Moisture In The Atmosphere

( Originally Published Early 1900's )

WITHOUT moisture in the air we could not live, unless nature had constructed us differently. Most plants cannot survive without fresh water every day; yet a few of them can live without water much longer than man, and some lower animals can do likewise. The cactus, for example, stores up moisture to enable it to survive droughts. Similarly, a few animals, such as the camel and the dromedary, can store water; but in their case the supply lasts only a few days, while the desert plants store up enough for weeks or months. We, however, have not acquired this ability to get along without a daily supply of water, and so nearly every minute of the day we are alert to the necessity for moisture.

The moisture of the atmosphere occurs, for the most part, in the form of an invisible vapor. This we know, because if we confine some air within a glass jar and lower its temperature sufficiently, droplets of water will accumulate upon the inside surface of the container. Or again, most of us have observed the appearance of water upon the outside of our dinner-table gob-lets when ice-water has been poured into them. This moisture has come from the air around the glass.

The water vapor of the air is derived from the oceans, seas, lakes, and rivers, from the bare earth itself, from objects upon the earth, and from vegetation. It is hardly necessary here to prove its great importance to all life and particularly to man, but it is well for us to reflect now and then that without water vapor we could have no rain and therefore no crops, no trees, or any other type of vegetation. Water vapor helps the atmosphere retain a portion of the sun's heat and checks the radiation of heat from the earth, thereby contributing to our warmth. As we shall note when we discuss the winds, water vapor modifies our climate by preventing very high temperatures and very low temperatures such as we might have if there were no moisture in the air. Each one of us is greatly affected by the daily variations in the amount of moisture, not only in the atmosphere out-of-doors, but within our homes, in our offices, and in public buildings. These effects we shall point out later.

Scientists can measure the exact amount of moisture which the air contains at any moment.

This quantity they call the "absolute humidity." For example, a certain volume of air may have distributed within it an ounce of water vapor. This is its absolute humidity. But since air expands and contracts readily, the relationship of this quantity of water vapor to the space occupied by the air changes under different temperature conditions. While the exact amount of water vapor in the atmosphere is a matter of importance, we are usually more interested in knowing what the relative humidity of the air is. When people say on a sultry summer day, "Wasn't the humidity terrible to-day?" they have in mind the relative humidity rather than the absolute humidity. They have experienced a day when the air has had a relative humidity of perhaps 80 to 90 per cent, and in addition the temperature has been high. Relative humidity expresses the ratio between the actual amount of water the air contains and the amount it could contain if it were saturated. For example, if the air were saturated, as it is when we have fogs, then its relative humidity would be 100 per cent, because it then holds all the moisture it can hold at the given temperature. But now suppose that the sun shone brightly above the fog and increased the temperature of the air. The air would expand and then would be capable of holding more moisture.

Consequently, the fog would gradually disappear and the relative humidity would decrease, because now the warmed air would not be saturated. The air might be holding three fourths of all it could hold at its new temperature, in which case its relative humidity would be only 75 per cent.

We have observed that the air receives some of its water vapor from the earth itself and objects upon it. The process whereby these minute water particles enter the air is known as evaporation. Evaporation is taking place constantly from the surface of all liquids and all solids at all temperatures. Every one of us feels the effect of evaporation when we fan ourselves upon a hot day or when we seek a breeze to cool us off. Some persons tell the direction of the wind by wetting a finger and exposing it to the wind. The side of the finger which feels coolest, indicates the direction from which the wind comes, because evaporation goes on more rapidly where the breeze is strongest.

If it were possible to magnify surfaces sufficiently to enable us to see at first hand every-thing that was happening upon them, we would observe the water vapor particles moving about constantly in motion like minute "busybodies" darting here and there. They would be moving at very high speeds. Then, too, the gases in the atmosphere against these surfaces would be seen moving rapidly. If the temperature of the air were suddenly increased we would see swarms of water vapor particles leave the surfaces and enter the air, because the air particles would have been separated that is, the air would have been expanded, and there would be more room available for more water vapor particles.

We mentioned the fact that when the air is heated, moisture may be taken up. The converse is equally true. When the air is cooled, water vapor is given up, that is, condensed. Condensation may result in many different water formations depending upon the conditions under which the process takes place. Before noting the most important of them, we should recall that condensation always occurs when the air is saturated and that saturation may happen at various temperatures. Whatever may be the temperature at which condensation begins to take place, this temperature is known as the dew-point.

When our ancestors first endeavored to explain the formation of dew, they thought it fell from the air as the temperature lowered at night and the water vapor just above the surface condensed into droplets, but they were mistaken, for dew never falls. Some persons may tell you that they have seen dew fall, but they have really seen a mist forming in the air similar to that in clouds, or they have witnessed mist falling just as rain does. But this was not dew. Our poets and other writers have helped to perpetuate the false notion of falling dew. Even James Whit-comb Riley, admired as he was by all, committed an error when he wrote :

One naked star has waded through
The purple shallows of the night,
And faltering as falls the dew
It drips its misty light.

The Beetle.

Dew forms only upon the surface of objects. It does so when the temperature of the surface falls below the dew-point of the air with which it is in contact. Usually we can tell the nights when the dew may form. They are still and clear. Then radiation may go on freely from the earth's surface, resulting in the cooling of solids and, consequently, of the air in contact with them. When, however, the night is windy or cloudy, we do not expect dew. The atmosphere is so stirred up that as fast as it cools off in one place the wind displaces the cool air with a new supply of warm air, and so the night passes before all the air can be cooled to the dew-point. Again, if the sky is overcast, the clouds serve as a blanket for the earth and check radiation, preventing the temperature of the air from being reduced to the dew-point.

Now and then dew is particularly heavy upon vegetation, and occasionally farmers will say that the heavy dew is the equivalent of a light shower. This, however, is very doubtful, because we find that much of the moisture which has been condensed from the air had previously been given off (transpired) from the plants and now re-turns in part to their surface as dew. While dew undoubtedly benefits plant life slightly, it may interfere with farming. When the corn is high or other crop plants are well out of the ground, they may have so heavy a coating of dew that the farm-hands cannot cultivate between the rows early in the morning without getting soaking wet. Then they must wait until the strong sun-shine or the wind evaporates the dew. Often-times farmers must delay their work an hour or two this, of course, is costly.

How picturesque and ornate frost is! Coating the window-pane, incasing the vegetation, or spread upon the streets, frost sparkles in the early morning light, freshens all the exposed surfaces, and at times converts an ordinary landscape into a veritable fairyland. Though frost appeals to those with an appreciation of the beautiful, yet it finds no friends among fruit-growers, market-gardeners, and other cultivators of the soil who produce plants sensitive to frost.

The formation of frost is not unlike that of dew, excepting for the temperature of the dew-point. This must be below 32° F., that is, below the freezing-point, when, as the water vapor of the air is condensed upon the surface of the earth, it passes instantly into the frost state without first becoming water. Some people incorrectly say frost is frozen dew, and once in a while dictionaries and encyclopedias give similar definitions. However, if we just stop to analyze that definition, we can see that if frost were frozen dew, it would take the form of small balls of ice instead of the fine variegated spicules of opaque or translucent soft ice.

The length of the growing season for most of the world's crops is determined by the dates for the last killing frost in spring and the first killing frost of autumn. The occurrence of frost is spasmodic, and therefore the length of the season may be different in places which are with-in a few miles of each other. Many devices are used to protect plants against killing frost and thereby to lengthen the growing season, as well as save the crops from any possible damage either because of late spring frosts or early autumn frosts. Locations which often accomplish this end are upon windward shores of lakes, southern slopes of mountains, or upon slopes short distances above valley floors. The moist air from water bodies checks the loss of heat at night, preventing the temperature of the dew-point from falling to the freezing-point. Planting upon slopes of valley sides provides for good air drain-age, permitting cold air to settle in the valley bottom, which in turn forces the warm air up the valley sides. Smudge-pots, lattice covering, electric heaters, hot-water ditches, flooding, and many other devices are employed to prevent frost occurrence. While not always successful, they give a sufficiently high percentage of worth-while results to warrant their use whenever heavy damage from frost is likely to occur.

A common form of atmospheric moisture is fog. In some localities it has economic value. Much of its moisture is deposited upon vegetation and upon the soil in amounts often equivalent to a light shower. For the most part, however, fog is a serious obstacle to man's freedom of movement. Shipwrecks, train-wrecks, slowed-up traffic, increased use of artificial light are all costly. In manufacturing centers fog mixes with smoke, preventing the latter from escaping high into the air, and so helps to cover buildings, and our clothes as well, with dirt and soot. Breathing the moist unclean air may injure our health, especially if fogs occur frequently. Lon-don is a city famous for fogs, but such fame costs her millions of dollars every year, because fogs handicap business and most other activities of the people.

Many of us have noticed how patchy fog-distribution may be. This is because of the exceedingly sensitive conditions responsible for its formation. When the temperature of the air is reduced to the dew-point, either by radiation or by a cold mass of air coming into contact with a warm mass, then condensation of the water vapor takes place. The resulting droplets are so fine that they may not fall to the earth. Such a gathering of minute drops at or nearly in con-tact with the earth we recognize as fog. The drop-lets really do not float in the air, but most of them probably evaporate to make the fog continuous until such time as the rate of evaporation is greater than the rate of formation, at which time we may say that the fog "is lifting"; in other words, it is disappearing.

Have you ever stretched yourself full length upon the green and gazed at the passing clouds above? What a sight they are in all their many forms and colors! Most persons think clouds are vast masses of floating water vapor, but alas ! this is merely another one of those mistaken impressions about the weather and atmosphere harbored by great numbers of persons. Clouds are just like fogs which we described in the preceding paragraphs, only clouds do not rest on the earth's surface. Frequently, when fogs "lift" we see them later high above us and then call them clouds. In a mountain valley we may see the clouds around the peaks, but if we climb the slopes as far as these clouds, we remark about the fog around us and not the clouds. In other words, clouds are made up of water vapor which has condensed at some distance above the earth's surface. These droplets of water composing the cloud are constantly evaporating and new ones being formed. If we observe a cloud very closely as it seemingly floats, we can often see the portion toward the wind disappear, while that part farthest away from the wind is forming anew. If our eyes were upon a level with the top of the cloud, we could notice the water particles evaporate; but as each droplet evaporated an-other would rise from the lower part of the cloud to take its place.

Clouds tell us a great deal about the coming weather and about what is taking place in the air above. This is such an interesting and useful story that we shall devote an entire chapter to it.

Clouds not only shade the sun upon hot days, thereby affording us some relief from extreme heat, but they may obscure the sun upon winter days, making the air colder than it would be otherwise. Clouds, of course, give us our rain, snow, hail, or sleet, and each of these, except the latter two, may be useful or not, according to the amount of precipitation and the time of the year when it occurs.

In contrast with clear skies which induce good cheer and happiness, cloudiness frequently depresses people. Our police records show a tendency toward more crimes during cloudy weather than in sunny weather, and even increased suicides on cloudy days. Students seem to think bet-ter when the days are clear than when overcast. Cloudy days are darker than clear days, and this means the use of more kerosene, or gas, or electricity. Naturally, such increases in the consumption of fuel for light pleases the companies that sell the fuel. Clouds at times serve effectively in preventing excess of strong sunshine for certain plants; but, on the other hand, they may decrease sunshine to an extent that damages crops by slowing down their growth or the ripening of their fruits. No doubt each one of us can think of still other influences of the clouds, some of them affecting our own selves. When it rains, many a boy or girl sings :

Rain, rain, go away,
Come again some other day

This appeal is offered particularly if an outing is scheduled for the rainy day. And yet, now that protecting raincoats, high boots, and even rainproof hats are available, it is possible to enjoy a rainy day as well as a clear one. Charles S. Brooks says in his delightful essay "On a Rainy Morning" : "If a rainy day lacks sunshine, it has vigor for a substitute. . . . There is so much life on wet and windy days."

Although rain, at times, may inconvenience us, particularly if it is of the drenching variety, remember that friend farmer needs rain. 'Without it he could not produce abundant crops, and with-out such crops our cost of living would rise so high that we should have to go without many foods. Rain supplies the water for our rivers, ponds, and lakes ; rain cleanses the air of soot, excessive dust, and many other impurities. Going out-of-doors after a shower, we often remark about the freshness of the air and the sharpness of shadows cast by trees and buildings.

The formation of rain represents merely one stage beyond the formation of clouds. You will recall that clouds form when the water vapor of the atmosphere at some distance above the earth's surface is condensed into water particles which remain in suspension in the air. If condensation above the freezing-point progresses far enough, the droplets will increase in size sufficiently so that the air no longer can support them; then they fall to the earth. Sometimes minute drops unite to make larger ones which are heavy enough to fall. These falling drops we call rain.

If you examine a rain drop under a micro-scope or fairly powerful pocket lens, you are more than likely to find a minute dust particle in the center. However, all drops do not form around dust particles. The atmosphere is full of electricity, which, as you may know, is composed of two kinds of charges, positive and negative. These charges, physicists have found, may attract water vapor just as dust does, and condensation of water vapor may occur about them as centers. When the drops of water around these charges become heavy enough they drop to the earth as rain.

We have noted the benefits which rain brings to man, and for these we surely are grateful. But rain may come down in such great quantities either in a day or a few days, or in the course of a month, that rivers fill to overflowing and their raging flood-waters destroy crops, carry away homes, drown live stock, and even cause loss of human life. The great flood in the Mississippi Valley in 1928 did millions of dollars of damage. The United States Government has set aside more millions of dollars to build strong dikes and other engineering works along the "Father of Waters" in an effort to prevent a repetition of disastrous floods.

We measure rainfall with a simple device called a rain-gage, which looks like an ordinary big tin can with a funnel in the top of it. The ratio between the diameter of the funnel and the can into which the water drops is carefully figured and from this is determined the amount of rain caught by the gage. By annual rainfall, or the more inclusive term precipitation, is meant not only rain but snow. Rain is measured in inches. When we say one inch of rain has fallen in any region we mean that if all the rain could have been caught and spread out upon a perfectly level surface, it would have measured exactly one inch in depth. If you live in the northeastern part of the United States, the rainfall for an entire year may be thirty-five or forty inches. In certain areas it may be more; in others less. These amounts, however, are small compared with a few places in the Puget Sound district, where the fall ranges from eighty to one hundred and twenty inches, and exceedingly small if compared with a town in the mountains of northeastern India or with localities in the Hawaiian Islands, where over four hundred inches of rain occur in a year. On the other hand, thirty to forty inches of rain is many times greater than that in hot deserts, where in some years there are less than five inches of rain and in other deserts no rain during a whole year or two or three years. In certain parts of the Atacama Desert of Chile rain falls about once in seven years.

Have you ever caught snowflakes upon the sleeve of your coat and examined them carefully before they melted? If you have, then you are acquainted with their beautiful lacelike hexagonal crystal forms. Snowflakes may be photographed if one has the right sort of a camera outfit, but such photography requires almost infinite patience and costly apparatus. One man, however, has devoted many years of his life to studying and photographing snowflakes. W. A. Bentley of Jericho, Vermont, made the micro-photographs reproduced here (frontispiece) . These are only a few samples from among. hundreds of photographs. You will note that although the edges of the flakes may be irregular in outline, nevertheless if straight lines be drawn connecting the outermost points they will form a hexagon.

One can well imagine many motifs for designs in lace, ornamental stone, wrought-iron, or other decorative materials which can be derived from these attractive snowflake crystals.

Snow forms in the same manner as rain, differing from the latter only in its temperature of condensation. If the water vapor of the air above the earth's surface condenses at a temperature of 32 F. or below, then it passes into snow at once.

We measure snow by catching it in a snow-gage, similar in form to a rain-gage. A known amount of water is added to the snow to melt it and then the depth of all of the water is measured just as though it had all been rain water. We then subtract the amount of water required to melt the snow from the total amount and the resulting figure is the equivalent in water of the amount of snow which fell. One inch of water equals ten inches of snow, so if we find for example that the water from the melted snow equals one-half inch, then we know that five inches of snow have fallen. Measuring snow accurately is difficult because of the uncertainty that we have caught all in the gage that should have entered it. Sometimes our gage readings are checked by finding a level surface where the snow has fallen, undisturbed by the wind, and measuring the depth of snow direct. This observation and that of melted snow in the gage give us a fairly good idea of the actual amount of snowfall. When we include snowfall in total :rainfall figures we count only the equivalent in water and not the depth of the snow itself.

Like rain, snow benefits man as well as handicaps him. A heavy snow covering the ground when spring arrives provides moisture for many crops. This supply is particularly valuable for wheat. As a soil covering, snow prevents alternate freezing and thawing. The soil does not heave that is, it does not push up into little knolls and force plants out of the ground. In this respect snow serves as an important help to agriculture. Then, again, in some of the hot semi-arid and arid regions of the earth where agriculture is possible with the aid of irrigation the water supply is frequently derived from the melting snow that has accumulated in the mountains all winter and far into the spring. In northwestern India the great Indus River flows across the desert, supplying water for the numerous irrigated areas in its valley. Its water supply in turn comes from the melting of the snow in the imposing Himalayas.

Out of the bosom of the Air,
Out of the cloud-folds of her garments shaken,
Over the woodlands brown and bare,
Over the harvest-fields forsaken,
Silent and soft and slow
Descends the snow.

From LONGFELLOW'S Snow-Flakes.

The poetic appeal of snow is lost to the practical minded when its large heavy flakes continue for hours and hours to accumulate in such quantities as to block street traffic and halt railroad trains, or pile up so high upon hill and mountain as to start great avalanches sliding down to the valley below, sweeping away soil, trees, fences, and even dwellings. Occasionally avalanches arrive at the base of a mountain just as a train appears from around a curve. Our western railroads build snow-sheds over their tracks for many miles to protect their trains against such possible havoc.

Although a single snowflake falls as gently to the earth as though it were but a feather, yet a great mass of flakes possesses much weight. As snow accumulates it packs, particularly if the temperature of the air at the surface fluctuates between temperatures slightly above freezing and slightly below, and the resulting water mixes with the rest of the snow. In certain localities, be-cause of the possible collection of a heavy weight of snow, flat roofs require a form of bracing which will prevent their collapsing. Pitched roofs often have small hooks or loops projecting from their surfaces at frequent intervals to prevent the snow from sliding off upon the heads of pedestrians. These slides might otherwise happen either during heavy snow-storms or when the sun begins melting the snow which has piled upon the roof slope. Heavy falls of snow accumulating upon the branches of trees may break them and do extensive damage.

Snow can injure us, but snow also can afford us much pleasure. Coasting, skiing, snow-shoeing, and cross-country hiking are all in order after a snowfall. Many persons impatiently await the first snow of the season, that they may indulge in one or more of these exhilarating sports. When we begin to lose interest in snow, then it is time for us to examine ourselves.

A vast industry depends for its success to a great extent upon a snow covering upon the ground all winter. This is the lumber industry. Trees are cut in winter and the logs brought out of the forest to the stream, where, in the following spring season, they are floated down to the sawmill. If the snow is absent, the logs cannot be easily skidded over the ground and so the cost of handling them is much higher than normally. The lumber companies of our Northern States and northern countries always wish for plenty of snow each winter. In countries such as Nor-way, Sweden, and Finland the national prosperity depends to a large degree upon favorable snows to permit of successful logging.

Hail occurs only in company with thunder-storms. No one seems to have discovered any benefits from hail, but, on the other hand, the damage done is familiar to most persons. Hail-stones of course, will break the glass roofs of greenhouses without any trouble at all, and if they come down at the right angle they may break the windows in our homes. Hailstones range in size from just a fraction of an inch in diameter to three or four inches or more. In Montana, Idaho, or in the regions often visited by tornadoes, they may reach these larger sizes. Some-times hailstones beat down crops. In the western plains of the United States they have been known to grow so large and to come down in such great quantities as to kill not only small animals like chickens and ducks or other poultry, but sheep, horses, and cattle. Losses to property from hail-storms may run into the hundreds of thousands of dollars during a single storm, and in the period of a year they may amount to ten or fifteen millions of dollars for the entire United States.

If you cut a hailstone in half, you will notice that it consists of a series of ringlike formations. One ring of ice will be clear, the next one cloudy, the next clear and so on throughout the whole stone. This interesting succession of ice layers has called forth many different ideas as to how hail is formed. We are certain of only one thing —namely, that hail is not frozen rain; that is, it is not the result of a raindrop freezing on its direct route to the earth. If we could just say, "Hail is frozen rain," we would have a very easy definition. We shall not try to give all the different theories here, but just mention one quite briefly.

A hailstone is supposed to start out from a high cloud as a water particle which is lifted by upward moving air currents into altitudes high enough to freeze it. Then the ice ball begins dropping until it reaches a warm air layer, when some of the water vapor freezes to it and upward currents carry it back into high levels, where ice is added to the hailstone, giving it a clear layer. Once more it starts toward the earth, and again it may add a layer of cloudy ice from the water vapor of the warm lower level air and finally drop to earth. So a hailstone may be carried up and down in the rising and falling air currents a half dozen times or more before coming to the earth. Aviators who have lived through a thunder-storm will testify to the capacity of the up-draft to carry hailstones from low levels to high levels.

Sleet is frozen rain. What a simple definition that is! We recognize sleet rapidly by the manner in which the little elongate ice pellets of which it is made cut our faces as they are blown against us. These pellets are rather small, never the size of a large hailstone.

Glaze or Glatteis, as the Germans call it, consists of the icy coating that sometimes gathers upon trees, fence-posts, and our streets and side-walks as well. We can almost see it form as the rain drops strike these objects and the splashing water freezes. Sometimes we have sleet and glaze at the same time. This occurs when sleet and rain are mixed and the temperature of the earth's surface is at the freezing-point or below.

An ice storm often converts the landscape into a new world where everything seems studded with brilliant diamonds. This is especially true in our parks or in the country, where every single twig incased in crystal-clear glossy ice reflects the dazzling sunbeams in a thousand different directions. The sight is one which we remember for years, particularly because of its beauty. Some of these ice accumulations are so thick that their weight breaks not only twigs but great branches. The delightful image formed in our minds is often spoiled by the thought of the tremendous amount of damage done and the ugly appearance which our trees may acquire after the ice melts and their numerous broken trunks and branches stick up awkwardly above the lawns or fields or meadows. Heavy ice storms break telegraph and telephone wires and poles, and make rails and roadways slippery and dangerous for traffic. Ice storms are most common in the northeastern quarter of the United States.