But there is another important form in which the moisture of the clouds may descend to the surface of the earth. When the weather is cold enough, there fall to the ground not drops of rain, but flakes of snow.
If you bring snow indoors, it soon melts into water. If you expose this water for a time it evaporates. Snow, water, and aqueous vapor are thus only different forms of the same substance. We say that water can exist in three forms—the gaseous, the liquid, and the solid. Snow is an example of the solid condition.
On a frosty night pools of water are covered with a hard, transparent crust, of what is called ice. You may break this crust into pieces, but if the cold continues, a new crust will soon be formed with bits of the old one firmly cemented in it. And the greater the cold the thicker will the crust be, until perhaps the whole of the water in the pools may become solid. If you take a piece of this solid substance, you find it to be cold, brittle, and transparent. Brought into a warm room it soon melts into water, and you may drive off the water as before into vapor. Ice is the general name given to water when it is in the solid state, such forms as snow and hail being only different appearances which ice puts on. Whenever water becomes colder than a certain temperature it passes into ice, or freezes, and this temperature is consequently known as the freezing-point.
The upper layers of the atmosphere are much colder than the freezing-point of water. In the condensation which takes place there, the clouds do not resolve themselves into rain. The vapor of the up-streaming currents of warm air from the earth’s surface is condensed and frozen in these high regions, and passes into little crystals, which unite into flakes of snow. Even in summer the fine white cloudlets which you see floating at great heights are probably formed of snow. But in those countries, such as ours, where in winter the air even at the surface is sometimes very cold, the snow falls to the ground, and lies there as a white covering, until returning warmth melts it away.
Besides rain and snow, the moisture of the air takes sometimes the form of hail, which consists of little lumps of ice like frozen rain; and of sleet, which is partially melted snow. But rain and snow are the most important, and it is these two forms which we must follow a little further.
Before doing so, let us gather together the sum of what has been said about the aqueous vapor of the air. We have learned that, as every sheet of water on the face of the globe evaporates, the air is full of vapor; that this vapor is condensed into visible form, and appears as dew, mist, and cloud. We have learned further, that the vapor of which clouds are formed is resolved into rain and snow, and, in one or other of these forms, descends to the earth again. There is thus a circulation of water between the solid earth beneath and the air above. This circulation is as essential to the earth in making it a fit habitation for living things, as the circulation of blood is in keeping our bodies alive. It mixes and washes the air, clearing away impurities, such as those which rise from the chimneys of a town. It moistens and quickens the soil, which it renders capable of supporting vegetation. It supplies springs, brooks, and rivers. In short, it is the very mainspring of all the life of the globe. So important a part of the machinery of the world deserves our careful consideration. Let us next attend, therefore, to what becomes of the rain and the snow after they have been discharged from the air upon the surface of the earth.
[To be continued.]
