Common Science Part 37

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 cla.s.s with the aid of the teacher._ Use a piece of gla.s.s 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.[7] 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.

[Footnote 7: If mercury spills on the floor or table during this experiment, gather it all into a piece of paper by brus.h.i.+ng even the tiny droplets together with a soft brush; squeeze it through a towel into a cup to clean it. It is expensive; so try not to lose any of 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.

[Ill.u.s.tration: FIG. 152. Filling the barometer tube with mercury.]

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_.

[Ill.u.s.tration: FIG. 153. Inverting the filled tube in the cup of mercury.]

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.

[Ill.u.s.tration: FIG. 154. Finding the pressure of the air by measuring the height of the mercury in the tube.]

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.

[Ill.u.s.tration: FIG. 155. The kind of mercury barometer that you buy.]

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.

[Ill.u.s.tration: FIG. 156. An aneroid barometer is more convenient than one made with mercury. The walls are forced in or spring back out according to the pressure of the air. This movement of the walls forces the hand around.]

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.

[Ill.u.s.tration: FIG. 157. Different forms of snowflakes. Each snowflake is a collection of small ice crystals.]

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 b.a.l.l.s 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 pus.h.i.+ng 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 s.p.a.ce. 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 gra.s.s 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 pa.s.ses 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 gla.s.s of water and cover the gla.s.s. Be sure the outside of the gla.s.s 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 gla.s.s of ice water come from? What made it condense?

[Ill.u.s.tration: FIG. 158. If you blow gently over ice, you can see your breath.]

_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.

[Ill.u.s.tration: FIG. 159. The gla.s.s does not leak; the moisture on it comes from the air.]

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.