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.