Another way in which a cold surface of the earth may produce condensation is shown by what takes place among mountains. When a warm moist wind blows upon a chill mountain top, the air is cooled, and its vapor becomes visible in the form of a mist or cloud. You can often see that the cloud is quite solitary, and even shapes itself to the form of the ground, as if it were a sort of fleecy cap drawn down over the mountain’s head. This is often well marked in the morning. As day advances, the ground, warmed by the sun, no longer cools the air, and hence the mist is gradually re-absorbed into the atmosphere. But by and by, at the coming on of night, when the ground is once more cooled by radiation, if there should be vapor enough in the air, the mist will re-form, and the mountain put on his cap again. Cold air, as well as cold ground, condenses the vapor of warmer air. If you watch what goes on along the course of a river, you will often see examples of this kind of condensation. The ground on either side of the river parts with its heat after sundown sooner than the river itself does, and consequently cools the air above it more than the air above the river is cooled. So when this colder air from either side moves over to take the place of the warmer damp air lying on and rising from the river, condensation ensues in the form of the mist or river-fog, which so commonly hangs at night and early morning over streams. A cloud is merely a mist formed by the cooling of warm moist air, when it loses its heat from any cause, such as expansion during ascent, or contact with currents of cooler air. If you watch what goes on in the sky, you may often see clouds in the act of forming. At first a little flake of white appears. By degrees this grows larger, and other cloudlets arise and flock together, until at last the sky is quite overcast with heavy clouds, and rain begins to fall. The vapor which is thus condensed in the air has all been obtained by the evaporation of the water on the earth’s surface. It rises with the warm air, which losing its heat as it ascends, and coming too in contact with colder layers of the atmosphere, can not hold all its vapor, and is obliged to get rid of the excess, which then condenses into cloud.