Читать книгу Meteorology: The Science of the Atmosphere - Charles Fitzhugh Talman - Страница 4

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Two quite different conceptions of the substance called “air” are current in the world. One has prevailed from time immemorial. The other is wholly modern. One is the popular view, the other the scientific.

Ancient philosophers regarded air as one of the four “elements” of which all things were supposed to be made. Average humanity, though it did not concern itself with philosophy, must have begun, almost as soon as it realized the existence of air at all, to think of it as something that, however it changed its state from hot to cold, dry to moist, pure to impure, was fundamentally uniform—a single entity. Certainly this idea is in full vigor today. The air that we breathe, supply to our fires, stir with fans, pump into bicycle tires, fly in—the air that asserts its independence of our will in the wind and the weather—gives us the impression of individuality. We instinctively rank it with water among the simple, definite things in the repertory of nature.

Even the man of science often finds it convenient to discuss and deal with air as if it were a single substance, but he is well aware that it is nothing of the kind. He knows that it is, in fact, a jumble of gases having very different properties. Some are heavy, others light. Some are chemically very active, others extremely inactive. Some are abundant, others very rare. These gases constitute the earth’s atmosphere. Other planets have atmospheres that are quite different in composition from ours. The sun itself has a very complex atmosphere.

The earth’s atmosphere is, then, a collection of gases, which are mixed but not chemically combined. Some of them are themselves chemical compounds. Each of these gases behaves very much the same as if the others were not present, and each of them has its separate business to perform in the economy of nature. For example, a tree draws upon the store of carbon dioxide gas in the atmosphere to build up its tissues. Presently the tree is cut down and its wood is burned for fuel. In this process a different atmospheric gas is brought into play. We often say that the “air” supports combustion—that we supply “air” with a bellows to make a fire burn more brightly—but it is not the air as a whole that enables things to burn. Four-fifths of the atmospheric substance takes no part in the process. We burn with oxygen alone. So it is with breathing. Oxygen and not air constitutes the breath of life.

Near the surface of the earth the proportions of the more abundant gases mixed together in the air are remarkably constant. Ignoring a variable admixture of water vapor, oxygen is always about 21 per cent, by volume, and nitrogen about 78 per cent. The remaining 1 per cent is mainly argon. At great altitudes, however, these percentages no longer obtain. The atmospheric gases differ greatly among themselves in weight, and in the high atmosphere, where they are not mixed by the winds, as they are below, the heavier tend to settle to the bottom and the lighter to float on top, as oil floats on water. It is calculated that at a height of thirty miles above sea level the percentage of nitrogen is about 86½ and of oxygen only 10, while at the same altitude the gas hydrogen, which at low levels constitutes less than one-hundredth of 1 per cent of the atmosphere amounts to more than 2½ per cent. Going higher, the percentage of hydrogen is supposed to increase rapidly, until, at an altitude of forty-eight miles, the atmosphere is more than half hydrogen, and at eighty miles above the earth this gas forms 99 per cent of the whole. These figures are not necessarily final; for some authorities believe that the atmosphere contains an unknown gas lighter than hydrogen, while others think that the hydrogen found in the lower air enters into chemical combinations before it can reach the higher levels; but it is beyond doubt that the composition of the upper atmosphere is quite different from that of the lower.

Of course almost any gas may be found locally and occasionally in the atmosphere, but there are several that are always found wherever a refined analysis of the air is made, and others that are generally present. The following is a fairly complete list: Nitrogen, oxygen, water vapor, argon, carbon dioxide, hydrogen, helium, neon, krypton, xenon, niton (radium emanation), ozone, hydrogen dioxide, ammonia and other compounds of nitrogen.

A number of these substances have only become known to science within the last quarter of a century. Argon, though it constitutes nearly 1 per cent of the atmosphere, escaped detection until the year 1894. The investigation of argon led to the discovery of some of the others. In 1895 it was found that the air, as well as certain minerals, contains helium. This substance was not new to science, but it had never before been found on earth. It was discovered in the atmosphere of the sun, by means of the spectroscope, as early as 1868. Terrestrial helium, neon, krypton, and xenon were all discovered by Sir William Ramsay, who also shared with Lord Rayleigh the distinction of discovering argon.

Ramsay has published the following figures for the proportions in which some of the rare gases exist in the atmosphere:

Helium	1	part in	245,320	by volume
Neon	1	“	80,800	““
Krypton	1	“	2,000,000	““
Xenon	1	“	17,000,000	““

Niton, or radium emanation, is one of the products of the disintegration of radium. Niton itself disintegrates very rapidly, one-half of any given quantity disappearing in about four days, and one of its products is helium. The amount of niton in the atmosphere is never more than an infinitesimal trace. Thus we are told that the total quantity of this substance present in the atmosphere of the whole earth up to an altitude of one kilometer (0.6 mile) weighs less than nine ounces, and that each cubic centimeter of air contains among its thirty million million million molecules only between one and two molecules of niton, on an average.

Turning, now, to the more abundant constituents of the atmosphere, we find that oxygen and nitrogen differ strikingly from each other in the fact that, while the former has a strong chemical affinity for nearly all other elements, the latter is chemically inert, having little tendency to unite directly with other elements, though by indirect processes, and chiefly through the agency of plants and animals, a large number of nitrogen compounds are produced. Oxides of nitrogen are formed directly from the atmospheric gases by lightning discharges, and these unite with the moisture of the air to form nitric and nitrous acids. A certain amount of ammonia (a compound of nitrogen and hydrogen) may also be formed by lightning from nitrogen and atmospheric water, but most of the ammonia in the air is derived from the decomposition of plant and animal matters. The compounds of nitrogen that occur in the air are washed down by rain in considerable quantities. Analyses of rain water made in different parts of the world show from one to nine pounds of such substances per acre per annum.

Carbon dioxide (more familiarly known as carbonic acid gas) occurs in the atmosphere in the almost constant proportion of three parts in 10,000 by volume. It is a little more abundant in the air of towns than in the open country or over the ocean, and it undergoes slight periodic variations, but the fact that it is not much more variable is rather surprising, considering that it is continually being added to and abstracted from the air by numerous agencies that have no dependence upon one another. It is supplied to the air by volcanoes, mineral springs, the combustion of fuel, the respiration of animals and plants, and the decay of organic matter. The amount supplied annually by the burning of coal alone is estimated to be equivalent to more than one-thousandth of the total volume of the gas present in the atmosphere at any one time. On the other hand, all green plants, in the presence of sunlight, withdraw carbon dioxide from the air, abstract the carbon from it for the use of the plant, and return the oxygen to the atmosphere. Thus it is estimated that an acre of beech forest takes a ton of carbon out of the air annually. A vast amount of atmospheric carbon dioxide enters into chemical combination with certain rocks at the earth’s surface. Lastly, a large quota of this atmospheric gas is absorbed by sea water, and certain authorities have seen in this process a regulator of the total amount in the atmosphere, the hypothesis being that the ocean gives back some of the carbon dioxide whenever this substance becomes deficient in the air.

Water vapor—i.e., water in an invisible gaseous form—is always present in the atmosphere, but its amount is subject to wide fluctuations. An important fact in this connection is that, at any given temperature, the air can hold only a definite amount of this vapor. This maximum amount increases rapidly with temperature. When the air is fully charged with water vapor it is said to be “saturated.” Properly speaking, the temperature limits the amount of the vapor that can occur in a given space, regardless of the presence of the other constituents of air, and in scientific language it is the vapor itself that is said to be saturated, and not the air; but in a popular book about the atmosphere, where much has to be said about atmospheric water vapor, adherence to scientific usage in this matter invariably leads to awkward complications. Speaking, then, in familiar terms—when the air is saturated with water vapor, a fall in temperature causes some of the vapor to condense in visible form, as cloud, fog, rain, dew, snow, hail, etc. As the sole source of these various forms of moisture, and on account of the important part it plays in many atmospheric processes, water vapor is, from a meteorological point of view, the most interesting constituent of the atmosphere.

One more atmospheric gas requires notice here, both on account of the great popular interest attaching to it, and because of recent scientific discoveries concerning it—viz., ozone. This substance may be described, in nontechnical language, as a concentrated form of oxygen. It is one of the most powerful oxidizing agencies known, and has found useful applications in medicine and various industries. Its popular renown, however, is due to the fact that for many years it was regarded as a great natural purifier of the atmosphere. “Life-giving ozone” was reputed to be abundant in the air of forests, mountains, and the seashore. Systematic observations were made of the prevalence of ozone at different places throughout the world, generally by noting the change of color of test-papers exposed to the air. These “ozonometric” observations are now a closed chapter in the history of meteorology, for it has been found that the reactions of so-called ozone papers are due chiefly or entirely to atmospheric substances other than ozone. Moreover, direct examination of the air by more accurate methods—including samples collected with the aid of kites and balloons up to a height of several thousand feet above the earth—shows that the amount of ozone in the whole of the lower atmosphere is exceedingly small—much too small to be of hygienic significance. Whatever ozone is produced from oxygen at such levels by lightning discharges or other possible agencies probably enters promptly into chemical union with oxidizable substances and therefore has only a brief existence.

On the other hand, the spectroscope has brought us evidence that far aloft in the atmosphere, many miles above the earth, ozone is quite abundant. Here it is supposed to be generated by two agencies—the electrical discharges of the aurora and ultra-violet radiations from the sun. The ultra-violet rays that help to produce it are prevented from reaching the earth, and astronomers are thus deprived of much interesting information they might otherwise obtain concerning the spectra of the sun and stars. However, as the present Lord Rayleigh has pointed out, we can console ourselves for this fact by reflecting that if the ozone did not shut off much of the ultra-violet light from the sun, this light would probably ruin our eyesight; or, rather, we should be put to the inconvenience of constantly wearing some sort of protective spectacles in the daytime.

The high-level ozone is further interesting because of exercising a certain control over the temperature of the lower air. It is more transparent for incoming solar radiation than for outgoing earth radiation. Hence, when it is unusually abundant, it should raise the general temperature of the earth. This presumably happens when the condition of the sun is such that an unusual amount of ultra-violet radiation reaches the upper atmosphere, a fact that must be taken into consideration in any attempt to establish a relation between climatic fluctuations and the sun-spot period.

The lowest part of our atmosphere is the densest because it is compressed by the weight of the air above it. Thus it happens that, although the atmosphere is at least several hundred miles in height, one-half of its mass—i.e., one-half of the quantity of matter in it, as expressed in terms of weight—lies below an altitude of about 3½ miles above sea level, while about seven-eighths lies below the ten-mile level. Above about five miles the atmosphere is too rare to support life. The highest clouds seldom occur higher than ten miles. Storms hardly ever reach that height. In short, the phenomena of life and the phenomena of weather are confined to a layer of air so shallow, in proportion to the dimensions of our globe, that on the surface of an orange it would be represented by a sheet of thin paper.

The actual height of the atmosphere is not even approximately known. There are theoretical reasons for believing that even at a height of thousands of miles above the earth there are molecules of atmospheric gases still under the control of the earth’s gravity, while at such levels yet other atmospheric molecules are constantly escaping into outer space. At an altitude of fifty miles the atmosphere is less than 1/75,000 as dense as at sea level—i.e., more than seventy-five times as attenuated as the best “vacuum” obtainable with an ordinary mechanical air pump. At 300 miles it is computed to be about one two-millionth as dense as at sea level.

The loftiest atmospheric phenomenon that we can observe directly is the aurora, which has been photographed up to heights of more than 300 miles. The altitude of the aurora is determined by simultaneous observations made at two or more points, and the same is true of shooting stars and their trails, which seem to be especially numerous between the levels of sixty and ninety miles. The so-called “noctilucent clouds,” which shone by reflected sunlight throughout the night for some years after the great eruption of Krakatoa and were supposed to consist of fine dust from that volcano, were probably about fifty miles above the earth. From the duration of twilight we infer that above about forty-five miles the air is so tenuous that it cannot reflect sunlight to the earth. Clouds furnish information concerning the movements of the air at various levels up to ten miles or more. Observations on mountains contribute further to our knowledge of the atmosphere above the ordinary levels of habitation.

Of all methods of exploring the atmosphere in a vertical direction, the most fruitful is the use of kites and balloons. In recent years investigations of this character have become so extensive and so highly specialized that they are regarded as forming a separate department of meteorology, known as Aerology. It is by virtue of developments in this field that meteorology has become “a science of three dimensions.” Formerly meteorologists could do but little more than study the bottom of the weather, so to speak; but now they observe it and chart it at all levels. The weather forecaster has daily reports of conditions aloft to aid his predictions both for dwellers on terra firma and for the aeronaut; while the accumulated data of upper-air observations are throwing new light on many difficult atmospheric problems.

Scientific balloon ascents are no novelty. Some were made in the eighteenth century, and many famous ones in the nineteenth, including those of Biot, Gay-Lussac, Glaisher, Tissandier, and other daring savants. The “record” height for such personal ascents was attained in 1901, when Berson and Süring rose to 35,400 feet above Berlin. Kites were sent up for meteorological purposes even before Benjamin Franklin’s immortal experiment in 1752. Modern aerological methods have, however, little in common with these pioneer undertakings. Existing types of box kites, pilot balloons, sounding balloons, and self-registering meteorological apparatus for upper-air research were developed in the latter part of the nineteenth century, but their use did not begin to bulk large in meteorology until about the beginning of the present century. The epoch-making event in these undertakings was the discovery of the isothermal layer.

It is a matter of common knowledge that the air is found to be colder the higher one ascends in the atmosphere. Thus, even in equatorial regions, the tops of high mountains are mantled in perpetual snow. The rate of this temperature decrease averages about 1 degree Fahrenheit per 300 feet. Previous to the year 1902 meteorologists supposed that the atmosphere continued to grow steadily colder in an upward direction indefinitely; but in that year a Frenchman, M. Teisserenc de Bort, who had sent aloft hundreds of small unmanned balloons carrying self-recording thermometers, announced that above a height of about six and one-half miles the temperature ceased to fall. In fact, he found that at about that level there was often a slight increase of temperature with increasing altitude for a certain distance upward, and then a nearly uniform temperature as high as the balloons ascended. This announcement was at first received with considerable skepticism, but very soon similar observations were reported from other parts of the world. A new “shell” of the atmosphere had been revealed—which, as subsequent investigations proved, differs from the lower air in other respects besides temperature—and it was at first named by its discoverer the isothermal layer. He afterward substituted the name stratosphere, now generally employed. In distinction from the stratosphere, the part of the atmosphere lying below it is called the troposphere.

The stratosphere has been explored in widely scattered parts of the earth, and information concerning it is daily accumulating. Although it extends over the whole world, the altitude at which it begins is by no means uniform. The altitude is greater in summer than in winter; it varies with the barometric pressure at the earth’s surface; and it is decidedly greater over the equator than over the poles. The last fact leads to an interesting paradox. Since over the equatorial regions the temperature keeps on falling with ascent to a greater height than in other latitudes, it is here that the lowest temperatures in the atmosphere are found. A sounding balloon sent up from Batavia, Java, in November, 1913, recorded 113° below zero Fahr., the lowest air temperature ever observed. In middle latitudes the temperature of the stratosphere averages something like 68° below zero Fahr.

The temperature of this interesting upper atmosphere varies a good deal, both vertically and horizontally, but never shows the steady vertical variation that characterizes the lower air. The stratosphere contains no clouds (except occasional dust clouds), and has a circulation quite distinct from that of the troposphere, the exact nature of which, however, has not yet been determined.

The sounding balloon, already mentioned, is one of the four principal types of aerial vehicle used in the study of the atmosphere, the others being the pilot balloon, the captive balloon, and the kite. The sounding balloon, or ballon-sonde, is a small free balloon that carries no human aeronaut, but instead a set of superhuman meteorological instruments, which register the temperature, the barometric pressure, and sometimes the humidity continuously and automatically through the whole course of their journey. The record is traced on a revolving drum or disk, usually coated with lampblack. In its commonest form the balloon is made of india-rubber, and when launched is inflated to less than its full capacity with hydrogen. As it rises to regions of diminished air pressure it gradually expands, and it finally bursts at an elevation determined approximately in advance. A sort of parachute, or sometimes an auxiliary balloon, insures a gentle fall to the ground. Attached to the apparatus there is generally a ticket offering the finder a reward for its return, and giving instructions as to packing and shipping. Sooner or later it generally comes back. In fact, the large percentage of records recovered, even in sparsely settled countries, is not the least remarkable feature of this novel method of research. Thus, of seventy-two balloons sent up by a Franco-Swedish expedition in Lapland, forty-one were eventually recovered with their instruments. One of these fell into a lake and was found after three years.

No instruments are carried by the pilot balloon, which merely serves to show, by its observed drift, the speed and direction of the air currents at different levels. The pilot balloon is sighted, while in flight, through a special form of theodolite, or, preferably, two theodolites some distance apart. Several ingenious methods have been devised for computing and plotting its actual course through the air. Such balloons, apart from their use in scientific research, have become one of the principal adjuncts of aeronautical undertakings all over the world, and are also used by artillerists to enable them to make proper allowance for the deflective effect of the wind on the flight of projectiles. Hundreds of thousands of pilot balloons were sent aloft for military purposes during the world war.

Meteorological instruments are sent up attached to kites or captive balloons whenever—as in connection with weather forecasting—the observations must be obtained more promptly than would be possible with the aid of sounding balloons, but such devices can attain only moderate altitudes. Kites have been raised to about four and one-half miles above sea level, as compared with nearly twenty-two miles reached by a sounding balloon and twenty-four miles by a pilot balloon. The average height of sounding-balloon ascents is about ten miles. As already stated, balloonists have risen to 6.7 miles. This is a little higher than the best aeroplane record.

The use of the aeroplane for making meteorological observations is still quite limited, but will inevitably increase. One other device gives promise of yielding valuable aerological information, on account of its ability to rise to extraordinary altitudes. This is a special form of rocket, recently invented by Prof. R. H. Goddard, which is propelled by several successive discharges of an explosive in the course of its upward flight, and with which the inventor thinks it will be possible to explore the whole vertical extent of the atmosphere. Meteorological apparatus for use with the Goddard rocket has been planned by Mr. S. P. Fergusson of the Weather Bureau.

The atmosphere presses down upon the earth with a weight that, at sea level, amounts to about 14.7 pounds to the square inch, on an average. This pressure is, at any point, exerted equally in all directions; it acts, for example, on the whole surface of the human body, and this means that a man of average size lives under a burden of some seventeen tons of air. He is not incommoded because the pressure from without is balanced by that of the air that permeates his body.

The pressure of the atmosphere decreases upward at nearly the same rate as its density. Thus on mountains and plateaus it is considerably less than in lowlands. At no place is the pressure invariable, nor is there a constant relation between pressure and altitude, but, knowing approximately the average atmospheric pressure over the earth’s surface, and knowing also the area of the latter, we can compute in round numbers the total weight of the atmosphere—about 5,000,000,000,000,000 tons. This is about 1/1,200,000 of the entire weight of the earth.