Actinometry


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actinometry

[‚ak·tə′näm·ə·trē]
(astrophysics)
The science of measurement of radiant energy, particularly that of the sun, in its thermal, chemical, and luminous aspects.

Actinometry

 

a branch of geophysics which studies radiation transfer and conversion in the atmosphere, in the hydrosphere, and on the surface of the earth. In a narrow sense the word “actinometry” is the totality of methods in meteorology for measuring the radiation of the earth.

The source of energy for the processes taking place on the earth and in the atmosphere is the sun. When the shortwave radiation of the sun (electromagnetic radiation in the wavelength range from 0.3 to 3 microns) passes through the earth’s atmosphere, chemical reactions, ionization, and the dissociation of molecules occur in the upper layers; absorption of the radiation (mainly by ozone, water vapor, and the earth’s surface) causes heating of the atmosphere. On the other hand, the earth, like every heated body, radiates energy into outer space. The increase and decrease of radiation energy of the atmosphere and the underlying surface are the ultimate cause of the appearance of different climatic zones on the earth and of changes in weather. The major task of actinometry in connection with this is the quantitative and qualitative study of direct, scattered, and reflected solar radiation, the longwave radiation of the earth’s surface and atmosphere, the radiation balance of the atmosphere, and the development of instruments and methods for measuring conversion of radiant energy in the atmosphere and hydrosphere and on the earth’s surface. Actinometry is closely associated with atmospheric optics and spectroscopy and has much in common with solar physics, the physics of the upper atmospheric layers, and the physics of the ground layer. The results of experimental and theoretical work in actinometry are used in climatology, agriculture and industry, medicine, architecture, transportation, aerology, and meteorology.

The development of actinometry began as early as the 17th century. The first measurements of solar heat (in certain relative units) were made by the English scientist E. Halley in 1693. In 1896 the Russian scientist R. N. Savel’ev was the first to carry out direct solar radiation measurements from a balloon, thus initiating actinometric investigations in the free atmosphere. However, it was only after the creation of the pyrheliometer (1887) and the pyrgeometer (1905) by the Swedish scientist K. Angstrom and the bimetallic actinometer (1905) by the Russian physicist V. A. Mikhel’son that investigations of solar and earthly radiation assumed a strictly quantitative character.

The history of the modern period of actinometry in Russia is closely associated with the name of S. I. Savinov and the Pavlovsk observatory. A permanent actinometry commission, under whose guidance a widespread network of actinometry stations was set up, was founded in 1925 in the USSR as part of the Main Geophysical Observatory (MGO). The MGO, one of the oldest observatories in the world, directs practically all work in the USSR in the fields of actinometric measurements on the earth’s surface and climatological studies of heat balance. In 1948, for the first time in the USSR, radiation measurements from an aircraft were started at the MGO. Extensive investigations in the field of actinometry have been made at the Central Aerological Observatory and at the Leningrad State University.

Since 1954 in the Federal Republic of Germany, the USA, the USSR, and Japan, studies of the free atmosphere were begun by means of actinometric radiosondes (ARS), which are instruments lifted with one or two small balloons up to 30 or 35 km, providing a height distribution of the descending and ascending fluxes of longwave radiation and effective radiation with an accuracy sufficient for the solution of many geophysical problems. In 1963 in the USSR, for the first time in the world, a network of actinometric radiosonde operations was started with regular releases of actinometric radiosondes. In addition, actinometric investigations of the free atmosphere by actinometric radiosondes were made from weather ships in the Antarctic.

Theoretical studies in actinometry cover a wide range of problems, in particular the question of the correlation between radiation and the temperature of the atmosphere and cloud cover and of changes in the weather and climate. The work of the Institute of Atmospheric Physics of the Academy of Sciences of the USSR (investigation of the correlation between radiation and cloud cover) and the Main Geophysical Observatory and the Hydrometeorological Scientific Research Center of the USSR (research on the theory of climate) occupies a prominent position.

Very great possibilities for actinometry were opened up with the advent of artificial earth satellites. From radiation measurements in the range between 8 and 12 microns, where the atmosphere has slight effect on the radiation from the earth’s surface, the radiation temperature of this surface has been determined, thus making it possible to establish in many cases whether cloud cover is present or absent; measurements of the outgoing shortwave (reflected) and longwave radiation yield the balance of the earth-atmosphere system, which plays a large part in climatological investigations. The possibilities of spectral radiation investigations with artificial earth satellites have given rise to the so-called reverse actinometry problems, in which an attempt is made to determine from the radiation energy measurements the atmosphere’s temperature profile and the height of its major absorbing components (water vapor, carbon dioxide, and ozone). These tasks pose new problems in mathematics, spectroscopy, the technology of actinometric instrument design, and the theory for the transfer of radiant energy, providing a new stimulus for the development of actinometry.

An important part was played in the development of actinometry by consolidating the efforts of a number of countries in carrying out studies according to an international program during the periods of the International Year of the Quiet Sun, the International Year of Geophysical Collaboration, the International Geophysical Year, and others. Fundamental information on actinometry is published in journals of atmospheric physics, aerology, and meteorology, and in the works of scientific research organizations.

REFERENCES

Kondrat’ev, K. Ia. Aktinometriia. Leningrad, 1965.
Khrgian, A. Kh. Ocherki razvitiia meteorologii, 2nd ed., vol. 1. Leningrad, 1959.
Ianishevskii, Iu. D. Aktinometricheskie pribory i metody nabliudenii. Leningrad, 1957.
Glavnaia geofizicheskaia observatoriia im. A. I. Voeikova za 50 let Sovetskoi vlasti. Leningrad, 1967.
Kondrat’ev, K. Ia., E. P. Borisenko, and A. A. Morozkin. Prakticheskoe ispol’zovanie dannykh meteorologicheskikh sputnikov. Leningrad, 1966.

G. N. KOSTIANOI

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