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Infinitely diversified as the fornis 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.
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 texture, 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 known by its irregular hemispherical or heaped guperstructure, hence its name 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.
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.
erally formed by the subsidence of vapor in the atmosphere, and has, therefore, been denominated 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, orbicu. lar 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, unpre. ceded by any other, or results from the changes of some other modification. Thus the cirrus or cirrostratus often changes into cirrocumulus, and vice versd. If it does not terminate with this kind of change, it subsides slowly as if by evaporation.
* See Indications, p. 95.
CIRROSTRATUS-Wane-cloud. Fig. 3, 4, 5, 6,7. This cloud is distinguishable by its flatness, and great horizontal extension in proportion to its perpendicular height. Under all its various forms, it preserves this characteristic. It often results from the fibres of the cirrus, after descending from a higher station in the atmosphere, subsiding into strata of a more regularly horizontal direction, and hence it is called cirrostratus. As it is generally changing its figure, and slowly subsiding, it has received the name of wane-cloud. It originates more frequently from cirrus than from any other, and less from cumulostratus than cirrocumulus. Being once formed, it sometimes re-assures the character of the modifica. tion from which it originated, but more frequently it evaporates by degrees, or by iposculating, with some other modification, produces the cumulostratus, and eventually the nimbus, falling in rain.
Sometimes this cloud is disposed in wavy bars or streaks, in close horizontal opposition, and these bars vary infinitely in size and color, generally blended in the middle, but distinct towards its edges, fig. 4. A variety not unlike this, is the mackerel-back sky of summer evenings. It is often very high in the atmosphere. Another common variety appears like a long streak, thickest in the middle, and wasting away at its edges. This, when viewed in the horizon, has the appearance of fig. 7. It often seems to lie on the summit of the cumulostratus, as represented in the engraving; in this case, the density of the latter increases in proportion as the former form and evaporate upon it. The result of this intermixture, and the consequent density, is the formation of the nimbus, and the fall of rain.
Another principal variety of the cirrostratus is one which consists of small rows of little clouds, curved in a peculiar manner; it is from this curvature called cymoid. fig. 5. (d*.)
Fig. 6 is the representation of a similar one, less perfectly formed, having more of the character of the cirrocumulus, and is often produced when a large cumulus passes under the variety marked fig. 7. (et.)
Another remarkable development of this varying genus is, that extensive and shallow sort of cloud, which occurs particularly in the evening and during night, through which the sun and moon but faintly appear. It is in this cloud that those peculiar refractions of the light of those bodies, called halos, mock suns, &c. usually appear. (ft.)
CUMULOSTRATUS—Twain-cloud. Fig. 9. The base of this modification is generally flat, and lies on the surface of an atmospheric stratum, the superstructure resembling a bulky cumulus overhanging its base in large fleecy protuberances, or rising into the forms of rocky mountains. Considerable masses of these frequently are grouped upon
* See Indications, p. 95.
a common stratum or Lase, from which it has been named cumulostratus. It derives the other appellation, twain-cloud, from the frequently visible coalescence of two other modifications, as, for example, the cirrus and the cumulus. Its density is always much greater than the cumulus. Cumulostratus sometimes forms spontaneously, but is generally produced by the retardation of the cumulus in its progress with the wind, which then increases in density and lateral dimensions, and finally protrudes over its base in large and irregular projections. Sometimes contiguous cumuli unite at their bases, and at once become cumulostratus. Sometimes the upper currents of air conduct cirrostratus near the summits of cuinuli, or pierce them, as is shown in the engraving. The effects of this junction have been described under the last modification.
Cumulostratus often evaporates, sometimes changes to cumulus, but, in general, it ends in nimbus, and falls in rain. In long ranges of these clouds it has been observed that part has changed into nimbus, and the rest remained unchanged.
NIMBUS—Rain-cloud. Fig. 11. This is not a modification depending upon a distinct change of form, but rather from increase of density and deepening of shade in the cumulostra. tus, indicating a change of structure, which is always followed by the fall of rain. This has been, therefore, called nimbus, (a rainy black cloud.) Any one of the preceding six modifications may increase so much as to obscure the sky, and, without falling in rain, “ dissolve,” and “ leave not a rack behind.” But when cumulostratus has been formed, it sometimes goes on to increase in density, and assume a black and portentous darkness. Shortly afterwards the intensity of this blackness yields to a more grey obscurity, which is an evidence that a new arrangement has taken place in the aqueous particles of the cloud; the nimbus is formed, and rain begins to fall. The shower continues until another interior change succeeds, when the nimbus is extinct, and more or less of other modifications re-appear: the cirrus, cirrostratus, or perhaps the cirrocumulus, is seen in the higher regions of the atmosphere, and the remaining cumulus, no longer retarded, sails along in a current of wind nearer the earth. These effects may be satisfactorily observed when showers fall at a distance; the nimbus can then be seen in profile, and the process of its formation and destruction followed through all its stages.
In addition to this sketch, it may be stated, that masses of cloud may present themselves to the observation so indefinite and shapeless, as to render it difficult to refer them at once to any of the preceding modifications; but it is believed that in every case, if the observations be attentively prolonged, a tendency to resolve into some of these forms will, sooner or later, be discovered. A circumstance which not only shows their distinct nature, but proves that there are some general causes why aqueous vapor,