When one of these arrives hastily with the wind, it brings but little rain, and frequently some hail or driven snow.
Since rain may be produced and continue to fall from the slightest obscuration of the sky by the nimbus, while a cumulus or a cumulo-stratus, of a very dark and threatening aspect, passes on without discharging any until some change of state takes place; it would seem as if nature had destined the latter as reservoirs, in which water is collected from extensive regions of the air for occasionally irrigating particular spots in dry seasons; and by means of which it is arrested, at times, in its descent in wet ones.
Although the nimbus is one of the least beautiful of clouds, it is, nevertheless, now and then adorned
by the splendid colouring of the rainbow, which can only be seen in perfection when the dark surface of this cloud forms for it a background.
The small ragged clouds which are sometimes seen sailing rapidly through the air, are called scud. They consist of portions of a rain-cloud, probably broken up by the wind, and are dark or light according as the sun shines upon them. They are the usual harbingers of rain, and, as such, are called by various names, such as messengers, carriers, and water-waggons.
In attempting to explain the production of clouds and rain, it is necessary to observe that the subject is beset with difficulties—the discussion of which does not belong to this little volume; but the following notice of Dr. Hutton’s theory may not be out of place.
It has been already stated, that the air supplies itself with moisture from the surface of the waters of the earth. This it continues to do at all temperatures, until it is so charged with vapour that it cannot contain any more. The air is then said to be saturated. Now, the quantity of moisture which a given bulk of air can contain, depends entirely
upon the temperature of the air for the time being. The higher the temperature of the air the greater will be the quantity of vapour contained in it; and, although it may be perfectly invisible to the eye, on account of the elasticity which the heat imparts to it, yet it can easily be made visible by subtracting a portion of the heat. If, for example, a glass of cold water be suddenly brought into a warm room, moisture from the air will be condensed upon the outside of the glass in the form of dew. A similar change is supposed to take place when two currents of air having different temperatures, but both saturated with vapour, are mingled together; an excess of vapour is set free, which forms a cloud or falls down as rain. If the currents continue to mingle uniformly, “the clouds soon spread in all directions, so as to occupy the whole horizon; while the additional moisture, incessantly brought by the warmer current, keeps up a constant supply for condensation, and produces a great and continued deposition of moisture in the form of rain. By degrees, the currents completely intermingle, and acquire a uniform temperature; condensation then ceases; the clouds are re-dissolved; and the whole face of nature, after being cooled and refreshed
by the necessary rain, is again enlivened by the sunshine, thus rendered still more agreeable by its contrast with the previous gloom.”
If the cloud, produced by the mingling of two differently heated currents of moist air, happen to form in the upper regions of the sky, it may be heavier than its own bulk of air, and will consequently begin to sink. Should the atmosphere near the earth be less dense than the cloud, the latter will continue to descend till it touches the ground, where it forms a mist. If the vapour has been condensed rapidly and abundantly, the watery particles will form rain, hail, or snow, according to the temperature of the air through which they pass. But it may happen that the cloud, in descending, arrives in a warmer region than that in which it was formed: in this case, the condensed moisture may again become vapour, and ascend again to a region where condensation may again take place.