Mr. Daniell’s explanation of the formation of rain differs from the above in some of its particulars, which are not sufficiently elementary to be given here; but it may be instructive to give a few of Mr. Howard’s illustrations respecting the
formation of the various clouds. If hot water be exposed to cool air, it steams—that is, the vapour given off from the surface is condensed in mixing with the air; and the water thus produced appears in visible particles, the heat of the vapour passing into the air. This effect may be seen about sunrise, in summer, on the surface of ponds warmed by the sun of the previous day, and also with water newly pumped from a well. But the small cloud formed in these instances usually disappears almost as soon as formed, the air being too dry to allow it to remain. But in the wide regions of the atmosphere the case is different, on account of the vast supply of vapour, and the ascent and descent of the cloud to regions which allow it to remain tolerably permanent. In the fine evenings of autumn, and occasionally at other seasons, mists appear suddenly in the valleys, gradually filling these low places, and even rising to a certain height, forming a foggy atmosphere for the following day. These collections of visible vapour resting on the earth, and often cut off so as to form a level surface above, so nearly resemble a sheet of water, as to have been occasionally mistaken for an inundation, the occurrence of the previous night. Such is the
origin and appearance of the stratus: it constitutes the fog of the morning, and sometimes, as at the approach of a long frost, occupies the lower atmosphere for several days. But the sun, we will suppose, has broken through and dissipated this obscurity, and cleared the lower air. On looking up to the blue sky, we see some few spots showing the first formation of a cloud there: these little collections increase in number, and become clouds, heaped, as it were, on a level base, and presenting their rounded forms upwards; in which state they are carried along in the breeze, remaining distinct from each other in the sky. This is the cumulus, or heap.
By and by, if the clouds continue to form, and enough vapour is supplied from above, these heaps are seen to grow over their base like a mushroom or cauliflower. Perhaps a flat top is seen forming separately, and this afterwards joins the simple heap of cloud; or the flat forms and the heaps become mixed irregularly among each other, occupying the spaces everywhere, till the sky becomes overcast, and presents the usual appearance of dense clouds. This is the cumulo-stratus, or heaped and flat cloud. It is not productive of rain, and it
forms, both in summer and in winter, the common scenery of a full sky.
On examining minutely the higher regions of the air, especially after the sky has been clear for some time, the spectator will probably see the cirrus descending from above in the form of threads or locks and feathers, which go on increasing until they fill the sky. They are more commonly seen above the two former kinds, which float upon the clear air below. On continuing to watch the cirri, they will be seen to pass to the intermediate form of cirro-cumulus, consisting of smaller rounded clouds attached to each other, or simply collected together in a flat aggregate, and forming the mottled or dappled sky.
The cumulo-stratus is more dense and continuous in its structure; thick in the middle, and thinned off towards the edges. Over-head it is a mere bed of haze, more or less dense. In the horizon, when seen sideways, it often resembles shoals of fish, as already noticed; but it is liable to put on the most ragged and patchy appearances, making a very ugly sky.
The nimbus, or rain cloud, is seen to the greatest advantage in profile, in the horizon, and at a great
distance, when it often resembles a lofty tower raised by its greater height to a conspicuous place among the dark threatening clouds, and catching the sun’s last rays upon its broad summit and sides. In its nearer approach, it may always be known by being connected below with an obscurity caused by the rain it lets fall, and which reaches down to the horizon.
In ascending from the lower valleys to the tops of lofty mountains, clouds may be traced through six modifications, the cirrus being seen from the loftiest summits, while the other forms are only skirting the sides of the mountains. Mr. Mason remarks, that clouds occasionally lie so low, that before the balloon seems to have entirely quitted the earth, it has been received between their limits, and entirely enveloped within their watery folds. Clouds, on the contrary, are sometimes at such a height, that the balloon either never comes into contact with them at all, or, if it passes through one layer, the aëronaut continues to behold another occupying a still remoter region of the skies above.