Atmospheric Optics

a branch of atmospheric physics that studies optical phenomena arising from the passage of light through the atmosphere. Atmospheric optics relates not only to such brilliant phenomena as dawn, rainbows, and the changing color of the sky but also to such less apparent but equally important occurrences as atmospheric scattering and emission of visible and invisible radiation, polarization of cosmic light, and visibility of objects. Atmospheric optics is a part of physical optics; it is closely related to the optics of colloids and aerosols, planetary atmospheres and seas, radiant heat transfer, and so on. The solution of problems in physical chemistry, astrophysics, oceanology, and engineering has provided results that are important to the development of atmospheric optics, and the methods and results of atmospheric optics often find application in these sciences.

The study of the optical properties of the air, oceans, and dry land directly relates to the task of atmospheric optics. The inverse task of atmospheric optics is the improvement of optical methods of probing the atmosphere—that is, the determination of other physical characteristics of air, sea, and land through their measured optical properties.

Optical phenomena in the lower and upper strata of the atmosphere (the ozone layer and higher) vary. In the upper strata, under the effect of solar radiation, mainly photochemical reactions occur. Excited particles generated through these reactions radiate accumulated energy (aurora borealis, noctilucent radiance, and so on). These phenomena are studied in aeronomy, which is not considered in this article.

Interest in optical phenomena in the atmosphere arose long ago. The colors of the sky and clouds, dawn, false suns, and other occurrences have been considered weather indicators since ancient times. Many such indexes exist, and at one time their study was considered to be the main task of atmospheric optics. This point of view was taken by the Russian geophysicist P. I. Brounov in the 1930’s. However, more detailed research has shown that although relationships undoubtedly exist among optical and other physical atmospheric phenomena, they are often very complex and ambiguous; optical signs of the weather sometimes contradict one another. Gradually it has become clear that the relationship between optical phenomena and the weather can be determined only through studying the nature of optical phenomena and simultaneously probing the mechanisms of physical phenomena responsible for changes in the weather.

The first attempts to explain the blue color of the sky date back to the 16th century. Leonardo da Vinci explained the blue celestial dome by the fact that the white air against the pitch-dark background of universal space appeared as blue. L. Euler (1762) believed that “particles of air themselves have a bluish hue and in aggregate mass create an intensive blue color.” At the beginning of the 18th century I. Newton explained the color of the sky as the interference reflection of solar light from minute drops of water always suspended in the air. In 1809 the French physicist D. Arago discovered that the color of the sky is strongly polarized.

The first accurate explanation of the blue color of the sky was given by the English physicist Rayleigh (J. W. Strutt, 1871, 1881). According to Rayleigh’s theory, the colored rays producing the solar spectrum are scattered by molecules of air proportionally to λ^-4 (where λ is the wavelength of the light). Blue rays are scattered, for example, 16 times more strongly than red rays. Therefore, the color of the sky (scattered solar light) is blue, and the color of the sun (direct solar light), when it is low above the horizon and the rays have a long distance to pass through the atmosphere, is red. Moreover, scattered light must be strongly polarized and polarization must be total at an angle of 90° from the direction of the sun.

Measurements of intensity, color, and the polarization of light in the sky have confirmed Rayleigh’s theory. However, in 1907 the Russian physicist L. I. Mandel’shtam showed that if a body—in which category air is included—is strongly homogeneous, then rays that are scattered by single molecules must, as a result of wave interference, attenuate one another in such a way that generally no scattering whatever would be observed. In actuality, as a result of thermal motion in a medium, there are always density fluctuations—that is, randomly distributed regions of compression and rarefaction—on which scattering occurs. The powerful theory of fluctuation scattering, developed by the Polish physicist M. Smoluchowski in 1908 and A. Einstein in 1910, led to the very formulas that were earlier derived through the molecular theories of Rayleigh. However, all these works did not take into account the dustiness of the atmosphere. Even the cleanest air—high in the mountains and in the arctic and antarctic—is always contaminated by organic and mineral dust, particles of smoke, and drops of water or other solutions. These particles are very small (radius about 0.1 nanometers), and their mass—and consequently their weight—are infinitesimal; therefore, they fall to earth so very slowly that the slightest current of air once again lifts them up. Since the air is constantly mixing, the atmosphere is always full of minute dust particles and droplets, which are most dense in the strata nearest the ground. This is atmospheric aerosol, which is generally considered to be the source of the opaqueness of the air. It decreases the limits of visibility in the actual atmosphere by a factor of approximately 20, as compared with an ideal atmosphere. In addition to aerosols, major roles in optical phenomena in the atmosphere are played by water vapor, carbon dioxide, and ozone, although they constitute only a small percentage of the volume of gases that mix to form air. Only these gases absorb solar and earth radiation and themselves generate radiation.

Aerosols have a significant effect on the scattering of light in the atmosphere. The German physicist E. Mie (1908) developed a theory of the scattering of light particles of arbitrary dimensions that was widely used in atmospheric optics. This theory was essentially developed and supplemented by the Soviet scientists V. V. Shuleikin (1924), V. A. Fock (1946), and K. S. Shifrin (1951) and the Dutch scientist van der Hulst (1957). Calculations indicate the character of scattering depends on the ratio of the radius of a particle a to the wavelength λ and on the material of the particle. Small particles (a/λ <<1) act as molecules as described in Rayleigh’s theory, but the larger the particles the less is the dependence of scattering on wavelength. Large particles (a/λ>>1) scatter light neutrally—all waves are scattered identically. This pertains particularly to cloud droplets, the radii of which are 10–20 times larger than the wavelengths of visible light. For this very reason clouds have a white color. Also as a result of this property, the sky becomes whitish if the air is dusty or contains water droplets. Major contributions to the study of the brightness and polarization of the sky have been made by the Soviet scientists V. E. Fesenkov, 1.1. Tikhanovskii, and E. V. Piaskovskaia-Fesenkova and to the study of the transparency of clouds, fogs, and lower strata of the atmosphere by A. A. Lebedev, I. A. Khvos-tikov, and S. F. Rodionov, the American scientists J. Strat-ton and H. G. Houghton, and the French scientists E. and A. Vassi and J. Bricard.

Along with experimental works, methods of calculating the distribution of illumination and polarization throughout the sky have also been created, which has necessitated making calculations of compound light scattering and of reflections against the earth’s surface. The Russian physicist O. D. Khvol’son (1890) developed an equation for this type of radiation transfer. For a cloudless sky the effect of compound scattering is not very great, but for clouds, which form a strongly opaque medium, it is a basic factor without which it is impossible to calculate the transparency of clouds or the reflection and light regimen within them. Major contributions to developing the transfer equation have been made by the Soviet scientists V. A. Ambartsumian (1941–43), v. V. Sobolev (1956), and E. S. Kuznetsov (1943–45) and by the Indian scientist G. Chandrasekhar (1950).

Visibility limitations are chiefly caused by the transparency of the air and also by its reflective properties. Reflection is diffuse—that is, scattered in all directions (except for reflection from the surface of calm water)—and it varies for different surfaces, as a result of which (for nonemitting bodies) a brightness contrast arises between the object and the background. If the contrast is greater than some threshold value, then the object is visible; if it is less than that value, then the object is indistinguishable from the background. The visibility distance of an object depends on the transparency of air and on illumination (at dusk and dawn the thresholds are not the same). Visibility (transparency of the atmosphere) is one of a number of basic meteorological factors that are observed at meteorological stations. Study of the conditions that affect horizontal and oblique visibility (against the background of the sky or the earth) is an important applied task of atmospheric optics. Significant results toward the solution of this problem have been obtained by the Soviet scientists V. V. Sharonov, N. G. Boldyrev, V. A. Berezkin, and V. A. Faas, the German scientist H. Koschmieder, and the Canadian scientist W. E. K. Middleton.

The study of the conditions of propagation in the atmosphere-of invisible infrared rays of wavelengths of 3–50 microns, which determine radiant heat transfer (this process consists of absorption and subsequent reradiation), is of great significance in atmospheric optics. Direct measurements, which can be made in the free atmosphere from airplanes or from artificial earth satellites, are very important. Essential results in the study of radiant heat transfer have been made by the Soviet scientists A. I. Lebedinskii, V. G. Kastrov, K. Ia. Kondrat’ev, B. S. Neporent, and E. M. Feigel’son and by the American scientists J. N. Howard and R. M. Hoody.

Two difficulties arise in carrying out the inverse tasks of atmospheric optics: first, it is necessary to ensure that the optical information consists of the needed data; second, it is necessary to show the way in which the data were obtained and to show the required accuracy in measurement. As early as 1923, V. G. Fesenkov showed that the structure of the atmosphere at altitudes greater than 30 km can be determined based on the changing intensity of the twilight sky. After 30 years information about the structure of the stratosphere and the ionosphere obtained directly with the use of rockets confirmed the data of the twilight method. Significant contributions in developing the twilight method were made by the Soviet scholars G. V. Rozenberg and N. M. Shtaude. Several methods that facilitate study of the structure of opaque media according to their light-scattering properties have been developed and have been applied in areas other than geophysics. The greatest interest has been generated by the development of methods of atmospheric probing from artificial satellites for determining the temperatures of the earth’s surface or of clouds based on infrared radiation sensed by the satellites. Methods of determining vertical temperature and humidity profiles on the basis of incident radiation are also being studied. Important results in the development of this method have been obtained by the Soviet scientist M. S. Maklevich, the American scientist L. Kaplan, and the Japanese scientist G. Yamamoto.

Work in the development and coordination of research in areas of atmospheric optics is conducted by the Academy of Sciences of the USSR in conjunction with the Main Administration of the Hydrometeorological Service of the USSR.