ACTINOMETRY (Greek aktis, aktinos — a beam and metreu — I measure) — set of methods of measurement of a radiant energy. To tasks And. the research of direct solar radiation, absorption and dispersion by its molecules of the atmosphere, various solid and liquid impurity, and also determination of long-wave radiation of the earth and the atmosphere belong.
The principle of transformation of one type of energy in another is the basis for methods of measurement of a radiant energy. At absorption of a radiant energy of the sun transition of a radiant energy to thermal happens the blackened surface of any receiver. Registering the amount of heat which is marked out at the same time or temperature increase of a reception surface of the device, it is possible to measure the size of the flow of solar radiation falling on a direct surface. Principles of this sort of measurement of a radiant energy are the basis for a calorimetric method. The phenomenon of photoeffect and photochemical influences are used in photo-electric and photographic methods of measurement.
At And. devices in which the flow of a radiant energy is defined by temperature difference of a reception surface and the environment, edge are used it is measured by the size of the current arising in a chain of consistently connected thermocouples. Such devices are relative and need graduation by comparison of their indications with indications of absolute devices.
A flow of a radiant energy of the sun which passes outside the atmosphere in unit of time (1 min.) through a surface unit (1 cm 2 ), perpendicular to sunshine and remote from the Sun on the distance equal to the average radius of a terrestrial orbit, carries the name of a solar constant. The international commission on radiation recommended to accept as the standard the value of a solar constant equal 1,98 kal/cm 2 ×мин.
In process of passing of a radiant energy of the sun through the atmosphere owing to absorption and dispersion the land surface is reached only by a part it — direct solar radiation. E.g., on the latitude of Leningrad 38% of the sum of radiation from possible in a year turn out on average.
In middle latitudes at the ideal atmosphere, i.e. the atmosphere deprived of water vapors and aerosols in summertime in okolopoludenny hours direct solar radiation can reach 1,65 kal/cm 2 ×мин. In the conditions of the real atmosphere on the Earth's surface the largest measured size was equal 1,51 kal/cm 2 ×мин.
Existence of considerable dust content and smoke of the atmosphere in the conditions of the city leads to essential reduction of solar radiation in comparison with rural areas. The most considerable influence of dust content and smoke of the atmosphere on a flow of direct solar radiation was revealed as a result of the aktinometrichesky measurements executed in Berlin. Here the flow of solar radiation on a perpendicular surface in the city is on average 20% less, than in the country. With small heights of standing of the sun it is distinction reached 50%. Most significantly influence of dust content of the atmosphere of the city on weakening of solar radiation in windless weather when over the city a large amount of dust and smoke accumulates affects.
The land surface is reached not only by a straight line, but p scattered solar radiation. The main difference of scattered radiation from a straight line consists that direct radiation is the directed flow whereas scattered goes from all points of the heavenly arch. The second difference consists in spectral structure of scattered radiation in comparison with a straight line. So, the maximum of radiation of direct solar radiation falls on a wave of 556 nanometers, and disseminated — on 480 nanometers. In the presence clouds the maximum of scattered radiation is displaced towards longer waves (680 nanometers).
In the summer scattered radiation is much less than straight line, in the fall and in the spring it is equal to it, and in the winter nearly 3 times bigger. The transparency of air is less, the scattered radiation since in this case in the atmosphere the additional centers of dispersion (motes or water drops) appear is more considerable. During dry and wet fogs and at increase in cloudiness intensity of scattered radiation rises to 0,4 — 0,6 кал/см1×мин. The role of scattered radiation for northern and polar regions where the ground mass of heat to Earth arrives at the expense of scattered radiation is especially big. For a bay Silent, e.g., direct solar radiation makes 30%, and disseminated — 70%. For Tashkent the relations opposite — direct solar radiation makes 70%, and disseminated — 30%.
In an integral flow of radiation of the Sun the relation of scattered radiation to a straight line makes 1:9, in ultra-violet area — 4:6 because dispersion of beams with smaller wavelength goes more vigorously, than with bigger wavelength.
The sum of flows of direct and scattered solar radiation makes total radiation. In case of continuous or partial cloudiness total radiation represents only scattered radiation. The spectral structure of total radiation does not depend on height of standing of the sun and remains to constants during the day.
In winter months bystry reduction of arrival of total radiation from midlatitudes to high is characteristic, and this distribution is zone. A summer half of year is characterized by existence of small gradients of total radiation on extensive spaces. During polar day total radiation in northern areas much more, than in the field of the equator. The western areas receive less radiation, than east.
Having reached the land surface, a part of radiation is reflected from it (albedo), forming a flow of reflected radiation. Albedo of various surfaces unequally. The difference between total and reflected radiation is called the coming radiation, and a difference between the coming radiation and radiation of the earth and the atmosphere — radiation balance which is the major climate factor. The main patterns of radiation balance are defined by duration of sunshine, cloudiness and transparency of the atmosphere, character and a condition of the spreading surface, etc. The maximum positive sizes of radiation balance are observed in okolopoludenny hours, and the maximum negative values — at night.
In a warm half of year the day positive sum of radiation balance surpasses the night negative sum and therefore the daily radiation balance is positive. In winter time the return picture — daily radiation balance negative takes place.
Geographical distribution of the annual sums of radiation balance in the territory of the USSR shows that it is positive everywhere and changes from 20 kcal/cm 2 ×год in the north to 60 kcal/cm 2 ×год in the south. Geographical distribution of radiation balance is zone. Aktinometrichesky researches in the Arctic and Antarctic showed that for the most part of the Arctic (except for its central part) the average annual radiation balance is positive, in the central Arctic it is close to zero (0,5 kcal/cm 2 ×год). The annual radiation balance of Antarctica, except for surfaces, free from ice and snow, is negative. The devices used at actinometry. Devices for measurement of intensity of a radiant energy are called actinometers. Distinguish devices for measurement of intensity of direct solar radiation. — pirgeliometr (fig. 1), devices for measurement of scattered solar radiation — piranometr (fig. 2), devices for measurement of the terrestrial (night) radiation (pirgeometra), the device for measurement of the solar radiation reflected from the land surface — albedometr (fig. 3), devices for measurement of radiation of artificial sources of a caloradiance — actinometers (fig. 4 and 5) which are used also for measurement of solar radiation. The size of a radiant energy is expressed in the gram-calories absorbed in 1 min. by a surface in 1 cm 2 , the insolation located perpendicularly to the direction of beams of a source (kal/cm 2 ×мин).
The devices used at And., it is possible to divide into devices for relative and absolute measurements which structurally differ from each other. Allows to carry out absolute measurements pirgeliometr.
The device consists of two tubes, one of which is blackened and open for direct solar radiation, and another is closed. Both tubes are washed by water. For equalizing of the water temperature following from two cameras, the closed tube is warmed with current that bigger force, than the water passing through a tube, open for solar radiation, heated up stronger. Knowing amount of heat generated in the first camera and the area of a reception opening, calculate intensity of solar radiation in absolute values.
The actinometer usually registers the size of direct solar radiation, previously having transferred it to other type of energy. The principle of operation of the actinometer is based on solar absorptivity by blackened surface and its transformation into warmth, edges either is registered directly, or turns into mechanical energy with registration of mechanical movements. So, in Michelson's actinometer blackened iron part of the bimetallic plate made of iron and an invar heats up. As the blackened iron party of a bimetallic plate heats up and extended, and the invar practically does not heat up and, therefore, there is no its lengthening, all bimetallic plate is bent, being stuck out by the blackened heating-up and extended party, and the radius of a bend is proportional to temperature. Movement of the quartz fiber placed on the edge of a plate serves as a measure of intensity of direct solar radiation. Operation of the actinometer of Yanishevsky is based on measurement of the thermoelectric electromotive force arising from a difference of heating of the thermoelements consisting of two in zigzag fashion connected strips of manganin and a constantan. Peripheral accustom to drinking are attached to the copper ring shaded from direct sunshine, and central accustom to drinking are attached to the central silver disk, blackened, affected by direct solar radiation.
The current resulting from a difference of heating of the thermocouple is proportional to temperature difference central and peripheral having soldered which is proportional to a flow of radiation.
Other devices which are used to carrying out relative and absolute measurements of solar radiation differ in design of their schemes and use of other sensitive elements.
Bibliography: Galanin N. F. Radiant energy and its hygienic value, L., 1969, bibliogr.; Kedrolivansky V. N. and Sternzat M. S. Meteorological devices, L., 1953; Kondratyev K. Ya. Actinometry, L., 1965; Minkh A. A. Methods of hygienic researches, M., 1971; Yanishevsky Yu. D. Aktinometricheskiye devices and methods of observation, L., 1957; Goody R. M of Atmospheric radiation, theoretical basis, Oxford, 1964.
Yu. V. Novikov; V. I. Belkevich (tekhn.).