Читать книгу Meteorology: The Science of the Atmosphere - Charles Fitzhugh Talman - Страница 8

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When the moralist reminds us that we are children of the dust and predestined to a dusty end, there is a grain of comfort in the discovery that modern science regards dust as one of the most important things in the whole economy of nature. No longer does dust seem an appropriate symbol of insignificance and humility when one surveys the bulk of serious literature that has been written about it, considers the caliber of the men who have devoted the better part of their lives to the study of it, or inspects the great array of ingenious apparatus that has been devised for its investigation.

The dust of which we have to speak in the present chapter embraces all small particles of solid matter found anywhere, or at any time, in the earth’s atmosphere. Particular kinds of dust have, of course, their special names. Soot, the visible part of smoke, is a form of dust that has played a very conspicuous part in human affairs; hence the separate mention of smoke in the heading of this chapter.

While there are many agencies that help to charge the atmosphere with dust, the most important of them all is the wind. Let us see what happens when the wind blows over the surface of a dusty road, for example. If the air flowed in a smooth horizontal stream over such a surface, its friction would drag the dust along on the ground, but would not lift it. Such surface drifting, due to the horizontal component of the wind’s motion, does, of course, occur, and its effects are strikingly visible in the shifting dunes that often form over a broad surface of sand or snow. All winds near the earth’s surface are, however, full of waves and eddies, and in many cases, as over a stretch of strongly heated soil, there are strong updrafts, sometimes extending to a great height in the atmosphere. All kinds of dust are heavier than air, and, contrary to popular belief, never truly “float” in the atmosphere. Dust may enter the atmosphere at high levels, through the disintegration of meteors, or it may be spouted up by volcanoes, but dust blown up from the earth’s surface rises only because the air is rising with it; and, in still air, all dust sinks more or less rapidly toward the ground. The rate of its fall depends upon its specific gravity, and upon the size and shape of the dust particles. Other things being equal, the finest particles fall most slowly. Exceedingly fine dust, even without upward air movements to support it, requires months or even years to fall to the ground from the higher levels of the atmosphere.

Upward movements in the air suffice to carry millions of tons of dust aloft every year, and horizontal air currents carry the same dust far and wide over the earth. The transportation of soil by the wind leads to some results of remarkable interest, practical as well as scientific. In the first place, far-reaching changes in topography are brought about by this process. Thus in China vast areas are covered to a depth of hundreds or even thousands of feet with a fine yellowish earth, called “loess,” which is believed to have been blown thither by the winds from the deserts of Central Asia. Less extensive deposits of this wind-borne material are found in many other parts of the world, including the Mississippi Valley. Another effect of wind transportation is the mixing of soils. There is a constant interchange of soil material between different regions, so that the composition of the soil on a particular farm, for instance, is not the same now that it was a few years ago or that it will be a few years hence. Lastly, the presence of dust in the atmosphere, whether derived from the soil or otherwise, has various interesting and important effects upon the heat and light we receive from the sun and modifies, in numerous ways, the conditions of human life upon our planet.

Several cases in which enormous quantities of solid matter have been carried to great distances by the wind have formed the subject of elaborate investigations on the part of meteorologists. Thus, during the three days, March 8–10, 1901, heavy dust storms occurred in the deserts of southern Algeria, and the sequel of these storms was carefully studied by Hellmann and Meinardus. A widespread cyclonic storm, central over Tunis at the time, sucked up the dust, which was carried northward by the winds at high altitudes. Deposits from this dust cloud occurred over an area extending as far as 2,500 miles from the place of origin. Reports collected from hundreds of observers indicated that 1,800,000 tons of dust fell over the continent of Europe, and one-third of this fell north of the Alps. As much more is believed to have fallen over the Mediterranean, while on the African coast itself the deposit is supposed to have amounted to 150,000,000 tons. In March, 1918, a shower of dust discolored falling snow at various places in the United States over an area of at least 100,000 square miles, extending in an east-west direction from Dubuque, Iowa, to Chelsea, Vt. Reports of this shower were collected by Messrs. E. R. Miller and A. N. Winchell, who estimate that the amount of dust could not have been less than a million tons, and may have been several hundred million. The dust is believed to have been blown up from the arid regions of the far southwestern United States and to have been transported a thousand miles or more.

Off the west coast of Africa, between the Canaries and the Cape Verde Islands, haze due to dust blown up from the Sahara Desert is frequently encountered by vessels, especially during the first four months of the year. This haze probably gave rise to the ancient legend of a Sea of Darkness—the Mare Tenebrosum—one of the mysterious terrors of the ocean reported by the navigators who first sailed toward the New World.

Extensive deposits of atmospheric dust have attracted attention from the earliest times. Ehrenberg, in 1849, collected records of 349 such cases, and published a map showing their distribution, which embraces the greater part of the world. Atmospheric dust is always brought down in greater or less quantities by rain. When it consists of fine powdery sand, the rain sometimes acquires a brownish or reddish tinge, staining objects on which it falls and constituting the “showers of blood” that have been regarded as prodigies from remote antiquity. Homer describes such a shower, and many similar occurrences are recorded by the Roman historians. Italy, owing to its proximity to the African coast, is often visited by these showers, which still strike superstitious terror into the hearts of the peasantry.

The millions of meteors that enter the earth’s atmosphere every day contribute their quota of dust, though the total amount is small compared with that of the material lifted from the earth. Fine ferruginous particles are often seen on the snowy summits of high mountains and the polar ice fields, and both their appearance and their composition indicate that they are derived from meteors.

Forest fires, burning peat beds, and other conflagrations on a large scale discharge quantities of dust into the atmosphere. Cinders from the great Chicago fire spread over a large part of the globe. They are said to have reached the Azores some forty days after the beginning of the catastrophe. In Europe, the once common practice of burning the moors to prepare them for cultivation gave rise to huge volumes of smoke, which was carried by the wind hundreds and even thousands of miles. The stronghold of this old custom—which still survives to some extent—was East Friesland, in northwestern Germany, and the characteristic haze to which it gave rise, known as “moor smoke” (German, Moorrauch), was sometimes observed as far away as Spain, Italy, and Greece.

The famous “dark days” that figure in both ancient and modern history, though in a few cases probably due to eclipses of the sun, have generally been the result of an abnormal accumulation of smoke or dust in the air; sometimes arising from volcanic eruptions, but more often from burning forests, moors, or prairies. Forest fires are the principal cause of dark days in the United States. Probably the most celebrated of such days was May 19, 1780, when, in consequence of great forest fires along Lake Champlain and down to the vicinity of Ticonderoga, darkness like that of night prevailed in New England. All but the most necessary business was suspended, the schools were dismissed, and the greater part of the population flocked to church to prepare for the end of the world, which was believed to be at hand. The great Idaho fire of August, 1910, was responsible for dark days over a larger area than in any other case on record in this country. Artificial light was required in the daytime over a broad belt, extending from Idaho to northern Vermont, but smoke was observed far beyond this area. The British ship Dunfermline reported that on the Pacific Ocean, 500 miles west of San Francisco, the smell of smoke was noticed and haze prevailed for ten days. When smoke in the air forms a rather thin layer, through which the sunlight penetrates feebly, we sometimes get an effect similar to the golden glow of sunset, a yellow or coppery tinge being cast over the landscape. Such was the cause of the “yellow day” still remembered in New England—September 6, 1881—attributed to the burning of the immense peat bogs of the Labrador barrens.

Another occasional cause of atmospheric dustiness is the eruption of volcanoes, especially those of an explosive character, which carry fine dust to heights at which it cannot be washed out of the atmosphere by rain. The remarkable dry fog of 1783—the most famous in history—which covered the greater part of Europe and North America for three or four months—was undoubtedly due to the violent eruptions of that year in Iceland and Japan. Its connection with the Iceland eruption was suggested even by contemporary writers. The outbreak of Krakatoa, in the East Indies, in 1883, spread a veil of dust over the greater part of the globe. For two or three years its presence in the air was the cause of striking optical phenomena, including gorgeous sunset glows. The story is told of an American fire brigade which, deceived by one of these brilliant sunsets, set out to extinguish what was mistaken for a great fire in a neighboring village. A large species of corona around the sun, known as “Bishop’s ring,” because it was first observed by the Rev. Sereno Bishop of Honolulu, appeared shortly after the eruption and reached its maximum intensity the following year. This was due to the diffraction of light by the exceedingly fine dust from the volcano, and the same phenomenon has been seen after other great explosive eruptions; e.g., that of Mont Pelée, in 1902. Some authorities believe that the finest particles of dust from the Krakatoa eruption were carried to an altitude of over fifty miles above the earth, and remained suspended at very high levels for several years, constituting the strange “noctilucent clouds,” seen on summer nights from 1885 onward. These clouds glowed with a silvery luster, attributed to reflected sunlight.

A persistent veil of volcanic dust in the upper air is thought to exercise marked effects upon terrestrial temperatures, and prolonged periods of intense vulcanism have been regarded as the cause, or one of the causes, of the recurrent ice ages of which geology furnishes the record. This explanation of ice ages was advanced by P. and F. Sarasin, in 1901, and was first put upon a scientific basis by Dr. W. J. Humphreys in 1913; but the idea that volcanic dust might be the cause of cold seasons was suggested by Benjamin Franklin as early as 1784. Franklin’s speculations on this subject were prompted by the cold winter of 1783–1784, which followed the extraordinary fog of 1783, already mentioned. Humphreys has published a list of all the great volcanic outbreaks recorded since 1750, and has shown that each of them registered itself in the temperatures of the earth and also, since accurate measurements began to be made of solar radiation, in these instrumental records. Thus, the intensely cold winters of 1783–1785 followed the tremendous eruptions of Asama, Japan, and Skaptar Jökull, Iceland, in 1783; the famous “year without a summer” (1816) was the sequel of the gigantic outbreak of Tomboro, in the Sunda Islands, in 1815, which is said to have hurled thirty-six cubic miles of solid matter into the atmosphere; and definite periods of low temperatures and reduced sunshine were observed after the eruptions of Mont Pelée, in 1902, and Mount Katmai, Alaska, in 1912.

The effect of a volcanic dust veil in lowering temperatures on earth is attributed chiefly to the fact that, while the fine grains of dust are able to reflect back into space the short waves of radiation coming from the sun, they do not bar the passage of the long heat waves radiated outward from the earth. According to Humphreys’s calculations, such a veil is about thirtyfold more effective in shutting solar radiation out than in keeping terrestrial radiation in. This process is just the reverse of the familiar effect of the greenhouse; where the glass lets in the short waves of solar radiation but does not readily let out the long waves of earth radiation.

A small contingent of atmospheric dust consists of common salt (sodium chloride) due to the evaporation of spray from the ocean. This substance is frequently found in rain, as well as in samples of air, not only near the seashore, but even in the interior of continents and on high mountains. According to Du Bois the amount of sodium chloride annually deposited on the dunes of Holland is at least 6,000,000 kilograms (more than 6,600 tons).

One of the striking phenomena of arid regions is the dust whirlwind; exemplified in the “devils” of India and South Africa, the “twisters” of Texas, etc. E. E. Free, in his treatise on “The Movement of Soil Material by the Wind” (U. S. Bureau of Soils, Bulletin 68), says of these whirls:

“They may be seen nearly every hot day, sometimes running rapidly over the surface; sometimes remaining nearly, if not quite, stationary, but never losing their rapid rotation. They usually last only a few minutes, but occasionally persist much longer. One observed by Pictet lasted for over five hours. They are largest and last longest on the flat, bare plains of the desert, and are usually seen in a calm or when only a light breeze is blowing, although their occurrence in windy weather is not unknown. These whirls have been noticed by many travelers in desert and steppe regions and have been carefully observed by Baddeley in India, and by Pictet in Egypt. They are frequent in China and on the pampas of South America, and occasionally occur during the dry season even in the humid regions. One of the most interesting phenomena in connection with the dust whirls is the occurrence of systems of several whirls, each revolving rapidly about its own center and also moving about a common center in a more or less perfect circle a few rods in diameter.”

The little whirls often seen on dusty roads are a miniature variety of the same phenomenon.

One very important class of dust particles in the atmosphere consists of organic matter, living or dead, including the pollen of plants and the countless myriads of microorganisms, as well as a variety of other products of the animal and vegetable kingdoms. An abundance of pollen in the air accounts for the occasional fall of yellow rain, described as “sulphur rain,” “golden showers,” etc. The promptness with which a piece of stale bread becomes moldy in a damp atmosphere is one of many proofs of the omnipresence in the atmosphere of the microscopic spores of fungi, ready to propagate their species with amazing rapidity as soon as they light upon a suitable nutrient medium. Last, but not least, bacteria, the most minute of all known organisms—so small that thousands or millions of them clustered together would make a mass not larger than the head of a pin—swarm in the air, as they do in water, the soil, and the bodies of animals. Fortunately, while certain species of bacteria carry disease and death with them, the great majority are harmless to mankind.