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READING THE WEATHER
OUR WELL-ORDERED ATMOSPHERE
IF there is anything that has been overlooked more than another it is our atmosphere. But it absolutely cannot be avoided in books on the weather. It deserves a chapter, anyway, because if it were not for the atmosphere this earth of ours would be a wizened and sterile lump. It would float uselessly about in the general cosmos like the moon.
To be sure the earth does not loom very large in the eye of the sun. It receives a positively trifling fraction of the total output of sunheat. So negligible is this amount that it would not be worth our mentioning if we did not owe our existence to it. It is thanks to the atmosphere, however, that the earth attains this (borrowed) importance. It is thanks to this thin layer of gases that we are protected from that fraction of sunheat which, however trifling when compared with the whole, would otherwise be sufficient to fry us all in a second. Without this gas wrapping we would all freeze (if still unfried) immediately after sunset. The atmosphere keeps us in a sort of thermos globe, unmindful of the burning power of the great star, and of the uncalculated cold of outer space.
Yet, limitless as it seems to us and inexhaustible, our invaluable atmosphere is a small thing after all. Half of its total bulk is compressed into the first three and a half miles upward. Only one sixty-fourth of it lies above the twentyone mile limit. Compared with the thickness of the earth this makes a very thin envelope.
Light as air, we say, forgetting that this stuff that looks so thin and inconsequential weighs fifteen pounds to the square inch. We walk around carrying our fourteen tons gaily enough. The only reason that we don't grumble is because the gases press evenly in all directions permeating our tissues and thereby supporting this crushing burden. A layer of water thirty-four feet thick weighs just about as much as this air-pack under which we feel so buoyant. But if these gases get in motion we feel their pressure. We say the wind is strong to-day.
As it blows along the surface of the earth this wind is mostly nitrogen, oxygen, moisture, and dust. The nitrogen occupies nearly eight-tenths of a given bulk of air, the oxygen two-tenths, and the moisture anything up to one-twentieth. Five other gases are present in small quantities. The dust and the water vapor occupy space independently of the rest. As one goes up mountains the water vapor increases for a couple of thousand feet and then decreases to the seven mile limit after which it has almost completely vanished. The lightest gases have been detected as high up as two hundred miles and scientists think that hydrogen, the lightest of all, may escape altogether from the restraint of gravity. One strange fact about all of these gases is that they do not form a separate chemical combination, although they are thoroughly mixed.
At first glance the extreme readiness of the atmosphere to carry dust and bacteria does not seem a point in its favor. In reality it is. Most bacteria are really allies of the human race. They benefit us by producing fermentations and disintegrations of soils that prepare them for plant food. It is a pity that the few disease breeding types of bacteria should have given the family a bad name. Without bacteria the sheltering atmosphere would have nothing but desert rock to protect.
Further, rain is accounted for only by the dust. Of course this sounds very near the world's record in absurdities. But it is a half truth at least, for moisture cannot condense on nothing. Every drop of rain, every globule of mist must have a nucleus. Consequently each wind that blows, each volcano that erupts is laying up dust for a rainy day. Apparently the atmosphere is empty. Actually it is full enough of dust-nuclei to outfit a full-grown fog if the dew-point should be favorable. If there were no dust in the air all shadows would be intensest black, the sunlight blinding.
But the dust particles fulfill their greatest mission as heat collectors, -- they and the particles of water vapor which have embraced them. It is in reality owing to these water globules and not to the atmosphere that supports them that we are enabled to live in such comfortable temperatures. For the air strata above seven miles where the tides of oxygen and nitrogen have rid themselves of water and dust absorb very little of the solar radiation. The heat is grabbed by the lowest layer of air as it goes by. The air snatches it both going and coming. The little particles get about half of it on the way down and when it is radiated back very little escapes them.
So it comes about that the heavy moist air near the earth is the warmest of all. It would, of course, get very warm if, as it collected its heat, it didn't have a tendency to rise. As it rises, moreover, it must fight gravity, that arch enemy of all rising things. And as it fights it loses energy, which is heat. So high altitudes and low temperatures are found together for these two reasons. But after the limit of moisture content has been reached the temperature gets no lower according to reliable investigations. Instead a monotony of 459° below zero eternally prevails -459° is called the absolute zero of space.
The vertical heating arrangements of the atmosphere appear somewhat irregular. But horizontally it is in a much worse way. The surface of the globe is three quarters water and one quarter land and irregularly arranged at that. The shiny water surfaces reflect a good deal of the heat which they receive, they use up the heat in evaporation and what they do absorb penetrates far. The land surfaces, on the contrary, absorb most of the heat received, but it does not penetrate to any depth. As a consequence of these differences land warms up about four times as quickly as water and cools off about four times as fast. Therefore the temperature of air over continents is liable to much more rapid and extreme changes than the air over the oceans.
The disparity of temperature is also rendered much greater because of differing areas of cloud and clear skies, because of interfering mountain masses, because of the change from day to night, or the constant progress of the seasons. At first blush it seems remarkable that the atmosphere should not be hopelessly unsettled in its habits, that there should belong to it any hint of system. As a matter of fact, in the main its courses are as well-ordered as the sun's. Cause and not caprice are at the bottom of the wind's listings. Its one desire is rest.
But rest it rarely succeeds in finding. Forever warming, rising, cooling, falling, it rushes about to regain its equilibrium. With so many opposing forces at work the calm day is the real marvel, our weeks of Indian Summer the ranking miracle of our climate. The very evolution of the myriad patches of air quilted over the earth with their different opportunities to become heated, to cool their heels, precludes stability in our so called Temperate Zone. But over great stretches of the earth's surface conditions are continuous enough to discipline the atmosphere into strict routine. Conjure the globe before your eyes and you will find the scheme of atmospheric circulation something like this:
A broad band of heated air perpetually rises from the sweltering equatorial belt of lands and seas. The supply never ceases, the warming process goes on night and day, and to a great height the light warm incense mounts. Then, cooling, from this altitude it begins to run down hill toward the poles. This is happening all the way around the globe. So naturally the common centers, the poles, cannot accommodate all this downrush of air. Therefore as it approaches the goal it falls into a majestic file about the center, very much as water does in running out of a hole in the center of a circular basin. The nearer north, the cooler this vast maelstrom grows and the nearer has it sunk to the earth. It descends circuitously and, by the force arising from the earth's rotation, is sheered to the right in the northern hemisphere, to the left in the southern.
Watching the water circle out of the basin you will notice the outside whirl is in no hurry to get to the center. This corresponds to the easterly trades of commerce, geography, and fiction. The direction of the upper currents flowing back to the poles is from southwest to northeast; but in our middle zones this becomes almost from west to east, is constant and is known to the profession as the prevailing westerlies.
Look up some day when wisps of clouds are floating very high. You will notice that their port is in the east, mattering not what wind may be blowing where you are. They are above the petty disturbances of the shallow surface winds. They follow a Gulf Stream of immeasurable grandeur. Onward, always onward, they sail, emblems of a great serenity.
Beneath this vast drift of air, which increases in velocity as it nears the pole, is an undertow from two to three miles thick. It is the movements of this undertow that affect our lives. These movements are influenced by all the changes of temperature and by the configurations of land. They take the form of whirls. These whirls may be small eddies, local in effect, or vast cyclones with diameters of fifteen hundred miles. Small or large they roll along under the Westerlies, translated by friction, and invariably moving for most of their course in an easterly direction, like their tractor above. They circle across the United States every few days. Their courses do not vary a great deal, and yet enough to make each one a matter for conjecture. And all the conjecturing centers upon the condition of the atmosphere, the changing atmosphere which is yet so dependable. The weather we are used to, the daily weather that catches us unprepared, and yet that does not mistreat us all the time is the product of these little whirls, which are so remotely connected with the grander atmospheric movements of our planet. Remembering this, we can at last come back to earth and set about our real business which is to see why certain kinds of weather come at such uncertain times and how to tell when they will arrive.