Section 44. _clouds, Rain, and Dew: Humidity._

Why is it that you can see your breath on a cold day?

Where do rain and snow come from?

What makes the clouds?

There is water vapor in the air all around us--invisible water vapor, its molecules mingling with those of the air--water that has evaporated from the oceans and lakes and all wet places.

This water vapor changes into droplets of water when it gets cool enough. And those droplets of water make up our clouds and fogs; they join together to form our rain and snow high in the air, or gather as dew or frost on the grass at night.

If the water vapor should suddenly lose its power of changing into droplets of water when it cooled,--well, let us pretend it has lost this power but that any amount of water can evaporate, and see what happens:

What fine weather it is! There is not a cloud in the sky. As evening closes in, the stars come out with intense brightness. The whole sky is gleaming with stars--more than we have ever seen at night before.

The next morning we find no dew or frost on the grass. All the green things look dry. As the day goes on, they begin to wilt and wither. We all wish the day were not quite so fine--a little rain would help things wonderfully. Not a cloud appears, however, and we water as much of our gardens as we can. They drink the water greedily, and that night, again no dew or fog, and not the faintest cloud or mist to dim the stars. And the new day once more brings the blazing sun further to parch the land and plants. Day after day and night after night the drought gets worse. The rivers sink low; brooks run dry; the edges of the lakes become marshes. The marshes dry out to hardened mud. The dry leaves of the trees rustle and crumble. All the animals and wood creatures gather around the muddy pools that once were lakes or rivers. People begin saving water and buying it and selling it as the most precious of articles.

As the months go by, winter freezes the few pools that remain. No snow falls. Living creatures die by the tens of thousands. But the winter is less cold than usual, because there is now so much water vapor in the air that it acts like a great blanket holding in the earth's heat.

With spring no showers come. The dead trees send forth no buds. No birds herald the coming of warm weather. The continents of the world have become vast, uninhabitable deserts. People have all moved to the shores of the ocean, where their chemists are extracting salt from the water in order to give them something to drink. By using this saltless water they can irrigate the land near the oceans and grow some food to live on. Each continent is encircled by a strip of irrigated land and densely populated cities close to the water's edge.

It is many years before the oceans disappear. But in time they too are transformed into water vapor, and no more life as we know it is possible in the world. The earth has become a great rocky and sandy ball, whirling through space, lifeless and utterly dry.

That which prevents this from really happening is very simple: In the world as it is, water vapor condenses and changes to drops of water whenever it gets cool enough.

HOW WATER VAPOR GETS INTO THE AIR. The water vapor gets into the air by evaporation. When we say that water evaporates, we mean that it changes into water vapor. As you already know, it is heat that makes water evaporate; that is why you hang wet clothes in the sun or by the fire to dry: you want to change the water in them to water vapor. The sun does not suck up the water from the ocean, as some people say; but it warms the water and turns part of it to vapor.

What happens down among the molecules when water evaporates is this: The heat makes the molecules dance around faster and faster; then the ones with the swiftest motion near the top shoot off into the air. The molecules that have shot off into the air make up the water vapor.

The water vapor is entirely invisible. No matter how much of it there is, you cannot see it. The weather is just as clear when there is a great deal of water vapor in the air as when there is very little, as long as none of the vapor condenses.

HOW CLOUDS ARE FORMED. But when water vapor condenses, it forms into extremely small drops of real water. Each of these drops is so small that it is usually impossible to see one; they are so tiny that you could lay about 3000 of them side by side in one inch! Yet, small as they are, when there are many of them they become distinctly visible. We see them floating around us sometimes and call them fog or mist. And when there are millions of them floating in the air high above us, we call them a cloud.

The reason clouds form so high in the air is this: When air or any gas expands, it cools. Do you remember Experiment 31, where you let the gas from a tank expand into a wet test tube and it became so cold that the water on the test tube froze? Well, it is much the same way with rising air. When air rises, there is less air above it to keep it compressed; so it expands and cools. Then the water vapor in it condenses into droplets of water, and these form a cloud.

Each droplet forms a gathering place for more condensing water vapor, and therefore grows. When the droplets of water in a cloud are very close together, some may be jostled against one another by the wind. And when they touch each other, they stick together, forming a larger drop. When a drop grows large enough it begins to fall through the cloud, gathering up the small droplets as it goes. By the time it gets out of the cloud it has grown to a full-sized raindrop, and falls to earth. The complete story of rain, then, is this:

HOW RAIN IS CAUSED. The surface of the oceans and lakes is warmed by the sun. The water evaporates, turning to invisible water vapor. This water vapor mingles with the air. After a while the air is caught in a rising current and swept up high, carrying the water vapor with it. As the air rises, there is less air above it to press down on it; so it expands. When air expands it cools, and the water vapor which is mingled with it likewise cools. When the water vapor gets cool enough it condenses, changing to myriads of extremely small drops of water. These make a cloud.

A wind comes along; that is, the air in which the cloud is floating moves. The wind carries the cloud along with it. More rising air, full of evaporated water from the ocean, joins the cloud and cools, and the water forms into more tiny droplets. The droplets get so close together that they shut out the sun's light from the earth, and people say that the sky is darkening.

Meanwhile some of the droplets begin to touch each other and to stick together. Little by little the drops grow bigger by joining together. Pretty soon they get so big and heavy that they can no longer float high in the air, and they fall to the ground as rain.

Part of the rain soaks into the ground. Some of it gradually seeps down through the ground to an underground stream. This has its outlet in a spring or well, or in an open lake or the ocean. But the rain does not all soak in. After the storm, some of the water again evaporates from the top of the ground and mixes with the warm air, and it goes through the same round. Other raindrops join on the ground to form rivulets that trickle along until they meet and join other rivulets; and all go on together as a brook. The brook joins others until the brooks form a river; and the river flows into a lake or into the ocean.

Then again the sun warms the surface of the ocean or lake; the water evaporates and mixes with the air, which rises, expands, and cools; the droplets form and make clouds; the droplets join, forming big drops, and they fall once more as rain. The rain soaks into the ground or runs off in rivulets, and sooner or later it is once more evaporated. And so the cycle is repeated again and again.

And all this is accounted for by the simple fact that when water evaporates its vapor mingles with the air; and when this vapor is sufficiently cooled it condenses and forms droplets of water.

THE BAROMETER. In predicting the weather a great deal of use is made of an instrument called the barometer. The barometer shows how hard the air around it is pressing. If the air is pressing hard, the mercury in the barometer rises. If the air is not pressing hard the mercury sinks. Just before a storm, the air usually does not press so hard on things as at other times; so usually, just before a storm, the mercury in the barometer is lower than in clear weather. You will understand the barometer better after you make one. Here are the directions for making a barometer:

EXPERIMENT 87. To be done by the class with the aid of the teacher. Use a piece of glass tubing not less than 32 inches long, sealed at one end. Fill this tube to the brim with mercury (quicksilver), by pouring the mercury into it through a paper funnel. Have the sealed end of the tube in a cup, to catch any mercury that spills. When the tube is full, pour mercury into the cup until there is at least half an inch of it at the bottom. Now put your forefinger very tightly over the open end of the tube, take hold of the sealed end with your other hand, and turn the tube over. Lower the open end, with your finger over it, into the cup. When the mercury in the cup completely covers your finger and the end of the tube, remove your finger carefully so that no air can get up into the tube of mercury. Let the open end of the tube rest gently on the bottom of the cup, and hold the tube upright with your hand or by clamping it to a ring stand. Hold a yardstick or meter stick beside the tube, remembering to keep the tube straight up and down. Measure accurately the height of the mercury column from the surface of the mercury in the cup. Then go to the regular barometer hanging on the wall, and read it.

The reason your barometer may not read exactly the same as the expensive laboratory instrument is that a little air and water vapor stick to the inside of the tube and rise into the "vacuum" above the mercury; also, the tube may not be quite straight up and down. Otherwise the readings would be the same.

Of course you understand what holds the mercury up in the tube. If you could put the cup of mercury into a vacuum, the mercury in the tube would sink down into the cup. But the pressure of the air on the surface of the mercury in the cup keeps the mercury from flowing out of the tube and so leaving a vacuum in there. If the air pushes down hard on the mercury in the cup, the mercury will stand high in the tube. This is called high pressure. If the air does not press hard on the mercury in the cup, the mercury stands low in the tube. This is called low pressure.

HOW WEATHER IS FORECAST. Weather forecasters make a great deal of use of the barometer, for storms are usually accompanied by low pressure, and clear weather nearly always goes with high pressure.

The reason storms are usually accompanied by low pressure is this: A storm is almost always due to the rising of air, for the rising air expands and cools, and if there is much water vapor in it, this condenses when it cools and forms clouds and rain. Now air rises only when there is comparatively little pressure from above. Therefore, before and during a storm there is not so much pressure on the mercury of the barometer and the barometer is low.

Clear weather, on the other hand, is often the result of air being compressed, for compressing air warms it. When air is being warmed, the water vapor in it will not condense; so the air remains clear. But when the air is being compressed, it presses hard on the mercury of the barometer; the pressure is high, and the mercury in the barometer rises high. Therefore when the mercury in the barometer is rising, the weather is usually clear.

These two statements are true only in a very general way, however. If weather forecasters had only their own barometers to go by, they would not be of much value; for one thing, they could not tell us that a storm was coming much before it reached us. But there are weather stations all over the civilized world, and they keep in touch with each other by telegraph. It is known that storms travel from west to east in our part of the world. If one weather man reports a storm at his station, and tells how his barometer stands, the weather men to the east of him know that the storm is coming their way. From several such reports the weather men to the east can tell how fast the storm is traveling and exactly which way it is going. Then they can tell when it will reach their station and can make the correct prediction.

Weather men do not have to wait for an actual storm to be reported. If the reports from the west show that the air is rising as it swirls along--that is, if the barometer readings in the west are low--they know that this low-pressure air is approaching them. And they know that low pressure usually means air that is rising and cooling and therefore likely to drop its moisture. In the same way, if the barometers to the west show high pressure, the eastern weather men know that the air that is blowing toward them is being compressed and warmed, and is therefore not at all likely to drop its moisture; so they predict fair weather.

The weather man is not ever certain of his forecasts, however. Sometimes the air will begin to rise just before it gets to him. Then there may be a shower of rain when he has predicted fair weather. Or sometimes the air that has been rising to the west, and which has made him predict bad weather, may stop rising; the storm may be over before it reaches his station. Then his prediction of bad weather is wrong. Or sometimes the storm unexpectedly changes its path. There are many ways in which a weather prophecy may go wrong; and then we blame the weather man. We are likely to remember the times that his prophecy is mistaken and to forget the many, many times when it is right.

HOW SNOW IS FORMED. The difference between the ways in which snow and rain are formed is very slight. In both cases water evaporates and its vapor mingles with the warm air. The warm air rises and expands. It cools as it expands, and when it gets cool enough the water vapor begins to condense. But if the air as it expands becomes very cold, so cold that the droplets of water freeze as they form and gather together to make delicate crystals of ice, snow is formed. The ice crystals found in snow are always six-sided or six-pointed, because, probably, the water or ice molecules pull from six directions and therefore gather each other together along the six lines of this pull. At any rate, the tiny crystals of frozen water are formed and come floating down to the ground; and we call them snowflakes. After the snow melts it goes through the same cycle as the rain, most of it finally getting back to the ocean through rivers, and there, in time, being evaporated once more.

Hail is rain that happens to be caught in a powerful current of rising air as it forms, and is carried up so high that it freezes in the cold, expanding air into little balls of ice, or hail stones, which fall to the ground before they have time to melt.

WHY ONE SIDE OF A MOUNTAIN RANGE USUALLY HAS RAINFALL. When air that is moving along reaches a mountain range, it either would have to stop, or rise and go over the mountain. The pressure of the air behind it, moving in the same direction, keeps it from stopping, and so it has to go up the slopes and over the range. But as it goes up, there is less air above it to push down on it; so it expands. This makes it cool, and the water vapor in it begins to condense and form snow or rain. Therefore the side of mountain ranges against which the wind usually blows, almost always has plenty of rainfall.

It is different on the farther side of the mountain range. For here the air is sinking. As it sinks it is being compressed. And as it is compressed it is heated. If you hold your finger over the mouth of a bicycle pump and compress the air in the pump by pushing down on the handle, you will find that the pump is decidedly warmed. When the air, sinking down on the farther side of the mountain range, is heated, the water vapor in it is not at all likely to condense. Therefore rain seldom falls on the side of the mountains which is turned away from the prevailing winds.

HOW DEW AND FROST ARE FORMED. The heat of the earth radiates out into the air and on out into space. At night, when the earth loses its heat this way and does not receive heat from the sun, it becomes cooler. When the air, carrying its water vapor, touches the cool leaves and flowers, the water vapor is condensed by the coolness and forms drops of dew upon them. Or, if the night is colder, the droplets freeze as they form, and in the morning we see the grass and shrubs all covered with frost.

THE CAUSE OF FOGS. When warm air is cooled while it is down around us, the water vapor in it condenses into myriads of droplets that float in the air and make it foggy. The air may be cooled by blowing in from the warm lake or ocean in the early morning, for at night the land cools more rapidly than the water does. This accounts for the early morning fogs in many cities that are on the coasts.

Likewise when the wind has been blowing over a warm ocean current, the surface of the warm water evaporates and fills the air with water vapor. Then when this air passes over a cold current, the cold current cools the air so much that the moisture in it condenses and forms fog. That is why there are fog banks, dangerous to navigation, in parts of the ocean, particularly off Labrador.

WHY YOU CAN SEE YOUR BREATH ON COLD DAYS. You really make a little fog when you breathe on a cold morning. The air in your lungs is warm. The moisture in the lungs evaporates into this warm air, and you breathe it out. If the outside air is cold, your breath is cooled; so some of the water vapor in it condenses into very small droplets, and you see your breath.

Here are two experiments in condensing water vapor by cooling the air with which it is mixed. Both work best if the weather is warm or the air damp.

EXPERIMENT 88. Put the bell jar on the plate of the air pump and begin to pump the air out of it. Watch the air in the jar. If the day is warm or damp, a slight mist will form.

As part of the air is pumped out, the rest expands and cools, as warm air does when it rises and is no longer pressed on so hard by the air above it. And as in the case of the rising warm air, the water vapor condenses when it cools, and forms the mist that you see. This mist, like all clouds and fog, consists of thousands of extremely small droplets.

EXPERIMENT 89. Hold a saucer of ice just below your mouth. Open your mouth wide and breathe gently over the ice. Can you see your breath?

Now put the ice into half a glass of water and cover the glass. Be sure the outside of the glass is thoroughly dry. Set it aside and look at it again in a few minutes.

What caused the mist when you breathed across the ice?

Where did the water on the outside of the glass of ice water come from? What made it condense?

APPLICATION 66. Explain why clouds are formed high in the atmosphere; why we have dew at night instead of in the daytime; why clothes dry more quickly in a breeze than in still air; why clothes dry more quickly on a sunny day than on a foggy one.

INFERENCE EXERCISE

Explain the following:

411. A gas-filled electric lamp gets hotter than a vacuum lamp.

412. You can remove a stamp from an envelope by soaking it in water.

413. We see our breath on cold days and not on warm days.

414. The electric arc is exceedingly hot.

415. Rock candy is made by hanging a string in a strong syrup left open to the air.

416. Dishes in which candy has been made should be put to soak.

417. Moisture gathers on eyeglasses when the wearer comes from a cold room into a warm one.

418. Sprinkling the street on a hot day makes the air cool.

419. You cannot see things in a dark room.

420. Where air is rising there is likely to be rain.