On a dry, bracing day, evaporation goes on rapidly, because the air has not nearly got all the quantity of vapor it can hold in solution. On a damp day, however, when the air contains about as much vapor as it can hold at that particular temperature, evaporation is quite feeble, or ceases altogether. This varying capacity of the air for vapor is the reason why laundresses find so much difference between days, in the ease with which they can have their clothes dried.
After sunset, when the sky is clear, you know that the grass gets wet with dew. In the morning you may see mists hanging over woods, and streams, and hills, and gradually melting away as the sun mounts in the sky. At all times of the year you may watch how clouds form and dissolve, and form again, ever changing their size and shape as they move through the air. Now these are all examples of the condensation of vapor. Let us see how the process takes place.
Condensation, as we have seen, results from a cooling of the air. When vapor is condensed, it does not at once take the form of running water. The cold glass brought into the warm room has first a fine film of mist formed upon it, and then by degrees the clear drops of water come. In reality mist is made up of exceedingly minute particles of water, and it is the running together of these which makes the larger drops. So in nature on the great scale, when condensation occurs the vapor first appears as a fine mist. This is always the result of cooling; so that, whenever you see a mist or cloud forming, you may conclude that the air in which it lies is being cooled.
Dew is the name given to the wetness which we notice appearing in the evening, or at night, upon grass, leaves, or stones, or even sometimes on our hair. In the morning you have, no doubt, often watched the little dewdrops sparkling upon the foliage and the delicate threads of gossamer. Now this wetness does not come out of the leaves or stones, nor out of your hair. It is all derived from the air by condensation, exactly as we see the film of mist form upon the cold tumbler in the warm moist air of a room. In fact, that film of mist was really dew, and all dew is formed in the same way, and from the same cause.
At night, when the sky is clear, the earth radiates heat rapidly; that is to say, it gives off into cold space a great part of the heat which it has received from the sun during the day. Its surface consequently becomes cold, as you may have felt when you put your hand upon leaves or stones after nightfall. The layer of air next the cooled ground is chilled below its point of condensation, and the excess of vapor is deposited as dew upon the grass, twigs, stones, and other objects. Hence it is that the temperature at which this condensation begins to take place is called the dew-point.
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.
On a summer morning the sky is often free from cloud. As the day advances, and the earth gets warmed, more vapor is raised; and as this vapor, borne upward by the ascending air-currents, reaches the higher and colder parts of the atmosphere, it is chilled into the white fleecy clouds which you see forming about mid-day and in the afternoon. Toward evening, when less evaporation takes place, the clouds cease to grow, and gradually lessen in size until at night the sky is quite clear. They have been dissolved again by descending and coming in contact with the warm air nearest to the earth. Again, you have often noticed that clouds move across the sky. They are driven along by upper currents of air, and of course the stronger these currents are the faster do the clouds travel. In this way the sky is sometimes completely overcast with clouds which have come from a distance.
You are well aware that rain always comes from clouds in the sky. When the sky is clear overhead, no rain falls. Only when it gets overcast does the rain come. You can watch a dark rain-cloud gather itself together and discharge a heavy shower upon the earth. When a cold glass is brought into a warm room, you will remember that the film of mist formed upon the glass is found by degrees to gather into drops, and trickles down the cold surface. Now the mist on the glass and the cloud in the sky are both formed of minute particles of water, separated by air. It is the running together of these particles which gives rise to these drops. In the one case, the drops run down the cold glass. In the other case, they fall as drops of rain through the air. Rain, therefore, is thus a further stage in the condensation of the aqueous vapor of the atmosphere. The minute particles of the cloud, as condensation proceeds, gather more moisture round them, until at last they form drops of water, too heavy to hang any longer suspended in the air. These then fall to the earth as rain-drops.