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If the globules of water which constitute a cloud, descend, in consequence of their weight, and come once more within the influence of an elevated temperature, the aqueous vapor necessarily becomes again invisible. In this way, the under surface of a stratum of clouds becomes nearly parallel, or rather concentric, with the surface of the sub-adjacent landscape over which it floats. Above this first range of clouds the temperature may still be considerably higher, and hence another large body of air must be passed through, before a temperature sufficiently low be arrived at, to cause a second deposition of clouds.

M. Fresnel ingeniously supposes that the air contained between the minute globules of vapor, or the very fine crystals of snow, which form a mass of clouds, is always of a higher temperature than the surrounding clear air. He supports this opinion on the well known facts, already alluded to, that the rays of the sun will pass throngh the air without heating it, unless the air be in contact with water, land, or some other reflecting object. The cloud accordingly forms such a body as will stop the sun's rays, and force them to warm, not only the air in external contact with it, but all the air in its interstices. It follows, therefore, that though the mass of waters in a cloud be heavier than the surrounding air, the warmer air in the interior of the cloud buoys it up, and causes it to float.*

M. Gay Lussac, on the other hand, refers the mounting of clouds in the air to the impulsion of the ascending currents, which result from the difference of temperature between the surface of the earth and the air in elevated regions.

The formation of clouds may be observed with most advantage in Alpine countries, as they are there so frequently produced under the eye, upon the sides or the summits of mountains, by the condensation of the vapor in the sheet of air immediately over them. A mountain cloud is at first of but small extent, but it enlarges insensibly, and is swept by the winds into the bosom of the air, where it either meets and unites with others, or various tufts of these are scattered over the sky.' These aërial groups appear, while drifting through the sky, to avoid dashing themselves upon the mountain peaks in their course, and, as if endowed with instinctive repulsion, they bound over the crest of a mountain in a concentric curve, and slide down into the valley on the other side. The French naturalists, with much plausibility, ascribe this beautiful phenomenon to electricity. M. Bory de St. Vincent thinks, that, when small tufts of cloud are carried towards the sides or the summit of a mountain, they move with less rapidity than the force (wind) which moves them, and this force consequently arriving sooner at the obstacle, is reflected, and meets and checks the cloud in its progress.

The mean height of the clouds may be conceived by the following extract from Mr. Leslie. “ We shall not err much, if we estimate the position

Annales de Chim. et de Phys. xxi. 260.

of extreme humidity at the height of two miles at the pole, and four miles and a half under the equator, or a mile and a half beyond the limit of congelation. This range is nearly parallel to the curve of perpetual congelation in the polar regions, but bends nearer to it in approaching the equatorial parts.

CLASSIFICATION OF CLOUDS.

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Infinitely diversified as the forms of clouds may appear to be, correct observers have stated that they may all be comprised in seven modifiations. Names and definitions have been given to these by Mr. Howard and Mr. Forster. By this classification and nomenclature their appearances may be noted down and transmitted to contemporary and future observers, for the purposes of comparison and record. A great advance has consequently been made in the perspicuous description which has succeeded to the vague and unintelligible generalities of preceding ages. Mr. Howard's' names are in Latin; to them we annex Mr. Forster's English nomenclature.

These following modifications are arranged in the order of their ordinary elevation, but which is very frequently deranged :Howard.

Forster.
CIRRUS.

Curl-cloud.
CIRROCUMULUS

Sonder-cloud.
CIRROSTRATUS

Wane-cloud.
CUMULOSTRATUS

Twain-cloud.
CUMULUS

Stacken-cloud.
NIMBUS

Rain.cloud.
STRATUS

Fall-cloud. In the annexed engraving are representations of the more usual forms of these genera, and we subjoin a few remarks on each to render their classification still more easy. In doing this, we shall depart from the above order, for the purpose of taking the simpler forms first.

CIRRUS-Curl-cloud. Fig. 1. The curling and flexuous forms of this cloud constitute its most obvious external character, and from these it derives its name. It may be distinguished from all others by the lightness of its appearance, its fibrous tex. ture, and the great and perpetually changing variety of figures which it presents to the eye. It is generally the most elevated, occupying the highest regions of the atmosphere.

The comoid cirrus cloud, vulgarly called the mare's tail, is the proper cirrus. It has, as represented in the engraving, somewhat the appearance of a distended lock of white hair, or of a bunch of wool pulled out into fine pointed ends (a *).

In variable and warm weather in summer, when there are light breezes, long and obliquely descending bands of cirrus are often observed, and seem sometimes to unite distinct masses of clouds together. Frequently, by means of the interposition of these cirri, between a cumulus and some other cloud (as, for example, cirrostratus), the cumulostratus, and ultimately the nimbus, is formed.

* See Indications of Weather, p. 95.

Upon a minute examination of the cirrus, every particle is found to be in motion, while the whole mass scarcely changes its place. Sometimes the fibres which compose it, gently wave backwards and forwards, to and from each other.

After a continuance of clear, fine weather, the cirrus is often observed as a fine whitish line of cloud, at a great elevation, like a white thread stretched across the sky; the ends of which seem lost in each horizon (b*).

To this line of cirrus others are frequently added laterally; and sometimes becoming denser by degrees, and descending lower in the atmosphere, inosculate † with others from below, and produce rain. To this kind the name of linear cirrus has been given. Sometimes on the sides of the first line of a cirrus, clouds of the same kind are propagated, and sent off in an oblique or transverse direction, so that the whole phenomenon has the appearance of net-work; this has been denominated reticular cirrus.

Though the above-mentioned varieties of the cirrus are all composed of straight lines of cloud, either parallel, or crossing each other in different directions; they are ranged under the head of cirrus, or curl-cloud, from their analogy of texture 10 the substance from which this cloud is named.

CUMULUS-Stacken-cloud. Fig. 9. This cloud is easily krown by its irregular hemispherical or heaped superstructure, hence its naine cumulus, a heap or pile. It has usually a flattened base. The mode of its formation is by the gathering together of detached clouds, which then appear stacked into one large and elevated mass, or stacken-cloud. The best time for viewing its progressive formation is in fine settled weather. About sunrise small thinly-scattered specks of clouds may be observed. As the sun rises, these enlarge, those near each other coalesce, and at length the cumulus is completed. It may be called the cloud of day, as it usually exists only during that period, dissolving in the evening, in a manner the exact counterpart of its formation in the morning. Cumuli, which are of a more regular hemispherical form, whitishcolored, and which reflect a strong silvery light when opposed to the sun, appear to be connected with electrical phenomena. Those seen in the intervals of showers are more variable in form, and more fleecy with irregular protuberances. When this kind of cloud increases so as to obscure the sky, its parts generally inosculate, and begin to assume that density of appearance which characterizes the cumulostratus.

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STRATUS-Fall-cloud.

Fig. 11. This kind of cloud rests upon the surface of the globe. It is of variable extent and thickness, and is called stratus, a bed or covering. It is gen

* See Indications, p. 95.
| Inosculation is a union by the conjunction of the extremities.

erally formed by the subsidence of vapor in the atmosphere, and has, therefore, been denoininated fall-cloud. This genus includes all fogs, and those creeping mists, which in summer evenings fill the valleys, remain during the night, and disappear in the morning. The best time for observing its formation is on a fine evening, after a hot summer's day: as the cumuli which have prevailed through the day decrease, a white mist forms by degrees close to the ground, or extends only for a short distance above it. This cloud arrives at its density about midnight, or between that time and morning, and it generally disappears about sunrise. It is, for this reason, called by some, the cloud of night. The coming in of autumn is generally marked by a greater prevalence and density of this cloud. In winter it is still denser. It has often been found to be electrified positively. The stratus should not be confounded with that variety of the cirrostratus, which is similar in external appearances; the test to distinguish them is, the stratus does not wet objects that it alights upon ;-the cirrostratus moistens every thing it touches.

CIRROCUMULUS. ,-Sonder-cloud. Fig. 2.

This consists of extensive beds of a number of little, well-defined, orbicular masses of clouds, or small cumuli, in close horizontal opposition ; but at the same time lying quite asunder (sonder-cloud), or separate from one another. It is to be distinguished from some appearances of the cirrostratus which resemble it, by the dense and compact form of its component nubeculæ (little clouds). From the intermediate nature of this cloud between the cirrus and cumulus, it has been called cirrocumulus. The word sonder-cloud is of Saxon derivation.

Sometimes the nubeculæ are very dense in their structure, very round in their form, and in very close opposition (c*).

At other times they are of a light, fleecy texture, and of no regular form.

The cirrocumulus of summer is of a middle nature between the two last; its nubeculæ vary in size and in proximity; and its picturesque appearance in this season often presents, by moonlight, as Bloomfield expresses it,

The beauteous semblance of a flock at rest.

The formation of this kind of cloud is either spontaneous, that is, unpreceded by any other, or results from the changes of some other modification. Thus the cirrus or cirrostratus often changes into cirrocumulus, and vice versâ. If it does not terminate with this kind of change, it subsides slowly as if by evaporation.

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* See Indications, p. 95.

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