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extensive sheets of this cloud, covering the welkin before its condensation into water, that the Halo appears.* It is this cloud which, under some known circumstances of atmospheric change, first in a diffused form obscures the sky, giving the sun, moon, or stars that dim light, and those peculiar refractions, spoken of in another place, and which often eventually becomes nimbiform, and ends in gentle and continued Rain. The sun often sets apparently shrouded in a dense feature of this modification, and this is a sure indication of a wet morning. But let us turn to more elegant varieties of the Wanecloud, which sometimes appear in longish irregular spots, or in bars in close horizontal position. Features of this kind are frequently of short duration, and move along very slowly in a high atmosphere, and appear subsiding by degrees; while perhaps other beds of it are forming in other places: a feature much like this appears in the intervals of Showers. There also appears in variable weather, and before
* See Chapter III.
+ Pl. II. Fig. 2.
What is called the mackerelback sky often consists of this feature spread over a large portion of the firmament: but a sort of cirrocumulus, in like manner spread aloft, likewise receives this whimsical appellation.
Storms, a feature of cirrostratus, like the cyma of architecture.* I have seen cirrostratus which did not lie, as it usually does, in a horizontal plane. A feature occurred on the 5th of March, 1810, in the North East, which was a long tapering inclined and curved column of dark lakecoloured specks; above it were cirri scattered about like loose hay. But to describe the cirrostratus in all its varieties of mottles, specks, streaks, and lines, would swell too much this chapter, and the meteorologist must observe them for himself.
Of the Varieties of the Stackencloud.
CUMULI vary in size and in the regularity of their forms; they have all the tendency to assume an irregular hemespherical figure: those which attend fair settled weather, which form soon after sunrise, become large and inosculate into extensive masses in the middle of the day, and subside in the evening, are of the most
*Pl. III. Fig. 2.
regular shape. When they increase rapidly, and become more irregular, with fleecy bases, they will soon be cumulostrati, and are to be considered as indicating variable or wet weather: in this case they are lower down in the air, and of denser appearance. In the intervals of, and before showers, I have seen them very large, and yet moving along in the wind, like immense hemispheres of cloud, dense in the middle, with silvery summits, and constantly tending to become cumulostratus, and to reproduce the Showers; which, when they last long, are nourished by dark flocky cumuli, entering into the raining nimbus from below. See Pl. V. Fig. 2.
Some of these little Stackenclouds are not so fleecy as the rest; they are more compact in form, and, flying along rapidly between the Showers, are considered as a foreboding of their return, and are called, by the vulgar, water waggons. The cumuli before keen March Showers of Snow, with North and East winds, have that look of transparency, and that definite though rugged edge, described in another place, as happening also to cumulostratus. Cumuli have sometimes appeared as it were tuberculated, and, though of their usual hemispherical sort of form, to be
composed of numerous eminences, or lobes of cloud. I have not observed what peculiarities of weather these cumuli accompany.
It is curious to watch the formation of cumuli in the morning, and trace them, when it is possible, from the minute specks of cloud which, here and there, seem to form out of the atmosphere, to those large masses which move majestically along in the wind, and convey water from place to place for the irrigation of the earth. In fair weather, soon after sunrise, a small cloud appears; this increases, others form near it, and they fall into one another as if attracted; a large mass is at length upraised, and then all the smaller ones which form in its neighbourhood are soon lost, while the large one is augmented, and the spectator, though he seldom sees it in actual congression, feels no doubt that the disappearance of the smaller, and augmentation of the larger cloud, be owing to the larger mass having attracted the smaller into itself. It becomes a question, however, why the smaller clouds are lost to appearance before they reach and are quite drawn into the larger one? Possibly when the small cloud is very near with most of its vapours drawn away, the rest rush into the larger; as a magnet, when
it has approached a larger one within a certain distance, is forcibly and suddenly attracted to the latter. When these ephemeral mountains of electrified vapour have increased much, as they do towards the middle of the day, large ones often inosculate, and form dense and extensive irregular masses. Something else besides this, however, seems necessary to cause that density and continuity of a base, common to several superstructures which constitute cumulostratus.
The rapid formation and disappearance of small cumuli is a process constantly going on in particular kinds of weather, particularly when the air is clear and dry, with light Easterly breezes. These little Stackenclouds seem to form out of the atmosphere, and to be resolved again as rapidly into it.
On the elevation of a Fallcloud in the morning, we often see cumuli forming at its upper part; probably the same particles of vapour, on the return of the vapour plane, take the form of the cloud of day, and subside in fog again in the evening See Plate II. Figs. 3, 4.