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A rainstorm comes, the walks are wet, the roads are muddy. Then the sun breaks through the clouds and soon the walks are no longer damp and the mud of the road is dried. Where did the water come from and where has it gone? Let us answer these questions.

A kettle on the stove is forgotten and soon a cracking is heard; the housewife jumps to her feet for the kettle is dry. The kettle was filled with water, but it has all boiled away; and where has it gone? Surely into the air of the room, for it can be seen issuing as "steam" and then disappearing from view, as if by magic. The heat of the fire has changed the liquid water to a gas as invisible as the air itself. This gas is water vapor.

Fig. 9. A glass of cold water on which vapor has condensed in drops.

Do you wish to prove that the water vapor is there, although unseen? Then, if the day is cool, watch the window and notice the drops of water collect upon it. Or, if the day is warm, bring an ice-cold glass or pitcher into the room and see the drops collect upon it (Fig. 9). People sometimes say, when drops of water collect on a glass of cold water, that the glass is "sweating;" but see whether the same thing will not happen with a cold glass that does not contain water.

These two simple observations teach us two very important facts: (1) That heat will change liquid water to an invisible vapor, or gas, which will float about in the air of a room; and (2) that cold will cause some of the vapor to change back to liquid water.

Let us observe a little further. The clothes upon the line on wash day are hung out wet and brought in dry. If the sun is shining they probably dry quickly; but will they not dry even if the sun is not shining? They will, indeed; so here is another fact to add to our other two, namely (3) that the production of vapor from water will proceed even when the water is not heated.

This change of water to vapor is called evaporation. The water evaporates from the clothes; it also evaporates from the walks after a rain, from the mud of the road, from the brooks, creeks and rivers, and from ponds, lakes, and the great ocean itself. Indeed, wherever water is exposed to the air some evaporation is taking place. Yet heat aids evaporation, as you can prove by taking three dishes of the same kind and pouring the same amount of water into each, then placing one on the stove, a second in the sun, and a third in a cool, shady place, as a cellar, and watching to see which is the last to become dry.

About three-fourths of the earth's surface is covered by water, so that the air is receiving vapor all the time. In fact, every minute thousands of barrels of water-vapor are rising into the atmosphere from the surface of the ocean. The air is constantly moving about, forming winds, and this load of vapor is, therefore, drifted about by the winds, so that the air you are breathing may have in it vapor that came from the ocean hundreds or even thousands of miles away. You do not see the vapor, you are perhaps not even aware that it is there; yet in a room 10 feet high and 20 feet square there is often enough vapor, if it could all be changed back to water to fill a two-quart measure.

There is a difference in the amount of vapor from time to time. Some days the air is quite free from it, and then clothes will dry rapidly. On other days the air is damp and humid; then people say it is "muggy," or that the "humidity is high." On these muggy days in summer the air is oppressive because there is so much vapor in it. Near the sea, where there is so much water to evaporate, the air is commonly more humid or moist than in the interior, away from the sea, where there is less water to evaporate.

We have seen that there is some vapor in all air, and that there is more at some times than at others. We have also seen how it has come into the air, and that cold will cause it to condense to liquid water on cold window panes and on water glasses. There are other ways in which the vapor may be changed to liquid.

After a summer day, even when there has been no rain, soon after the sun sinks behind the western horizon the grass becomes so damp that one's feet are wet in walking through it. The dew is "falling." During the daytime the grass is warmed by the sun; but when the sun is gone it grows cooler, much as a stove becomes cool when the fire is out. This cool grass chills the air near it and changes some of the vapor to liquid, which collects in drops on the grass, as the vapor condenses on the outside of a glass of ice water.

In the opposite season of the year, on a cold winter's day, when you step out of a warm house into the chilly air, a thin cloud, or fog, forms as you expel the air from your lungs, and you say that you can "see your breath." What you really see are the little drops of water formed as the vapor-laden breath is chilled on passing from the warm body to the cold air. The vapor is condensed to form a tiny mist.

Fig. 10. A wreath of fog settled in a valley with the hilltops rising above it.

Doubtless you have seen a wreath of fog settling in a valley at night; or in the morning you may have looked out upon a fog that has gathered there during the night (Fig. 10). If your home happens to be upon a hillside, perhaps you have been able to look down upon the fog nestled there like a cloud on the land, which it really is. Such a fog is caused in very nearly the same way as the tiny fog made by breathing. The damp air in the valley has been chilled until the vapor has condensed to form tiny mist or fog particles. Without doubt you can tell why this fog disappears when the sun rises and the warm rays fall upon it.

On the ocean there are great fogs, covering the sea for hundreds of miles; they make sailing dangerous, because the sailors cannot see through the mist, so that two vessels may run together, or a ship may be driven upon the coast before the captain knows it. Once more, this is merely condensed vapor caused by chilling air that has become laden with vapor. This chilling is often caused when warm, damp winds blow over the cold parts of the ocean.

This leads the way to an understanding of a rain storm; but first we must learn something about the temperature of the air. The air near the ground where we live is commonly warmer than that above the ground where the clouds are. People who have gone up in balloons tell us so; and now scientific men who are studying this question are in the habit of sending up great kites, carrying thermometers and other instruments, in order to find out about the air far above the ground.

Fig. 11. Fog clouds among the valleys in the mountains, only the mountain peaks projecting above them.

It is not necessary, however, to send up a kite or a balloon to prove this. If your home is among mountains, or even among high hills, you can prove it for yourself; for often, in the late autumn, when it rains on the lower ground, it snows upon the mountain tops, so that when the clouds have cleared away the surface of the uplands is robed in white (Fig. 12). In the springtime, or in the winter during a thaw, people living among these highlands often start out in sleighs on a journey to a town, which is in the valley, and before they reach the valley their horses are obliged to drag the sleigh over bare ground. It is so much warmer on the lower ground that the snow melts away much more quickly than it does among the hills.

The difference in temperature is, on the average, about one degree for every three hundred feet, so that a hill top rising twelve hundred feet above a valley would have an average temperature about four degrees lower than the valley. Now some mountains, even in New York, rise thousands of feet above the surrounding country. They rise high into the regions of cold air, so that they are often covered with snow long before any snow has fallen on the lowlands; and the snow remains upon them long after it has disappeared from the lower country (Fig. 12).

Fig. 12. A mountain whitened by snow on the top, while there is no snow at the base.

Fig. 13. A mountain peak snow capped, and covered on the very crest by a cloud.

Some mountains are so lofty that it never rains upon them, but snows instead; and they are never free from snow, even in mid-summer. If one climbs to the top of such peaks he finds it always very cold there. While he is shivering from the cold he can look down upon the green fields where the birds are singing, the flowers blossoming and the men, working in the fields, are complaining of the heat.

One who watches such a mountain as this, or in fact any mountain peak, will notice that it is frequently wrapped in clouds (Fig. 13). Damp winds blowing against the cold mountains are chilled and the vapor is condensed. If one climbs through such a cloud, as thousands of people have done when climbing mountains, he often seems to pass through nothing but a fog, for really many clouds are only fogs high in the air. (Fig. 14).

But very often rain falls from these clouds that cling to the mountain sides. The reason for this is easy to understand. As the air comes against the cold mountains so much vapor is condensed that some of the tiny fog particles grow larger and larger until they become mist particles, which are too heavy to float in the air. They then begin to settle; and as one particle strikes against another, the two unite, and this continues until perhaps a dozen have joined together so as to form a good-sized drop, which is so heavy that it is obliged to fall to the ground as rain.

Fig. 14. Clouds clinging to the mountain sides. If one were climbing these mountains he would find himself, in passing through the clouds, either in a fog or a mist.

Let us now look at our summer storms. These do not form about mountain peaks; yet what has been said about the mountains will help us to understand such showers.

It is a hot summer day. The air is muggy and oppressive, so that the least exertion causes a perspiration, and even in the shade one is uncomfortably hot. Soon great banks of clouds appear (Fig. 15),—the "thunder heads,"—and people say "a thunder shower is coming, so that we will soon have relief from this oppressive heat." The clouds draw near, lightning is seen and thunder heard, and from the black base of the cloud, torrents of water fall upon the earth. If we could have watched this cloud from the beginning, and followed it on its course, we would have seen some facts that would help explain it. Similar clouds perhaps began to form over your head in the early afternoon and drifted away toward the east, developing into thunder storms many miles to the east of you.

On such a day as this, the air near the ground is so damp that it gives up vapor easily, as you can prove by allowing a glass of ice water to stand on a table and watching the drops of water gather there, causing the glass to "sweat" (Fig. 9). The sun beats down upon the heated ground and the surface becomes like a furnace, so that the air near the ground is warmed.

Fig. 15. A "thunder head," or cumulus cloud.

Air that is warm is lighter than cool air, and, being lighter, will rise, for the heavy cool air will settle and push it up, as a chip of wood will rise in a pail of water, because it is lighter than the water which pushes it to the top. This is why the warm air rises from a furnace, or a stove, or a lamp. It is the reason why the hot air rises through a house chimney; undoubtedly you can find other illustrations, as ventilation, and can find abundant opportunity to prove that warm air will rise.

The warm, moist air near the ground becomes so light that the heavy air above settles down and pushes it up, so that an uprising current of air is formed above the heated ground, much as an uprising current of hot air rises through the chimney when the stove is lighted. Rising thousands of feet into the sky the warm air reaches such a height, and finally comes to a place so cool, that some of the vapor must be condensed, forming fog particles, which in turn form a cloud.

On such a day, if you will watch a cloud, you will notice that its base is flat (Fig. 15); and this flat base marks the height above ground where the temperature of the atmosphere is low enough to change the vapor to fog particles. Of course the air still rises somewhat above this base and continues to get cooler, and to have more and more vapor condensed. This makes a pile of clouds resting on a level base, but with rounded tops (Fig. 15). Sometimes the base of these summer clouds, called cumulus clouds, is a mile above the ground and their tops fully a mile higher than this.

Fig. 16. Photograph of a lightning flash.

Just as on the mountain side, where the drops grow larger until they must fall, so here, fog particles grow to drops of such a size that they are too heavy to float. This growth is often aided by the violent currents of air, which sometimes tumble and toss the clouds about so that you can see the commotion from the ground. These currents blow one particle against another, forming a single drop from the collision of two; then still others are added until the rain drop is so heavy that it must fall.

But sometimes the air currents are so rapid that the drops are carried on up, higher and higher, notwithstanding the fact that they are heavy. Then they may be carried so high, and into air so cold, that they are frozen, forming hail. These "hailstones" cannot sink to the ground until they are thrown out of the violent currents, when they fall to the ground, often near the edge of the storm.

Some hailstones are of great size; you will find it interesting to examine them. If you do this, notice the rings of clear and clouded ice that are often to be seen. These are caused when the hail, after forming, settles to a place where it melts a little, then is lifted again by another current, growing larger by the addition of more vapor. This continues until finally the ice ball sinks to the ground.

There is thunder and lightning in such storms. Few things in nature are grander than these, and those who will watch the lightning flash will see many beautiful and interesting sights (Fig. 16). Sometimes the flash goes from cloud to cloud, again from the cloud to the ground. No one knows exactly why the lightning comes; but we do know that it is an electric spark, something like that which one can often see pass from the trolley to the wire of an electric car line. The main difference is that the spark in a thunder storm is a powerful lightning bolt that passes over a space of thousands of feet and often does great damage where it strikes.

The thunder is a sound which may be compared to the crack heard when a spark passes from the trolley, though of course the noise is very much louder. The crack of the lightning echoes and reverberates among the clouds, often changing to a great rumble; but this rumbling is mainly caused by the echo, the sound from the lightning being a loud crack or crash like that which we sometimes hear when the lightning strikes near by.

Some of the vapor of the air, on condensing, gathers on solid objects like grass, or glass; but some, as fog, floats about in the air. Really this, too, is often gathered around solid objects. Floating about in the air are innumerable bits of "dust" which you can see dancing about in the sunlight when a sunbeam enters a dark room. Some of these "dust" particles are actual dust from the road, but much of it is something else, as the pollen of plants, microbes, and the solid bits produced by the burning of wood or coal.

Each bit serves as a tiny nucleus on which the vapor condenses; and so the very "dust" in the air aids in the formation of rain by giving something solid around which the liquid can gather. The great amount of dust in the air near the great city of London is believed to be one of the causes for the frequent fogs of that city.

That there is dust in the air, and that the rain removes it, is often proved when a dull hazy air is changed to a clear, bright air by a summer shower. Watch to find instances of this. Indeed, after such a hazy day, when the rain drops first begin to fall, if you will let a few drops fall upon a sheet of clean white paper, and then dry it, you will find the paper discolored by the dust that the rain brought with it. So the rain purifies the air by removing from it the solids that are floating in it.

These are only a few of the things of interest that you can see for yourself by studying the air. Watch the sky; it is full of interest. See what you can observe for yourself. Watch especially the clouds, for they are not only interesting but beautiful (Fig. 17). Their forms are often graceful, and they change with such rapidity that you can notice it as you watch them. Even in the daytime the colors and shadows are beautiful; but at sunrise and at sunset the clouds are often changed to gorgeous banks of color.

Fig. 17. A sky flecked with clouds high in the air.

Watch the clouds and you will be repaid; look especially for the great piles of clouds in the east during the summer when the sun is setting (Fig. 18). Those lofty banks, tinged with silver and gold, and rising like mountains thousands of feet into the air, are really made of bits of fog and mist. Among them vapor is still changing to water and rain drops are forming, while violent currents are whirling the drops about, and perhaps lifting them to such a height that they are being frozen into hailstones. Far off to the east, beneath that cloud, rain is falling in torrents, lightning is flashing and thunder crashing, though you cannot hear it because it is so far away.

Fig. 18. The cloud banks of a thunder storm on the horizon.

You see the storm merely as a brightly lighted and beautifully colored cloud mass in the sky; but the people over whom it is hanging find it a threatening black cloud, the source of a furious wind, a heavy rain, and the awe-inspiring lightning. To them it may not be beautiful, though grand in the extreme; and so, too, when the summer thunder shower visits you in the early evening, you may know that people to the west of you are probably looking at its side and top and admiring its beauty of form and color.

The storm passes on, still to the eastward, and finally the cloud mass entirely disappears beneath the eastern horizon; but if you watch, you will see signs that it is still there, though out of sight; for in the darkness of the night you can see the eastern horizon lighted by little flashes, the source of which cannot be seen. You call it "heat lightning," but it is really the last signal that we can see of the vanishing thunder storm, so far away that the sound of the crashing thunder cannot be heard.

You watch the mysterious flashes; they grow dimmer and dimmer and finally you see them no more. Our summer shower is gone. It has done what thousands of others have done before, and what thousands of others will do in the future. It has started, moved off, and finally disappeared from sight; and as it has gone it has told us a story. You can read a part of this story if you will; and in reading it will find much that interests.