Fig. 202.
Fig. 203.
289. Shapes of Clouds.—Clouds have a very great variety of shape, the causes of which are for the most part not understood. They are generally divided into four classes: Cirrus, Cumulus, Stratus, and Nimbus. The Cirrus is represented in Fig. 202 (p. 225). It is a light, fleecy cloud, having graceful turns like curls, and hence its name, which is the Latin word for curl. Such clouds are commonly very high up in the air. The Cumulus (Latin for heap) you see in Fig. 203 (p. 225). Clouds taking this form appear as heaps rounded upward, and often appear like mountains of snow when they are illuminated by the sun. We see such clouds mostly in summer. The Stratus (Latin for covering) is seen in the same figure under the Cumulus. Clouds of this form lie low in the horizon, stretched along like a sheet. They often form in the latter part of the day, and increase in the night, but the rising sun dissipates them. The Nimbus, or rain-cloud, is represented in Fig. 204 (p. 226). It has a uniform gray or dark color. We often have two forms of cloud mingled together. Thus in Fig. 205 (p. 226) we have a mixture of the Stratus and the Cirrus, termed Cirro-Stratus. This is commonly called the mackerel-sky, and is quite a sure prognostic of rain. Then we have the Cirro-Cumulus, Fig. 206 (p. 227), and the Cumulo-Stratus, Fig. 207 (p. 227).
Fig. 204.
Fig. 205.
Water is gathered into clouds undoubtedly, in part at least, from the influence of attraction. But what the circumstances are that give them all these various shapes we know not. Whatever they are, they sometimes operate very extensively, giving a similar shape to all the clouds that cover the whole arch of the heavens; and at other times they operate variously in different localities, producing different shapes, sometimes even in near neighborhood to each other. Sometimes the edge of a cloud is irregular, or curved, or feathery; and at others it is a well-defined line, stretching along over a large portion of the horizon. In all these cases we have only divers arrangements of the same thing—a collection of vesicles of water containing air, which is made lighter than the air outside of the cloud by means which I shall speak of in another part of this chapter.