Clouds, in any one of the preceding forms, at the same degree of elevation, or two or more of these forms at different elevations, may increase and become so dense as completely to obscure the sky; this, to an inexperienced observer, would seem to indicate the speedy commencement of rain. But Mr. Howard is of opinion that clouds, while
in any of the states above described, never let fall rain.
Before rain the clouds always undergo a change of appearance, sufficiently remarkable to give them a distinct character. This appearance, when the rain happens overhead, is but imperfectly seen; but from the observations of aëronauts, it appears that whenever a fall of rain occurs, and the sky is at the same time entirely overcast with clouds, there will be found to exist another stratum of clouds at a certain elevation above the former. So, also, when the sky is entirely overcast and rain is altogether or generally absent, the aëronaut, upon traversing the canopy immediately above him, is sure to enter upon an upper hemisphere either perfectly cloudless or nearly so. These remarks were, we believe, first made by Mr. M. Mason, and he states that they have been verified during many hundred ascents.
In October, 1837, two ascents were made by Mr. Mason, which well illustrate what has been said. On the 12th, “the sky was completely overspread with clouds, and torrents of rain fell incessantly during the whole of the day. Upon quitting the earth, the balloon was almost immediately
enveloped in the clouds, through which it continued to work its way upwards for a few seconds. Upon emerging at the other side of this dense canopy, a vacant space, of some thousand feet in breadth, intervened, above which lay another stratum of a similar form and observing a similar character. As the rain, however, still continued to pour from this second layer of clouds, to preserve the correctness of the observation, a third layer should, by right, have existed at a still further elevation; which, accordingly, proved to be the case. On the subsequent occasion of the ascent of the same balloon, (October 17th,) an exactly similar condition of the atmosphere, with respect to clouds, prevailed; unaccompanied, however, with the slightest appearance of rain. No sooner had the balloon passed the layer of clouds immediately above the surface of the earth, than, as was anticipated, not a single cloud was to be found in the firmament beyond; an unbroken expanse of clear blue sky everywhere embracing the frothy plain that completely intercepted all view of the world beneath.”
Mr. Howard had not the advantages of a balloon to assist his observations. He has noticed that during rain and before the arrival of the denser
and lower clouds, or through their interstices, there exists, at a greater height, a thin light veil or a hazy appearance. When this has considerably increased, the lower clouds are seen to spread till they unite in all points and form one uniform sheet. The rain then commences, and the lower clouds arriving from the windward, move under this sheet and are successively lost in it. When the latter cease to arrive, or when the sheet breaks, letting through the sun-beams, every one’s experience teaches him to expect that the rain will abate or leave off.
But there often follows an immediate and great addition to the quantity of cloud. At the same time the darkness becomes less, because the arrangement, which now returns, gives free passage to the rays of light; the lower broken clouds rise into cumuli, and the upper sheets put on the various forms of the cumulo-stratus, sometimes passing to the cirro-cumulus.
The various phenomena of the rain-cloud are best seen in a distant shower. If the cumulus be the only cloud at first visible, its upper part is seen to become tufted with cirri. Several adjacent clouds also approach and unite at its side. The cirri
increase, extending upwards and sideways, after which the shower is seen to commence. At other times, the cirro-stratus is first formed above the cumulus, and their sudden union is attended with the production of cirri and rain. In either case the cirri spring up in proportion to the quantity of rain falling, and give the cloud a character by which it is easily known at great distances, and which has long been called by the name of nimbus.