Cloud
a mass of small globules of water, or of ice and snow, suspended in the air. Clouds are formed by the evaporation of water from land and sea. There is a theory that the condensation of the water vapor, which is so fine as to be invisible, into the larger globules of visible clouds, is due not only to a cooling of the air, but, in part at least, to the presence of dust particles and the repelling and attracting influence of electricity. Scientists have found that pure, dry air may be laden with vapor and be cooled without condensation but that, if at this point in the experiment, dust be injected without a change of temperature or the addition of more vapor, a mist forms at once.
Clouds form and float at widely different heights. It is not difficult, especially when a storm is coming up, to note clouds at different altitudes moving in two or more entirely different and even opposite directions, showing that they are floating in different layers of air. The warmer the air, the more moisture it can carry. When air has all the moisture it can carry it is said to be saturated. If saturated air be warmed it can take up more moisture. If saturated air be cooled it must drop part of its moisture, and we have a shower, or a rain, or a dew.
Clouds are not merely of great benefit, but are indispensable in carrying moisture from the place of their formation to the places where moisture is needed, and where shade is welcome. Shelley expresses this thought beautifully in his Cloud:
From the seas and the streams;
I bear light shade for the leaves when laid
In their noon day dreams.
From my wings are shaken the dews that waken
The sweet buds, every one
When rocked to rest on their mother's breast,
As she dances about the sun.
Clouds are classified according to their form and height. The chief three kinds are: 1. The cirrus, a light, feathery cloud frequently seen streaming across the sky. Cirrus clouds move swiftly, and are high up, six to ten miles, in a wind-swept region.
2. The cumulus or fleecy cloud is piled up like a fleece of wool. The bases of cumulus clouds are flat and the upper parts are dome-shaped. They pile up and tumble about as if tossed by strong winds and take innumerable forms of mountains, castles, minarets, and towers.
3. The nimbus, a dark, low, rain cloud. Rents in a nimbus frequently give magnificent views of the upper regions and other clouds.
It would be a mistake to suppose that all clouds can be referred to these three forms; for clouds occur in all sorts of combinations and intermediate forms, delightful to study.
A cloud record is kept readily by a numerical scale of from 1 to 8. An entry of 0 indicates a clear sky; 8 a dark, clouded sky in which no blue can be seen, and in which the sun cannot be located.
The brilliant colors of the clouds, gray, purple, orange, yellow, olive, and pink, are due probably to the refraction of light by minute dust particles rather than to the globules of water.
The height of clouds is found by two observers located at two ends of a given base line. Each observer measures the angle formed between the base and a line directed toward a part of the cloud agreed upon. From the length of the base and the value of the two angles adjacent to the base, the altitude of the triangle, that is to say, the height of the cloud, is easily computed.