PHYSICAL PHENOMENA OF THE UPPER REGIONS OF THE ATMOSPHERE.' By Prof. ALFRED CORNU, D. C. L., F. R. S., Officer of the Legion of Honor, Vice-President of the Academy of Sciences, Paris. The primary and effective cause for almost all the physical phenomena that occur in the earth's atmosphere is the heat of the sun. The atmosphere may then be considered as an immense heating apparatus that has for its fire the sun, for its boiler the earth or the clouds heated by the solar rays, and for its condenser the radiation that occurs toward interplanetary space. The means at the disposal of physicists and meteorologists for studying the different regions of the atmosphere are very limited; they are usually obliged to content themselves with very indirect observations. and to proceed by induction. Most interesting phenomena do indeed occur in the upper regions at almost inaccessible heights. The purpose of this paper is to show by a few experiments that physical meteorologists are beginning to attain a true explanation of natural phenomena. You will see, indeed, that in certain cases they can not only exactly produce those phenomena, but often they are able to effect a true synthesis of them by using means in every way analogous to those actually operative in nature. I will commence by enumerating the means in use among meteorolo gists for studying the different regions of the atmosphere. The most direct method is founded upon the use of the aerostat. The aerostat, or balloon, allows us, in fact, to transport our meteorological instruments into the very midst of the atmospheric strata we wish to study. Unhappily this method is difficult, expensive, and even dangerous; therefore it is employed only in exceptional cases. The aerostatic ascensions most productive of results have been those of Gay-Lussac (1804), of Glaisher (1862), and recently that of Dr. Berson of Stassfurt (1894), who ascended more than 9,000 meters. 'Translated from the Proceedings of the Royal Institution of Great Britain, Vol. XIV, pp. 638-648. The most important facts observed from the balloon were entirely unexpected. I will briefly state them: 1. Clouds formed of ice crystals occur very frequently; they constitute the cirrus clouds that float at very great heights. 2. The direction of the wind changes at different heights. 3. The temperature does not always decrease regularly with the increase in altitude, cold strata and warm strata often alternating with each other. The second method of studying the atmosphere is by the establishment of mountain observatories, upon isolated peaks when possible. At these stations the unexpected inversion of the temperature at various altitudes is daily verified. The ice clouds are too high to be directly reached by mountain observatories. A view of the principal mountain observatories of France will prob ably interest you. Photographs of the following observatories were then thrown on the screen: Pic du Midi (altitude 2,800 meters), in the Pyrenees. This last observatory, thanks to the lightness of its construction of open work, may almost be considered as a captive balloon fixed permanently at 300 meters above the ground. Halos. Since ice clouds are situated at altitudes (6,000 to 10,000 meters) greater than that of the highest mountain observatories, we are condemned to the use of the balloon alone for all observations upon them. Fortunately the presence of ice crystals is revealed by an optical phenomenon that can be observed even at ordinary levels-the halo. This is a brilliant circle having a radius of about 22 degrees that surrounds the sun or moon. It has a reddish tint within and is slightly bluish at its outer border. It is explained, as are many appearances of a similar kind, by the refraction of the light of the sun or the moon in passing through icy needles. In fact the ice crystals are hexagonal prisms, the faces of which are inclined to each other, two by two, at an angle of 60 degrees. These, scattered through the air and facing in every direction, refract the light, but the refracted rays can not pass beyond the angle of 22 degrees imposed upon them by the minimum of deviation discovered by Sir Isaac Newton. The limit of the refracted rays is then a cone of 22 degrees around the line that passes from the eye to the luminous body. Experiment imitating a halo.-By forming crystals in a transparent medium made by mixing appropriate liquids, there is exactly reproduced the mingling of the warm, moist strata of the atmosphere with the cold ones which produces ice crystals. To do this place in a glass jar a saturated aqueous solution of potash alum, and send through the jar a luminous beam projecting the image of a circular opening like that of the sun upon the dark sky. Then add to the contents of the jar a quarter of its total volume of rectified spirits; the alum, insoluble in the alcoholic mixture, precipitates in very minute crystals that float within the liquid. The image of the sun first becomes dim as in a fog, but soon a brilliant and slightly iridescent circle is seen, simulating very closely the appearance of a halo. The experiment is brilliant and instructive. This phenomenon is well known to country people; it is a certain sign of rain when it appears during a warm day, even when no other sign predicts a meteorological disturbance. Alteration and inversion of temperature. -In neighboring observatories situated at widely different altitudes, like those of Puy-de-Dôme and Clermont, we often find that warm currents exist in the upper regions. It is to successive inversions of this character that Mr. Amsler, of Schaffouse, attributes the beautiful phenomenon known in Switzerland as the "Alpenglühen," which consists in a renewal of the illumination of the snowy summits of the Alps some moments after the setting of the sun has darkened them. There was thrown on the screen a photograph of the summits of the Bernese Oberland, the Jungfrau, the Mönch, the Eiger; the view being taken from St. Beatenberg, near the Lake of Thun. A picturesque imitation of the phenomenon just cited was given by means of a colored glass and suitable diaphragms. The explanation of Mr. Amsler is founded on the change of direction of curvature that is given to the trajectory of the luminous rays according as the air at the bottom of the valleys is warmer or colder than that of the more elevated regions. Before sunset, the earth's surface, heated by the solar rays, gives the trajectory a curvature, S A M B, like that of a mirage; that is, convex toward the earth; the sun, while setting at S', causes the shadow of the summit, A, to be projected upon the summit, B, which it would seem ought henceforward to remain in shadow, since the sun continues to descend and its last ray is S'A M'B'. But if, during the interval, the air of the valley becomes sufficiently cool, the trajectory curves in the opposite direction, S" A M" B", and the summit, B, is illumined anew. Experiments showing the inversion of curves of luminous trajectories.— By using some care we can place in a transparent jar, 20 cm. in diameter, three strata of liquid, a lower one of chloride of zinc, heavy but less refringent, and an upper one of diluted glycerin, lighter but also less refringent than the middle one. A movable mirror, LL, throws a beam of light through an opening, S, of a diaphragm. By throwing this beam in appropriate directions, it is reflected either from the upper or the lower stratum. A little fluorescein lights up the trajectories of the beams and makes their curvatures quite visible; we can thus represent the Alpenglühen with some accessory features. Scintillation of the stars.-This phenomenon is also a proof of the alterations of temperature and movement that occur in the higher strata of air. Spectrum analysis shows that the scintillation is produced by the disappearance of the successive colors of the spectrum following a somewhat regular course, according to the distance of the star from the zenith. Imitation of this phenomenon.-A very striking experiment showing this can be made by projecting with a lens, L, the image of a luminous opening, O, upon a small silvered ball, B, 3 to 4 inches in diameter, placed upon black velvet. We thus obtain the appearance of a fixed star of remarkable brilliancy. But the luminous opening, O, is made in a cardboard upon which is projected the spectral image of a slit, F, dispersed by a prism for direct vision, P. The cardboard, CO, is not exactly at the focus of the spectrum; that being formed farther away, in the plane of the lens, L. It hence results that the iridescent image of the slit in the cardboard has at its middle, where is placed the opening, O, a white region. The light thrown upon the ball, B, is therefore perfectly colorless. But on leaving the opening, the beam expands into a spectrum upon the projection lens, L. which recomposes it at B, as in the celebrated Newtonian experiment. Then by shifting before the lens, L, a grating with large meshes, certain radiations are intercepted, and the star, B, appears colored. A diverging half lens, D, having the same focus as L, annuls its effect, and the spectrum of the star, with the artificial bands caused by the grating, appears on a white screen beside the ball. This is an imitation of the spectrum analysis of the scintillation of the stars. We see by these few examples that the study of the optical phenomena of the atmosphere aided by physical analysis and synthesis, may and must teach us much concerning the calorific phenomena of inaccessible regions. Dynamic phenomena of the atmosphere.-The phenomena we have hitherto studied are due to states of almost complete equilibrium in the atmospheric strata; we might call them static. But the calorific action of the sun, combined with the cooling action of radiation into space, may produce phenomena of movement presenting every degree of intensity, from the weakest to the most violent. We will call these dynamic phenomena. They are manifested under very diverse forms: 1. Under the form of mechanical energy; which results in the forma tion of winds, whirlwinds, cyclones, waterspouts, etc. 2. Under the form of calorific energy; which results in the formation of clouds, rain, and hail, corresponding to the changes of state of water, the ever variable element of the atmosphere. 3. Under the form of electrical energy; lightning, thunder, etc. In fact, the transformation of solar energy into mechanical energy is the fundamental phenomenon and the one that leads to all others. For the sake of brevity this is the only transformation that we will consider here. The most simple mechanical phenomenon that is produced in the atmosphere is the wind. The wind has for its origin a difference of pressure between two more or less distant points. We have known since Pascal that the pressure of air is measured by the barometer. We might, then, think that the direction of the wind could always be determined by the indications of that instrument; that is to say, that the wind ought to go from the point where the barometrical pressure is greatest to the point where that pressure is least. But, strange to say, this is almost never the case; the real direction of the wind is always oblique to that theoretical direction. This fact has only been known for a few years. It has been put beyond doubt by the general meteorological charts which, conceived thirty years ago by Le Verrier, are to-day so widely circulated. The wind seems to move around the point in the chart where the minimum is found, its direction, in the northern hemisphere, being the reverse of that taken by the hands of a watch, or in the same direction with the hands around the point of maximum pressure. In the southern hemisphere these directions are reversed. In a word, the most ordinary movement of the atmosphere is a gyratory one, that which is called cyclonic. This whirling movement of the air was noted long ago. We see it occurring quite frequently around us; dust and dead leaves are raised SM 96-9 |