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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 cirro. stratus. As it is generally changing its figure, and slowly subsidiog, 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-assumes 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 hori. zontal 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
a common stratum or Lase, froin 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. Sometiines 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.
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 fail. 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,
suspended in the air, should assume, though with great varieties of size and form, certain definable and constant modifications.
INDICATIONS OF APPROACHING CHANGES OF THE WEATHER FROM
THE FORM OF THE CLOUDS.
The prognostics of coming weather must always be deduced from those clouds which ultimately prevail; for, in variable weather, all the modifica tions may be seen in a single day.
CIRRUS. The comoid, or mare's tail, variety of this cloud described a, p. 90, is an accompaniment of a variable state of weather, and forebodes wind and rain. In very changeable seasons, the direction of the fine and almost evanescent tails varies considerably in a few hours. When the tails have had a constant direction towards the same point of the compass for any considerable time, it has been frequently observed that a gale has sprung up from the quarter to which they had previously pointed.
The variety b, p. 91, is the first indication of a change to wet.
CIRROCUMULUS. When this cloud prevails, we may, in general, anticipate, in summer, an increase of temperature; in winter, it often precedes the breaking up of a frost, and indicates warm and wet weather.
The variety c, p. 92, is striking before, or about the time of thunder storms in summer. It is commonly a forerunner of storms, and has been remarked as such by the poets.
CIRBOSTRATUS. The prevalence of this cloud is always a sign of rain
The cymoid arrangement d, p. 93, is a sure indication of stormy weather, and the variety c, p. 93, is almost always so.
The variety f, p. 93, is the surest prognostic we are acquainted with, of an impending fall of rain or snow.
CUMULOSTRTus may always be regarded as a stage towards the production of rain or snow, and, in this case, always ends in the nimbus.
NIMBUS is always followed by a fall of rain or snow.
Fogs and mists, when they extend over large surfaces varied with land and water, are generally produced in fine calm weather, after the sun's rays have ceased to warm the earth, by the higher masses of air, which have been rapidly cooled in the more elevated regions of the atmosphere, de. scending by their weight, and intermixing with the lower, and lighter, and still warmer strata. These are gradually chilled, until the undermost stratum is affected, first to dampness, then to a slight precipitation scarcely visible to the eye, and finally, to mist and fog. The earth, during clear nights, immediately on the withdrawing of the heat-imparting energies of the sun, begins to emit the particles of heat it had acquired during the day, or, in ordinary language, to cool. The atmosphere does the same, but at a much slower rate. In the race, therefore, between the cooling powers of these
two bodies, the start is usually made at sunset with the earth's surface warmer than the incumbent air. The first, by its more rapid descent, overtakes the latter at some point of equal temperature, and passing its sluggish competitor, becomes colder, and, of course, instead of warming the stratum of air in contact, as it did in the first part of its course, it now, on the contrary, becomes an absorber of heat, and, consequently, cools the contiguous bodies. In both these cases the process is favorable to the formation of mist, but in different modes. In the first, it assists the intermixture of the two differently warmed bodies of air, by keeping up the temperature of the lower one, and by thus increasing its disposition to ascend, the mingling and the deposition are more rapid and complete. In the latter case, it is in the same condition with relation to the air near the surface, as the strata above it, namely, cooler, and therefore it acts similarly; the surface-air being now between two cooling inasses, the rate of its condensation and consequent precipitation of moisture is at least continued, and perhaps increased.
The phenoinenon of mists forming over lakes and rivers, when the atmosphere of their banks and adjacent land is entirely free from visible vapor, is a very remarkable one, and has excited considerable atte on. The late Sir H. Davy observed and communicated to the public some curious facts, which have contributed very much to our knowledge on the subject. The principal operating cause in producing this singular effect, is the difference of the rate of cooling, in the absence of the sun, in fluid and in solid bodies. In the surface of the former the particles, as they are cooled, sink, and give place to those beneath, which then are warmer, and therefore lighter, producing thus a renewal of surface, and a very slow decrease of its general temperature, compared with those of solid bodies, whose particles are motionless among themselves. When these on their surface are cooled, they remain in their place, and are affected by the superior warmth of the internal particles, only in the degree of the conducting power of the body. And this conducting power is found to be extremely feeble in most of the substances which form the solid crust of our globe.
These conditions being understood, it will be easy to imagine, that the portion of the atmosphere which reposes on the surface of water, will continue warmer after sunset on a clear night, than the contiguous parts which rest upon the adjacent land. From its position, too, with regard to an aqueous surface, it will also obtain a greater load of moisture. If we now suppose the cooled air of a superior stratum to descend in the usual manner upon the masses of air lying upon the earth and the water, which, though closely adjoining, are, with regard to heat and humidity, very differently compounded, we shall find that its descent might produce little or no visible change in the land-stratum, while, by its superior weight, it would fall into and partially displace that over the water, intimately intermixing with it, and condensing its moisture, and thus creating in the air a visible
river or lake of vapor, whose boundaries in a still night would exactly coincide with the banks of the water beneath, however irregular their outline. Mr. Harvey observed a mist of this kind hovering in a beautiful stratum over the stream which supplies Plymouth with water. The mist moved in the direction of the running stream, but with a velocity much greater, while it accommodated itself, in a most singular manner, in its course, to all the turns and windings of the channel. The breadth of the mist was nearly the same as that of the stream, and its average altitude about five feet. The water of the stream was observed to stand at 56°, the air over the water 475°, the ground near the mist 45°, the air above it 49o.
The following facts also corroborate the view we have taken of the theory of the formation of mists over water. Sir H. Davy, on descending the Danube during three nights in June, 1818, observed, that the mist regularly appeared over the water in the evening, when the temperature of the air on the shore was from 3° to 6o lower than that of the stream, and that it as regularly disappeared when the temperature of the atmosphere on the banks surpassed that of the river. At six o'clock in the morning of the last day mentioned, Sir H. Davy observed, at the junction of the rivers Inn and Ilz with the Danube, the respective temperatures of the water of the three rivers, and that of the air on the land. He found them, and the existing state of the atmosphere over the waters, to be as follows :Temperature of the Temperature of
State of the atmosphere over the
Rivers. 62° Danube.
Thick fog on the whole breadth. 54° 56° Inn.
Dense mist ditto. 55° Ilz.
Light mist. This observation strikingly exhibits the precipitation in its varied proportion produced by the intermixture of the cooler air of the land with that of the floating strata of air over the rivers, at their different temperatures.
If we suppose that we have enumerated above all the causes of the formation of mists, it would be difficult to account for the fact of mists increasing in density and extent after their first formation, or for their continuance after the difference of the temperatures of the air and water had been reduced to nearly nothing. It is evident that the conditions we have mentioned are not sufficient for the production of this effect, which yet may often be observed. Sir H. Davy thinks that this increase and prolonged existence depend, not only upon the operation of the causes which originally produced them, but likewise upon heat which is evolved from the superficies of the particles of water composing the mist. This produces a descending current of cold air in the very body of the mist, whilst the warmer water continually sends up vapor. This decrease of temperature in the middle of the body of mist was remarked also by Mr. Harvey, during a dense mist, which shrouded not only over the whole of
air on Land.