c.u.mulo-stratus, when well formed and seen singly, and in profile, is quite as beautiful an object as the c.u.mulus. 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 c.u.muli and the commencement of rain, while the lower atmosphere is comparatively dry, and during the approach of thunder storms. The appearance of the c.u.mulo-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 ma.s.sy Egyptian columns which support the flat-roofed temples of Thebes. But when a whole sky is crowded with these clouds, and the c.u.mulus rises behind them, and is seen through the interstices, the whole, as it pa.s.ses 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_ c.u.mulo-stratus is formed in the interval between the first appearance of the fleecy _c.u.mulus_ 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 c.u.mULO-CIRRO-STRATUS; NIMBUS OR RAIN-CLOUD.