- general characteristics solar radiation
- Straight solar radiation
- Total solar radiation
- Absorption of solar radiation in the atmosphere
Radiant energy from the Sun, or solar radiation, is the main source of heat for the Earth's surface and for its atmosphere. The radiation coming from the stars and the Moon is negligible in comparison with solar radiation and does not make a significant contribution to thermal processes on Earth. The heat flux directed to the surface from the depths of the planet is also negligible. Solar radiation propagates in all directions from the source (the Sun) in the form of electromagnetic waves at a speed close to 300,000 km / sec. In meteorology, thermal radiation is mainly considered, which is determined by the temperature of the body and its emissivity. Thermal radiation has wavelengths ranging from hundreds of micrometers to thousandths of a micrometer. X-rays and gamma radiation are not considered in meteorology, since they practically do not enter the lower atmosphere. Thermal radiation is usually subdivided into shortwave and longwave. Short-wave radiation is called radiation in the wavelength range from 0.1 to 4 microns, long-wave - from 4 to 100 microns. Solar radiation reaching the Earth's surface is 99% shortwave. Shortwave radiation is subdivided into ultraviolet (UV) radiation, with wavelengths ranging from 0.1 to 0.39 microns; visible light (BC) - 0.4 - 0.76 microns; infrared (IR) - 0.76 - 4 microns. Sun and infrared radiation provide the highest energy: the sun accounts for 47% of the radiant energy, infrared - 44%, and UV - only 9% of the radiant energy. This distribution of thermal radiation corresponds to the distribution of energy in the spectrum of an absolutely black body with a temperature of 6000K. This temperature is considered conditionally close to the actual temperature on the sun's surface (in the photosphere, which is the source of the sun's radiant energy). The maximum radiant energy at this temperature of the emitter, according to Wien's law, l = 0.2898 / T (cm * deg). (1) falls on blue-blue rays with lengths of about 0.475 microns (l. Is the wavelength, T is the absolute temperature of the emitter). The total amount of radiated heat energy is proportional, according to the Stefan-Boltzmann law, of the fourth degree absolute temperature emitter: E = sT 4 (2) where s = 5.7 * 10-8 W / m 2 * K 4 (Stefan-Boltzmann constant). The quantitative measure of solar radiation arriving at the surface is the irradiance, or the density of the radiation flux. Energy illumination is the amount of radiant energy delivered per unit area per unit of time. It is measured in W / m2 (or kW / m2). This means that 1 J (or 1 kJ) of radiant energy is supplied to 1 m 2 per second. The energy illumination of solar radiation falling on an area of a unit area perpendicular to the sun's rays per unit time at the upper boundary of the atmosphere at an average distance from the Earth to the Sun is called the solar constant Sо. In this case, the upper boundary of the atmosphere is understood as the condition for the absence of the effect of the atmosphere on solar radiation. Therefore, the magnitude of the solar constant is determined only by the emissivity of the Sun and the distance between the Earth and the Sun. Modern research using satellites and rockets has established the Sо value equal to 1367 W / m2 with an error of ± 0.3%, the average distance between the Earth and the Sun in this case is determined as 149.6 * 106 km. If we take into account the changes in the solar constant due to the change in the distance between the Earth and the Sun, then with an average annual value of 1.37 kW / m2, in January it will be equal to 1.41 kW / m2, and in June - 1.34 kW / m2 2, therefore, the northern hemisphere receives slightly less radiation at the border of the atmosphere during a summer day than the southern hemisphere during its summer day. Due to constant change solar activity the solar constant may fluctuate from year to year. But these fluctuations, if they exist, are so small that they lie within the measurement accuracy of modern instruments. But during the existence of the Earth, the solar constant most likely changed its value. Knowing the solar constant, you can calculate the amount of solar energy entering the illuminated hemisphere at the upper boundary of the atmosphere. It is equal to the product of the solar constant by the area large circle Earth. With an average radius of the earth equal to 6371 km, the area of the great circle is p * (6371) 2 = 1.275 * 1014 m 2, and the radiant energy arriving at it is 1.743 * 1017 W. For a year, this will amount to 5.49 * 1024 J. The arrival of solar radiation on a horizontal surface at the upper boundary of the atmosphere is called a solar climate. The formation of a solar climate is determined by two factors - the duration of sunshine and the height of the Sun. The amount of radiation at the border of the atmosphere per unit of horizontal surface area is proportional to the sine of the Sun's height, which changes not only during the day, but also depends on the season. As you know, the height of the Sun for the days of the solstice is determined by the formula 900 - (j ± 23.50), for the days of the equinox - 900 -j, where j is the latitude of the place. Thus, the height of the Sun at the equator changes throughout the year from 90 ° to 66.50 °, in the tropics - from 90 to 43 °, in the polar circles - from 47 to 0 ° and at the poles - from 23.5 ° to 0 ° ... In accordance with such a change in the height of the Sun in winter in each hemisphere, the influx of solar radiation to the horizontal area rapidly decreases from the equator to the poles. In summer, the picture is more complex: in the middle of summer, the maximum values are not at the equator, but at the poles, where the day length is 24 hours. In the annual course in the extratropical zone, one maximum (summer solstice) and one minimum ( winter solstice). In the tropical zone, the influx of radiation reaches a maximum twice a year (equinox days). Annual amounts of solar radiation vary from 133 * 102 MJ / m2 (equator) to 56 * 102 MJ / m2 (poles). The amplitude of the annual cycle at the equator is small, in the extratropical zone it is significant.
2 Direct solar radiation Direct solar radiation refers to radiation that arrives at the earth's surface directly from the solar disk. Despite the fact that solar radiation spreads from the Sun in all directions, it comes to the Earth in the form of a beam of parallel rays emanating from infinity. The inflow of direct solar radiation to the earth's surface or to any level in the atmosphere is characterized by irradiance - the amount of radiant energy supplied per unit of time per unit of area. The maximum inflow of direct solar radiation will enter the site perpendicular to the sun's rays. In all other cases, the irradiance will be determined by the height of the Sun, or the sine of the angle that forms a sunbeam with the surface of the site S ’= S sin hc (3) In the general case, S (the irradiance of a site with a unit area perpendicular to the sun's rays) is So. The flux of direct solar radiation falling on a horizontal area is called insolation.
3. Scattered solar radiation Passing through the atmosphere, direct solar radiation is scattered by molecules of atmospheric gases and aerosol impurities. During scattering, a particle located in the path of propagation of an electromagnetic wave continuously absorbs energy and re-radiates it in all directions. As a result, the stream of parallel sun rays going in a certain direction is re-emitted in all directions. Scattering occurs at all wavelengths of electromagnetic radiation, but its intensity is determined by the ratio of the size of the scattering particles and the wavelengths of the incident radiation. In an absolutely pure atmosphere, where scattering is produced only by gas molecules whose dimensions are less than the radiation wavelengths, it obeys Rayleigh's law, which states that the spectral irradiance of the scattered radiation is inversely proportional to the fourth power of the wavelength of the scattered rays Dl = a Sl / l 4 ( 4) where Sl is the spectral density of the irradiance of direct radiation with a wavelength of l, Dl is the spectral density of the irradiance of scattered radiation with the same wavelength, and is the coefficient of proportionality. In accordance with Rayleigh's law, shorter wavelengths prevail in scattered radiation, since red rays, being twice as long as violet ones, are scattered 14 times less. Infrared radiation is scattered very little. It is believed that about 26% of the total solar radiation flux undergoes scattering, 2/3 of this radiation comes to the earth's surface. Since the scattered radiation does not come from the solar disk, but from the entire firmament, its irradiance is measured on a horizontal surface. The unit for measuring the irradiance of scattered radiation is W / m2 or kW / m2. If scattering occurs on particles commensurate with the radiation wavelengths (aerosol impurities, ice crystals and water droplets), then the scattering does not obey Rayleigh's law and the energy illumination of the scattered radiation becomes inversely proportional not to the fourth, but to the smallest powers of wavelengths - i.e. the scattering maximum shifts to the longer wavelength part of the spectrum. With a high content of large particles in the atmosphere, scattering is replaced by diffuse reflection, in which the light flux is reflected by the particles as mirrors, without changing the spectral composition. Since white light falls, then a flux of white light is also reflected. As a result, the color of the sky becomes whitish. There are two associated interesting phenomena- this is the blue color of the sky and twilight. The blue color of the sky is the color of the air itself, due to the scattering of sunlight in it. Since scattering in a clear sky obeys Rayleigh's law, the maximum energy of scattered radiation coming from the firmament falls on the blue color. The blue color of the air can be seen when looking at distant objects that appear to be shrouded in a bluish haze. With height, as the density of the air decreases, the color of the sky becomes darker and turns into a deep blue, and in the stratosphere - into purple. The more impurities are contained in the atmosphere, the greater the proportion of long-wave radiation in the spectrum sunlight, the whitish the sky becomes. Due to the scattering of the shortest waves, direct solar radiation is depleted in waves of this range, therefore, the maximum energy in direct radiation shifts to the yellow part and the solar disk turns yellow. At low angles of the Sun, scattering is very intense, shifting to the long-wavelength part of the electromagnetic spectrum, especially in a polluted atmosphere. The maximum direct solar radiation shifts to the red part, the solar disk turns red, and bright yellow-red sunsets occur. After sunset, darkness does not come immediately, similarly in the morning, it becomes light on the earth's surface some time before the appearance of the solar disk. This phenomenon of incomplete darkness in the absence of a solar disk is called evening and morning twilight. The reason for this is the illumination of the high layers of the atmosphere by the Sun below the horizon and the scattering of sunlight by them. Distinguish astronomical twilight, which continues until the sun drops below the horizon by 180 and at the same time it becomes so dark that the faintest stars will be distinguishable. The first part of the evening astronomical twilight and the last part of the morning astronomical twilight is called civil twilight, in which the Sun sinks below the horizon of at least 80. The duration of astronomical twilight depends on the latitude of the area. Above the equator, they are short, up to 1 hour, in temperate latitudes they are 2 hours. In high latitudes during the summer season, evening twilight merges with morning twilight, forming white nights.
4 Absorption of solar radiation in the atmosphere. On the upper boundary of the atmosphere, solar radiation comes in the form of direct radiation. About 30% of this radiation is reflected back into space, 70% - into the atmosphere. Passing through the atmosphere, this radiation undergoes changes associated with its absorption and scattering. About 20-23% of direct solar radiation is absorbed. Absorption is selective and depends on the wavelengths and material composition of the atmosphere. Nitrogen, the main gas in the atmosphere, absorbs radiation only at very short wavelengths in the ultraviolet part of the spectrum. The energy of solar radiation in this part of the spectrum is very small and the absorption of radiation by nitrogen practically does not affect the value of the total energy flux. Oxygen absorbs slightly more in two narrow parts of the visible part of the spectrum and in the ultraviolet part. Ozone absorbs radiation more vigorously. The total amount of radiation absorbed by ozone reaches 3% of direct solar radiation. The main part of the absorbed radiation falls on the ultraviolet part, at wavelengths shorter than 0.29 microns. In small quantities, ozone also absorbs visible radiation. Carbon dioxide absorbs radiation in the infrared range, but due to its small amount, the proportion of this absorbed radiation is generally small. The main absorbers of direct solar radiation are water vapor, clouds and aerosol impurities concentrated in the troposphere. Water vapor and aerosols account for up to 15% of absorbed radiation, and clouds up to 5%. Since the bulk of the absorbed radiation falls on such variable components of the atmosphere as water vapor and aerosols, the level of absorption of solar radiation varies considerably and depends on the specific conditions of the state of the atmosphere (its humidity and pollution). In addition, the amount of absorbed radiation depends on the height of the Sun above the horizon, i.e. from the thickness of the layer of the atmosphere that the sunbeam passes through.
5. Visibility, radiation attenuation law, turbidity factor. The scattering of light in the atmosphere leads to the fact that distant objects at a distance become poorly distinguishable not only due to their decrease in size, but also due to the turbidity of the atmosphere. The distance at which the outlines of objects cease to be distinguished in the atmosphere is called the visibility range, or simply visibility. The visibility range is most often determined by eye according to certain, pre-selected objects (dark against the sky), the distance to which is known. In very clean air, the visibility range can reach hundreds of kilometers. In air containing many aerosol impurities, the visibility range can be reduced to several kilometers or even meters. So, in weak fog, the visibility range is 500-1000 m, and in heavy fog or sandstorms, it drops to several meters. Absorption and scattering leads to a significant weakening of the solar radiation flux passing through the atmosphere. Radiation is attenuated in proportion to the flux itself (with other equal conditions, the greater the flux, the greater the energy loss) and the number of absorbing and scattering particles. The latter depends on the length of the beam path through the atmosphere. For an atmosphere that does not contain aerosol impurities (ideal atmosphere), the transparency coefficient p is 0.90-0.95. In a real atmosphere, its values range from 0.6 to 0.85 (somewhat higher in winter, lower in summer). With an increase in the content of water vapor and impurities, the transparency coefficient decreases. With an increase in the latitude of the area, the transparency coefficient increases due to a decrease in water vapor pressure and less dustiness in the atmosphere. All attenuation of radiation in the atmosphere can be divided into two parts: attenuation by constant gases (ideal atmosphere) and attenuation by water vapor and aerosol impurities. The ratio of these processes is taken into account by the turbidity factor 6. Geographic patterns of distribution of direct and scattered radiation... The flux of direct solar radiation depends on the height of the Sun above the horizon. Therefore, during the day, the flux of solar radiation, first quickly, then slowly increases from sunrise to noon, and at first slowly, then rapidly decreases from noon to sunset. But the transparency of the atmosphere changes during the day, so the curve of the daytime movement of direct radiation is not smooth, but has deviations. But on average, over a long observation period, changes in radiation during the day take the form of a smooth curve. During the year, the irradiance of direct solar radiation for the main part of the Earth's surface changes significantly, which is associated with changes in the height of the Sun. For the northern hemisphere, the minimum values of both direct radiation to the perpendicular surface and insolation fall on December, the maximum values are not summer period , and in the spring, when the air is less turbid by condensation products and a little dusty. Average midday power illumination in Moscow in December is 0.54, April 1.05, June-July 0.86-0.99 kW / m 2. The daily values of direct radiation are maximum in summer, with the maximum duration of sunshine. The maximum values of direct solar radiation for some points are as follows (kW / m2): Tiksi Bay 0.91, Pavlovsk 1.00, Irkutsk 1.03, Moscow 1.03, Kursk 1.05, Tbilisi 1.05, Vladivostok 1, 02, Tashkent 1.06. The maximum values of direct solar radiation grow little with decreasing latitude, despite an increase in the height of the Sun. This is due to the fact that in the southern latitudes the moisture content and dustiness of the air increases. Therefore, at the equator, the maximum values are slightly higher than the maximums of temperate latitudes. The highest annual values of direct solar radiation on Earth are observed in the Sahara - up to 1.10 kW / m2. The seasonal differences in the arrival of direct radiation are as follows. In the summer period, the highest values of direct solar radiation are observed under 30-400 latitudes of the summer hemisphere, towards the equator and towards the polar circles the values of direct solar radiation decrease. To the poles for the summer hemisphere, the decrease in direct solar radiation is small, in the winter it becomes zero. In spring and autumn, the maximum values of direct solar radiation are observed in 10-200 of the spring hemisphere and 20-300 in the autumn. Only the winter part of the equatorial zone receives the maximum values of direct solar radiation for a given period. With altitude, the maximum values of radiation increase due to a decrease in the optical thickness of the atmosphere: for every 100 meters of altitude, the amount of radiation in the troposphere increases by 0.007-0.14 kW / m 2. The maximum radiation values recorded in the mountains are 1.19 kW / m 2. The scattered radiation entering the horizontal surface also changes during the day: it increases before noon and decreases in the afternoon. The magnitude of the scattered radiation flux in general depends on the length of the day and the height of the Sun above the horizon, as well as the transparency of the atmosphere (a decrease in transparency leads to an increase in scattering). In addition, scattered radiation varies over a very wide range depending on the cloud cover. The radiation reflected by the clouds is also scattered. The radiation reflected by the snow is also scattered, which increases its share in winter. Scattered radiation with average cloud cover is more than twice its value on a cloudless day. In Moscow, the average noon value of scattered radiation in summer with a clear sky is 0.15, and in winter with a low sun - 0.08 kW / m 2. With discontinuous clouds, these values are 0.28 in summer and 0.10 kW / m 2 in winter. In the Arctic, with relatively thin clouds and snow cover, these values in summer can reach 0.70 kW / m2. The values of scattered radiation in Antarctica are very high. Scattered radiation decreases with increasing altitude. Scattered radiation can significantly complement direct radiation, especially when the sun is low. Due to the scattered light, the entire atmosphere during the day serves as a source of illumination: during the day it is light both where the sun's rays do not directly fall, and when the Sun is hidden by clouds. Scattered radiation increases not only the illumination, but also the heating of the earth's surface. The scattered radiation is generally less than the direct one, but the order of magnitude is the same. In tropical and middle latitudes, the amount of scattered radiation is from half to two-thirds of the values of direct radiation. At 50-600, their values are close, and closer to the poles, scattered radiation prevails.
7 Total radiation All solar radiation reaching the earth's surface is called total solar radiation. In a cloudless sky, total solar radiation has a daily variation with a maximum around noon and an annual variation with a maximum in summer. Partial cloudiness, which does not cover the solar disk, increases the total radiation in comparison with a cloudless sky, full cloudiness, on the contrary, decreases it. On average, cloudiness reduces radiation. Therefore, in summer, the arrival of total radiation in the pre-noon hours is greater than in the afternoon and in the first half of the year it is greater than in the second. The noon values of the total radiation in the summer months near Moscow with a cloudless sky averaged 0.78, with an open Sun and clouds 0.80, with continuous clouds - 0.26 kW / m 2. The distribution of total radiation values over the globe deviates from the zonal , which is explained by the influence of the transparency of the atmosphere and clouds. The maximum annual values of the total radiation are 84 * 102 - 92 * 102 MJ / m2 and are observed in deserts North Africa... Above areas of equatorial forests with high clouds, the values of the total radiation are reduced to 42 * 102 - 50 * 102 MJ / m2. Towards higher latitudes of both hemispheres, the values of the total radiation decrease, amounting to 25 * 102 - 33 * 102 MJ / m2 under the 60th parallel. But then they grow again - a little over the Arctic and significantly - over Antarctica, where in the central parts of the continent they are 50 * 102 - 54 * 102 MJ / m2. Over the oceans as a whole, the values of the total radiation are lower than over the corresponding latitudes of the land. In December, the highest values of total radiation are observed in the deserts of the Southern Hemisphere (8 * 102 - 9 * 102 MJ / m2). Above the equator, the values of total radiation decrease to 3 * 102 - 5 * 102 MJ / m 2. In the Northern Hemisphere, radiation rapidly decreases towards the polar regions and is equal to zero beyond the Arctic Circle. In the Southern Hemisphere, the total radiation decreases to the south to 50-600 S latitude. (4 * 102 MJ / m2), and then increases to 13 * 102 MJ / m2 in the center of Antarctica. In July, the highest values of total radiation (over 9 * 102 MJ / m2) are observed over northeastern Africa and the Arabian Peninsula. Above the equatorial region, the values of the total radiation are low and equal to those in December. To the north of the tropic, the total radiation decreases slowly to 600 N, and then increases to 8 * 102 MJ / m2 in the Arctic. In the southern hemisphere, the total radiation from the equator decreases rapidly to the south, reaching zero values at the Arctic Circle.
8. Reflection of solar radiation. Albedo of the Earth. When arriving at the surface, the total radiation is partially absorbed in the upper thin layer of soil or water and turns into heat, and partially is reflected. The conditions for the reflection of solar radiation from the earth's surface are characterized by the albedo value, equal to the ratio reflected radiation to the incoming stream (to the total radiation). А = Qref / Q (8) Theoretically, albedo values can vary from 0 (absolutely black surface) to 1 (absolutely white surface). The available observational materials show that the albedo values of underlying surfaces vary over a wide range, and their changes cover almost the entire possible range of reflectivity values of various surfaces. In experimental studies, albedo values have been found for almost all common natural underlying surfaces. These studies show, first of all, that the conditions for absorption of solar radiation on land and on water bodies differ markedly. The highest albedo values are observed for clean and dry snow (90-95%). But since the snow cover is rarely completely clean, the average values of the snow albedo in most cases are equal to 70-80%. For wet and dirty snow, these values are even lower - 40-50%. In the absence of snow, the highest albedos on the land surface are characteristic of some desert regions, where the surface is covered with a layer of crystalline salts (the bottom of dried lakes). Under these conditions, the albedo is 50%. Few less value albedo in sandy deserts. The albedo of wet soil is less than that of dry soil. For wet chernozems, the albedo values are extremely small - 5%. The albedo of natural surfaces with a continuous vegetation cover varies within relatively small limits - from 10 to 20-25%. Moreover, the albedo of the forest (especially coniferous) is in most cases less than the albedo of meadow vegetation. The conditions for absorption of radiation on water bodies differ from the conditions for absorption on the land surface. Pure water is relatively transparent to short-wave radiation, as a result of which the sun's rays penetrating into the upper layers are repeatedly scattered and only after that are largely absorbed. Therefore, the process of absorption of solar radiation depends on the height of the Sun. If it is high, a significant part of the incoming radiation penetrates into the upper layers of the water and is mainly absorbed. Therefore, the albedo of the water surface is the first few percent at a high Sun, and at a low Sun, the albedo increases to several tens of percent. The albedo of the "Earth-atmosphere" system is of a more complex nature. Solar radiation entering the atmosphere is partially reflected as a result of atmospheric backscattering. In the presence of clouds, a significant part of the radiation is reflected from their surface. The albedo of clouds depends on the thickness of their layer and averages 40-50%. In the complete or partial absence of clouds, the albedo of the "Earth-atmosphere" system substantially depends on the albedo of the earth's surface itself. The nature of the geographic distribution of the planetary albedo from satellite observations shows significant differences between the albedos of the high and middle latitudes of the Northern and Southern Hemispheres. In the tropics, the highest albedo values are observed over deserts, in convective cloud zones over Central America and over ocean areas. In the Southern Hemisphere, in contrast to the Northern, a zonal variation of the albedo is observed due to the simpler distribution of land and sea. The highest albedo values are found at polar latitudes. The predominant part of the radiation reflected by the earth's surface and the upper boundary of the clouds goes into world space. A third of the scattered radiation also goes away. The ratio of reflected and scattered radiation leaving space to the total amount of solar radiation entering the atmosphere is called the planetary albedo of the Earth or the albedo of the Earth. Its value is estimated at 30%. The main part of the planetary albedo is radiation reflected by clouds. 6.1.8. Own radiation. Counter radiation. Effective radiation. Solar radiation, absorbed by the upper layer of the Earth, heats it up, as a result of which the soil and surface waters themselves emit long-wave radiation. This terrestrial radiation is called the intrinsic radiation of the terrestrial surface. The intensity of this radiation, with some assumptions, obeys the Stefan-Boltzmann law for an absolute black body with a temperature of 150C. But since the Earth is not an absolutely black body (its radiation corresponds to the radiation of a gray body), in the calculations it is necessary to introduce a correction equal to e = 0.95. Thus, the Earth's own radiation can be determined by the formula Ez = esТ 4 (9) It is determined that at an average planetary temperature of the Earth of 150C, the Earth's own radiation Ez = 3.73 * 102 W / m2. Such a large return of radiation from the earth's surface would lead to its very rapid cooling, if this was not prevented by the reverse process - the absorption of solar and atmospheric radiation by the earth's surface. The absolute temperatures on the earth's surface are in the range of 190-350K. At such temperatures, its own radiation has wavelengths in the range of 4-120 microns, and the maximum energy falls on 10-15 microns. The atmosphere, absorbing both solar radiation and the earth's own radiation, heats up. In addition, the atmosphere is heated in a non-radiation way (by heat conduction, during condensation of water vapor). The heated atmosphere becomes a source of long-wave radiation. Most of of this radiation of the atmosphere (70%) is directed to the earth's surface and is called counter radiation (Ea). Another part of the radiation from the atmosphere is absorbed by the overlying layers, but as the content of water vapor decreases, the amount of radiation absorbed by the atmosphere decreases, and part of it goes into world space. The earth's surface absorbs the oncoming radiation almost entirely (95-99%). Thus, the counter-radiation is for the earth's surface important source heat in addition to absorbed solar radiation. In the absence of clouds, the long-wave radiation of the atmosphere is determined by the presence of water vapor and carbon dioxide. The influence of atmospheric ozone, in comparison with these factors, is insignificant. Water vapor and carbon dioxide absorb long-wavelength radiation in the range from 4.5 to 80 microns, but not entirely, but in certain narrow spectral regions. The strongest absorption of radiation by water vapor occurs in the wavelength range of 5-7.5 microns, while in the region of 9.5-12 microns. 4.1. Atmospheric transparency windows in the optical range, absorption is practically absent. This wavelength range is called the atmospheric transparency window. Carbon dioxide has several absorption bands, of which the most significant is the band with wavelengths of 13-17 microns, which account for the maximum of terrestrial radiation. It should be noted that the content of carbon dioxide is relatively constant, while the amount of water vapor varies very significantly, depending on the meteorological conditions. Therefore, a change in air humidity has a significant effect on the amount of atmospheric radiation. For example, the largest counter radiation is 0.35-0.42 kW / m 2 on average near the equator, and towards the polar regions it decreases to 0.21 kW / m 2, on flat territories Ea is 0.21-0.28 kW / m 2 and 0.07-0.14 kW / m 2 - in the mountains. The decrease in the counter radiation in the mountains is explained by the decrease in the water vapor content with height. The counter radiation of the atmosphere usually increases significantly in the presence of clouds. Clouds of the lower and middle tiers, as a rule, are quite dense and emit as a black body at the appropriate temperature. High clouds, due to their low density, usually emit less than a black body, so they have little effect on the ratio of their own and counterpropagating radiation. The absorption of long-wavelength self-radiation by water vapor and other gases creates a "greenhouse effect", i.e. keeps the sun's heat in the earth's atmosphere. The increase in the concentration of these gases and, above all, carbon dioxide as a result economic activity a person can lead to an increase in the share of the remaining heat on the planet, to an increase in mean planetary temperatures and a change in the global climate of the Earth, the consequences of which are still difficult to predict. But it should be noted that water vapor plays the main role in the absorption of terrestrial radiation and the formation of the oncoming radiation. Through the transparency window, part of the long-wavelength terrestrial radiation escapes through the atmosphere into world space. Together with the radiation from the atmosphere, this radiation is called outgoing radiation. If the inflow of solar radiation is taken as 100 units, then the outgoing radiation will be 70 units. Taking into account 30 units of reflected and scattered radiation (planetary albedo of the Earth), the Earth gives off to outer space as much radiation as it receives, i.e. is in a state of radiant equilibrium.
9. Radiation balance of the earth's surface The radiation balance of the earth's surface is the difference between the arrival of radiation on the earth's surface (in the form of absorbed radiation) and its consumption as a result of thermal radiation (effective radiation). The radiation balance changes from night negative values to daytime positive in the summer at a Sun height of 10-15 degrees and vice versa, from positive to negative - before sunset at the same Sun heights. In winter, the transition of values of the radiation balance through zero occurs at large angles of the Sun (20-25 degrees). At night, in the absence of total radiation, the radiation balance is negative and equal to the effective radiation. The distribution of the radiation balance over the globe is fairly uniform. The annual values of the radiation balance are positive everywhere, except for Antarctica and Greenland. Positive annual values of the radiation balance mean that the excess of absorbed radiation is balanced by non-radiative heat transfer from the earth's surface to the atmosphere. This means that there is no radiation equilibrium for the earth's surface (the arrival of radiation is greater than its return), but there is thermal equilibrium ensuring the stability of the thermal characteristics of the atmosphere. The highest annual values of the radiation balance are observed in the equatorial zone between 200 north and south latitude. Here it is more than 40 * 102 MJ / m2. Towards higher latitudes, the values of the radiation balance decrease and around the 60th parallel range from 8 * 102 to 13 * 102 MJ / m2. Further to the poles, the radiation balance decreases even more and amounts to 2 * 102 - 4 * 102 MJ / m2 in Antarctica. The radiation balance is greater over the oceans than over land at the same latitudes. Significant deviations from the zonal values are also observed in deserts, where the balance is below the latitudinal value due to the large effective radiation. In December, the radiation balance is negative over a significant part of the Northern Hemisphere north of the 40th parallel. In the Arctic, it reaches values of 2 * 102 MJ / m2 and below. South of the 40th parallel, it increases to the Southern Tropic (4 * 102 - 6 * 102 MJ / m2), and then decreases to South Pole, amounting to 2 * 102 MJ / m2 on the Antarctic coast. In June, the radiation balance is maximum over the Northern Tropic (5 * 102 - 6 * 102 MJ / m2). To the north, it decreases, remaining positive to the North Pole, and to the south, it decreases, becoming negative off the coast of Antarctica (-0.4 -0.8 * 102 MJd / m2).
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Date the page was created: 2017-06-30
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The main source of energy coming to the Earth is radiant energy coming from the Sun. The flow of electromagnetic waves emitted by the Sun is commonly called solar radiation. This radiation is practically the only source of energy for all processes occurring in the atmosphere and on the earth's surface, including for all processes occurring in living organisms.
Solar radiation provides plants with energy, which they use in the process of photosynthesis to create organic matter, affects the processes of growth and development, the arrangement and structure of leaves, the duration of the growing season, etc. Quantitatively, solar radiation can be characterized by a radiation flux .
Radiation flux - it is the amount of radiant energy that is delivered per unit of time per unit of surface.
In the SI system of units, the radiation flux is measured in watts per 1m 2 (W / m 2) or kilowatts per 1m 2 (kW / m 2). Previously, it was measured in calories per cm 2 per minute (cal / (cm 2 min)).
1 cal / (cm 2 min) = 698 W / m 2 or 0.698 kW / m 2
The flux density of solar radiation at the upper boundary of the atmosphere with an average distance from the Earth to the Sun is called solar constant S 0... According to the international agreement of 1981 S 0 = 1.37 kW / m 2 (1.96 1 cal / (cm 2 min)).
If the Sun is not at its zenith, then the amount of solar energy falling on a horizontal surface will be less than on a surface located perpendicular to the Sun's rays. This amount depends on the angle of incidence of the rays on the horizontal surface. To determine the amount of heat received by a horizontal surface per minute, the following formula is used:
S ′ = S sin h ©
where S ′ is the amount of heat received per minute by the horizontal surface; S is the amount of heat received by the surface perpendicular to the beam; h© - the angle formed by the sunbeam with a horizontal surface (the angle h is called the height of the sun).
Passing through the earth's atmosphere, solar radiation is attenuated due to absorption and scattering by atmospheric gases and aerosols. The attenuation of the solar radiation flux depends on the length of the path traversed by the beam in the atmosphere and on the transparency of the atmosphere along this path. The length of the beam path in the atmosphere depends on the height of the sun. When the sun is at its zenith, the sun's rays travel the shortest path. In this case, the mass of the atmosphere traversed by the sun's rays, i.e. the mass of a vertical column of air with a base of 1 cm 2 is taken as one conventional unit (m = 1). As the sun descends to the horizon, the path of the rays in the atmosphere increases, and, consequently, the number of passable masses also increases (m> 1). When the sun is near the horizon, the rays travel the longest path in the atmosphere. Calculations show that m is 34.4 times greater than at the position of the Sun at its zenith. The attenuation of the direct solar radiation flux in the atmosphere is described by the Bouguer formula. Transparency coefficient p shows what fraction of solar radiation arriving at the upper boundary of the atmosphere reaches the earth's surface at m = 1.
S m = S 0 p m ,
where S m is the direct solar radiation flux reaching the Earth; S 0 - solar constant; p - transparency coefficient; m- the mass of the atmosphere.
The transparency coefficient depends on the content of water vapor and aerosols in the atmosphere: the more there are, the lower the transparency coefficient for the same number of passable masses. The transparency coefficient ranges from 0.60 up to 0.85.
Types of solar radiation
Direct solar radiation(S ′) - radiation arriving at the earth's surface directly from the Sun in the form of a beam of parallel rays.
Direct solar radiation depends on the sun's height above the horizon, the transparency of the air, cloud cover, the altitude of the place above sea level, and the distance between the Earth and the Sun.
Scattered solar radiation(D) – part of the radiation scattered by the earth's atmosphere and clouds and arriving at the earth's surface from the firmament. The intensity of scattered radiation depends on the height of the sun above the horizon, cloudiness, air transparency, altitude above sea level, and snow cover. Cloudiness and snow cover have a very large effect on scattered radiation, which, due to the scattering and reflection of direct and scattered radiation falling on them and their re-scattering in the atmosphere, can increase the scattered radiation flux several times.
Scattered radiation substantially complements direct solar radiation and significantly increases the flow of solar energy to the earth's surface.
Total radiation(Q) - the sum of the fluxes of direct and scattered radiation entering the horizontal surface:
Before sunrise, during the daytime and after sunset, in case of continuous clouds, the total radiation reaches the earth completely, and at low solar heights it mainly consists of scattered radiation. In a cloudless or slightly cloudy sky, with an increase in the height of the Sun, the proportion of direct radiation in the total composition rapidly increases and in the daytime the flux is many times greater than the flux of scattered radiation.
Most of the total radiation flux entering the earth's surface is absorbed by the top layer of soil, water and vegetation. In this case, the radiant energy is converted into heat, heating the absorbing layers. The rest of the total radiation flux is reflected by the earth's surface, forming reflected radiation(R). Almost the entire flux of reflected radiation passes through the atmosphere and goes into world space, but some of it is scattered in the atmosphere and partially returns to the earth's surface, increasing the scattered radiation, and, consequently, the total radiation.
The reflectivity of various surfaces is called albedo... It is the ratio of the reflected radiation flux to the total flux of total radiation incident on this surface:
The albedo is expressed in fractions of a unit or as a percentage. Thus, the earth's surface reflects a part of the total radiation flux, equal to QA, and is absorbed and converted into heat - Q (1-A). The last quantity is called absorbed radiation.
The albedo of various land surfaces depends mainly on the color and roughness of those surfaces. Dark and rough surfaces have lower albedos than light and smooth surfaces. The albedo of soils decreases with increasing moisture content, since their color becomes darker. The albedo values for some natural surfaces are given in Table 1.
Table 1 - Albedo of various natural surfaces
The reflectivity of the upper surface of the clouds is very high, especially at their high power. On average, the albedo of clouds is about 50-60%, in individual cases- more than 80-85%.
Photosynthetically active radiation(PAR) - part of the total radiation flux that can be used by green plants in photosynthesis. The PAR flow can be calculated using the formula:
PAR = 0.43S '+ 0.57D,
where S ′ - direct solar radiation entering the horizontal surface; D - diffused solar radiation.
The PAR flux falling on the sheet is mostly absorbed by it, much smaller portions of this flux are reflected by the surface and passed through by the sheet. The leaves of most tree species absorb about 80%, reflect and transmit up to 10-12% of the total PAR flow. Of the part of the PAR flux absorbed by the leaves, only a few percent of the radiant energy is used by plants directly for photosynthesis and is converted into chemical energy of organic substances synthesized by the leaves. The rest, more than 95% of radiant energy, is converted into heat and is spent mainly on transpiration, heating the leaves themselves and their heat exchange with the surrounding air.
Long-wave radiation of the Earth and the atmosphere.
Radiation balance of the earth's surface
Most of the solar energy entering the Earth is absorbed by its surface and atmosphere, some of it is emitted. Radiation from the earth's surface occurs around the clock.
Part of the rays emitted by the earth's surface is absorbed by the atmosphere and thus contributes to the heating of the atmosphere. The atmosphere, in turn, sends rays back to the surface of the earth, as well as into outer space. This property of the atmosphere to retain heat emitted by the earth's surface is called greenhouse effect... The difference between the arrival of heat in the form of counter radiation of the atmosphere and its consumption in the form of radiation from the active layer is called effective radiation active layer. The effective radiation is especially large at night, when the heat loss by the earth's surface significantly exceeds the heat flux emitted by the atmosphere. In the daytime, when the total solar radiation is added to the radiation of the atmosphere, an excess of heat is obtained, which goes to heating the soil and air, evaporating water, etc.
The difference between the absorbed total radiation and the effective radiation of the active layer is called radiation balance active layer.
The incoming part of the radiation balance is made up of direct and scattered solar radiation, as well as the counter radiation of the atmosphere. The expendable part is made up of reflected solar radiation and long-wave radiation of the earth's surface.
The radiation balance is the actual arrival of radiant energy on the surface of the Earth, on which it depends whether it will be heated or cooled.
If the arrival of radiant energy is greater than its consumption, then the radiation balance is positive and the surface heats up. If the income is less than the flow rate, then the balance is negative and the surface is cooled. The radiation balance of the earth's surface is one of the main climate-forming factors. It depends on the height of the Sun, the duration of the sunshine, the nature and condition of the earth's surface, the turbidity of the atmosphere, the content of water vapor in it, the presence of clouds, etc.
Instruments for measuring solar radiation
Thermoelectric actinometer М-3(Fig. 3) is designed to measure the intensity of direct solar radiation on a surface perpendicular to the sun's rays.
The actinometer receiver is a thermopile of alternating manganin and constantan plates, made in the form of an asterisk. The internal junctions of the thermopile are glued to the disk made of silver foil through an insulating gasket, the side of the disk facing the sun is blackened. External joints are glued to a massive copper ring through an insulating gasket. It is protected from heating by radiation with a chrome cap. The thermopile is located at the bottom of a metal tube, which is directed towards the sun during measurements. The inner surface of the tube is blackened, and 7 diaphragms (ring-shaped constrictions) are arranged in the tube to prevent scattered radiation from entering the actinometer receiver.
For observations, the arrow on the base of the device 11 (Fig. 2) is oriented to the north, and to facilitate tracking the sun, an actinometer is installed according to the latitude of the observation site (along the sector 9 and the risk at the top of the appliance rack 10 ). Aiming at the sun is done with a screw 3 and handles 6 located at the top of the appliance. The screw allows the tube to be rotated in a vertical plane; when the handle is rotated, the tube is guided behind the sun. A small hole is made in the outer diaphragm for precise aiming at the Sun. There is a white screen opposite this hole at the bottom of the instrument. 5 ... With the correct installation of the device, the sunbeam penetrating through this hole should give a bright spot (spot) in the center of the screen.
Rice. 3 Thermoelectric actinometer M-3: 1 - cover; 2, 3 - screws; 4 - axis; 5 - screen; 6 - handle; 7 - tube; 8 - axis; 9 - latitude sector; 10 - rack; 11 - base.
Universal pyranometer M-80M(Fig. 4) is designed to measure the total (Q) and scattered (D) radiation. Knowing them, it is possible to calculate the intensity of direct solar radiation on the horizontal surface S ′. The M-80M pyranometer has a device for overturning the instrument stand with the receiver downward, which allows you to measure the intensity of the reflected radiation and determine the albedo of the underlying surface.
Pyranometer receiver 1 is a thermoelectric battery, arranged in the shape of a square. Its receiving surface is painted in black and white in the form of a checkerboard. Half of the thermopile junctions are under the white cells, the other half under the black cells. The top of the receiver is covered with a hemispherical glass to protect it from wind and precipitation. To measure the intensity of scattered radiation, the receiver is shaded by a special screen 3 ... During measurements, the receiver of the device is installed strictly horizontally; for this, the pyranometer is equipped with a circular level 7 and set screws 4. At the bottom of the receiver there is a glass dryer filled with a water-absorbing substance, which prevents moisture condensation on the receiver and the glass. When inoperative, the pyranometer receiver is closed with a metal cap.
Rice. 4 Universal pyranometer M-80M: 1 - pyranometer head; 2 - locking spring; 3 - shade hinge; 4 - set screw; 5 - base; 6 - hinge of the folding tripod; 7 - level; 8 - screw; 9 - rack with a dehumidifier inside; 10 - thermopile receiving surface.
Traveling albedometer(Fig. 5) is designed to measure the intensities of the total, scattered and reflective radiation in the field. The receiver is the pyranometer head 1 mounted on a self-balancing gimbal 3 ... This suspension allows you to install the device in two positions - with the receiver up and down, and the horizontal position of the receivers is provided automatically. With the position of the receiving surface of the device upward, the total radiation Q is determined. Then, to measure the reflected radiation R, the handle of the albedometer is turned by 180 0. Knowing these values, you can determine the albedo.
Thermoelectric balance meter M-10M(Fig. 6) is designed to measure the total radiation balance of the underlying surface. The receiver of the balancer is a thermopile square shape consisting of many copper bars 5 wrapped in constantan tape 10 ... Half of each screw of the tape is electroplated silver plated, the beginning and end of the silver layer 9 are thermal junctions. Half of the junctions are glued to the upper, the other half to the lower receiving surfaces, which are used as copper plates 2 painted black. The balance meter receiver is housed in a round metal frame 1 ... When measuring, it is located strictly horizontally using a special overlay level. For this, the balance meter receiver is mounted on a ball joint. 15 ... To increase the measurement accuracy, the balance meter receiver can be shielded from direct solar radiation by a round shield 12 ... The intensity of direct solar radiation is measured in this case with an actinometer or pyranometer.
Rice. 5 Traveling albedometer: 1 - pyranometer head; 2 - tube; 3 - gimbal; 4 - handle
Rice. 6 Thermoelectric balance meter M-10M: a) - schematic cross-section: b) - separate thermopile; c) - appearance; 1 - receiver frame; 2 - receiving plate; 3, 4 - joints; 5 - copper bar; 6, 7 - insulation; 8 - thermopile; 9 - silver layer; 10 - constantan tape; 11 - handle; 12 - shadow screen; 13, 15 - hinges; 14 - bar; 16 - screw; 17 - cover
Instruments for measuring the duration of the solar
shine and illumination
The duration of sunshine is the time during which direct solar radiation is equal to or greater than 0.1 kW / m 2. Expressed in hours per day.
The method for determining the duration of sunshine is based on recording the time during which the intensity of direct solar radiation is sufficient to obtain a burn-through on a special tape, fixed in the optical focus of a ball glass lens, and is not less than 0.1 kW / m 2.
The duration of sunshine is measured by a heliograph instrument (Fig. 7).
Universal heliograph model GU-1(fig. 7). The base of the device is a flat metal plate with two posts 1 ... Between the posts on the horizontal axis 2 reinforced the movable part of the device, consisting of a column 3 with limb 4 and bottom stop 7 , staples 6 with a cup 5 and the upper stop 15 and a glass ball 8 which is a spherical lens. A sector is fixed at one end of the horizontal axis 9 with a scale of latitudes. When moving the horizontal axis 2 the instrument from west to east and turning the upper part of the instrument around it, the column axis 3 is installed parallel to the axis of rotation of the Earth (axis of the world). A screw is used to secure the set angle of inclination of the column axis 11 .
Top part the instrument can be rotated around the column axis 3 and fixed in four specific positions. For this, a special pin is used. 12 , which is inserted through the hole of the dial 4 into one of the four holes of the disc 13 fixed on the axis 2 ... The alignment of the holes in the limb 4 and disk 13 determined by the coincidence of marks A, B, C and D on the dial 4 with index 14 on disk.
Rice. 7 Heliograph universal model GU – 1.
1 - rack; 2 - horizontal axis; 3 - column; 4 - limb; 5 - cup; 6 - bracket; 7 - emphasis; 8 - glass ball; 9 - sector; 10 - latitude indicator; 11 - screw for fixing the angle of inclination of the axis; 12 - pin; 13 - disk; 14 - index on the disk; 15 - top stop.
At the meteorological site, the heliograph is installed on a concrete or wooden pillar 2 m high, on the upper part of which there is a platform made of boards with a thickness of at least 50 mm, so that at any position of the Sun relative to the sides of the horizon, separate buildings, trees and random objects do not obscure it. It is installed strictly horizontally and oriented along the geographic meridian and latitude of the meteorological station; the axis of the heliograph must be strictly parallel to the axis of the world.
The heliograph ball must be kept clean, since the presence of dust, traces of precipitation, dew, frost, frost and ice on the ball weakens and distorts the burn-through on the heliograph tape.
Depending on the possible duration of sunshine, the recording for one day should be made on one, two or three tapes. Depending on the season, straight or curved bands should be used and placed in the top, middle or bottom slots of the cup. Bookmark ribbons should be matched in the same color throughout the month.
For the convenience of working with the heliograph, a ladder with a platform is installed to the south of the support (pillar) with the device. The ladder should not touch the post and should be comfortable enough.
Light meter U-16(Fig. 8) is used to measure the illumination created by light or artificial light sources.
Rice. 8 Luxmeter U – 16. 1 - photocell; 2 - wire; 3 - meter; 4 - absorber; 5 - terminals; 6 - switch of measurement limits; 7 - proofreader.
The device consists of a selenium photocell 1 connected by a wire 2 with meter 3 , and absorber 4 ... The photocell is enclosed in a plastic case with a metal frame; to increase the measurement range by 100 times, an absorber made of milk glass is put on the case. The light meter is a magnetoelectric dial gauge mounted in a plastic case with a scale window. In the lower part of the body there is a corrector 7 for setting the arrow to zero, in the upper part - terminals 5 for connecting the wires from the photocell and the knob for switching the measurement limits 6 .
The scale of the meter is divided into 50 divisions and has 3 rows of numbers corresponding to the three measurement limits - up to 25, 100 and 500 lux (lx). When using an absorber, the limits are increased to 2500, 10000 and 50,000 lux.
When working with a light meter, it is necessary to carefully monitor the cleanliness of the photocell and absorber; if they get dirty, wipe them with a cotton swab dipped in alcohol.
The photocell is placed horizontally during measurements. The corrector set the meter arrow to zero division. Connect the photocell to the meter and take measurements after 4-5 s. To reduce overloads, they start with a larger measurement limit, then move to smaller limits until the arrow is in the working part of the scale. The reading is taken in scale divisions. In case of small deviations of the arrow, to improve the measurement accuracy, it is recommended to switch the meter to a lower limit. To prevent fatigue of the selenium photocell, shade the photocell for 3-5 minutes every 5-10 minutes of device operation.
Illumination is determined by multiplying the reading by the scale division value and by the correction factor (for natural light it is 0.8, for incandescent lamps -1). The scale division is equal to the measurement limit divided by 50. When using one or two absorbers, the resulting value is multiplied by 100 or 10000, respectively.
1 Get acquainted with the device of thermoelectric devices (actinometer, pyranometer, albedometer, balance meter).
2 To get acquainted with the device of the universal heliograph, with the methods of its installation at different times of the year.
3 Get acquainted with the device of the light meter, measure the natural and artificial illumination in the audience.
Place the entries in a notebook.
The Earth receives from the Sun 1.36 * 10-24 calories of heat per year. In comparison with this amount of energy, the rest of the arrival of radiant energy to the surface of the Earth is negligible. Thus, the radiant energy of stars is one hundred millionth of the solar energy, cosmic radiation is two billionths of a fraction, the internal heat of the Earth at its surface is equal to one five-thousandth of the solar heat.
Radiation from the Sun - solar radiation- is the main source of energy for almost all processes occurring in the atmosphere, hydrosphere and in the upper layers of the lithosphere.
The unit of measurement of the intensity of solar radiation is the number of calories of heat absorbed by 1 cm2 of an absolutely black surface perpendicular to the direction of the sun's rays in 1 minute (cal / cm2 * min).
The flow of radiant energy from the Sun reaching the earth's atmosphere is very constant. Its intensity is called the solar constant (Io) and is taken on average equal to 1.88 kcal / cm2 min.
The value of the solar constant fluctuates depending on the distance of the Earth from the Sun and on solar activity. Its fluctuations during the year are 3.4-3.5%.
If the sun's rays fell everywhere on the earth's surface vertically, then in the absence of atmosphere and with a solar constant of 1.88 cal / cm2 * min, each square centimeter would receive 1000 kcal per year. Due to the fact that the Earth is spherical, this number is reduced by 4 times, and 1 sq. cm receives an average of 250 kcal per year.
The amount of solar radiation received by a surface depends on the angle of incidence of the rays.
The maximum amount of radiation is received by the surface perpendicular to the direction of the sun's rays, because in this case all the energy is distributed over an area with a cross section equal to the cross section of the beam of rays - a. With an oblique incidence of the same beam of rays, the energy is distributed over a large area (section c) and a unit of surface receives a smaller amount of it. The smaller the angle of incidence of the rays, the lower the intensity of solar radiation.
The dependence of the intensity of solar radiation on the angle of incidence of the rays is expressed by the formula:
I1 = I0 * sin h,
where I0 is the intensity of solar radiation with a sheer incidence of rays. Outside the atmosphere - the solar constant;
I1 is the intensity of solar radiation when the sun's rays fall at an angle h.
I1 is as many times less than I0 as the section a is less than the section b.
Figure 27 shows that a / b = sin A.
The angle of incidence of the sun's rays (the height of the Sun) is 90 ° only at latitudes from 23 ° 27 "N to 23 ° 27" S. (i.e. between the tropics). At other latitudes, it is always less than 90 ° (Table 8). Accordingly to a decrease in the angle of incidence of the rays, the intensity of solar radiation entering the surface should also decrease. different latitudes... Since the height of the Sun does not remain constant throughout the year and during the day, the amount of solar heat received by the surface is constantly changing.
The amount of solar radiation received by the surface is in direct proportion from the duration of its illumination by the sun's rays.
In the equatorial zone outside the atmosphere, the amount of solar heat during the year does not experience large fluctuations, while at high latitudes these fluctuations are very large (see Table 9). In winter, the differences in the arrival of solar heat between high and low latitudes are especially significant. In the summer, under conditions of continuous illumination, the polar regions receive the maximum amount of solar heat per day on Earth. On the day of the summer solstice in the northern hemisphere, it is 36% higher than the daily sum of heat at the equator. But since the length of the day at the equator is not 24 hours (as at this time at the pole), but 12 hours, the amount of solar radiation per unit time at the equator remains the greatest. The summer maximum of the daily total solar heat, observed at about 40-50 ° latitude, is associated with a relatively long day (greater than at this time by 10-20 ° latitude) at a significant height of the Sun. The differences in the amount of heat received by the equatorial and polar regions are smaller in summer than in winter.
The southern hemisphere receives more heat in summer than the northern hemisphere, and vice versa in winter (the change in the distance of the Earth from the Sun affects). And if the surface of both hemispheres were completely uniform, the annual amplitudes of temperature fluctuations in the southern hemisphere would be greater than in the northern.
Solar radiation in the atmosphere undergoes quantitative and qualitative changes.
Even perfect, dry and clean, the atmosphere absorbs and scatters rays, reducing the intensity of solar radiation. The weakening effect of a real atmosphere containing water vapor and particulate matter on solar radiation is much greater than the ideal one. The atmosphere (oxygen, ozone, carbon dioxide, dust and water vapor) absorbs mainly ultraviolet and infrared rays. The radiant energy of the Sun absorbed by the atmosphere is converted into other types of energy: thermal, chemical, etc. In general, absorption weakens solar radiation by 17-25%.
Rays with relatively short waves - violet, blue - are scattered by molecules of gases in the atmosphere. This explains the blue color of the sky. The impurities scatter equally beams with waves of different lengths. Therefore, with their significant content, the sky acquires a whitish tint.
Due to the scattering and reflection of sunlight by the atmosphere, daylight is observed on cloudy days, objects in the shade are visible, and the phenomenon of twilight occurs.
The longer the path of the ray in the atmosphere, the greater its thickness it must pass and the more significantly the solar radiation is attenuated. Therefore, with the rise, the influence of the atmosphere on radiation decreases. The path length of the sun's rays in the atmosphere depends on the height of the sun. If we take the length of the path of the sun ray in the atmosphere as a unit at a height of the Sun of 90 ° (m), the ratio between the height of the Sun and the length of the path of the ray in the atmosphere will be as shown in Table. ten.
The general attenuation of radiation in the atmosphere at any height of the Sun can be expressed by the Bouguer formula: Im = I0 * pm, where Im is the intensity of solar radiation at the earth's surface changed in the atmosphere; I0 - solar constant; m is the path of the beam in the atmosphere; at a height of the Sun of 90 °, it is equal to 1 (the mass of the atmosphere), p is the transparency coefficient (a fractional number showing what fraction of radiation reaches the surface at m = 1).
At a height of the Sun of 90 °, at m = 1, the intensity of solar radiation at the earth's surface I1 is p times less than Io, that is, I1 = Io * p.
If the height of the Sun is less than 90 °, then m is always greater than 1. The path of the sunbeam can consist of several segments, each of which is equal to 1. The intensity of solar radiation at the border between the first (aa1) and second (a1a2) segments I1 is, obviously, Io * p, radiation intensity after passing the second segment I2 = I1 * p = I0 p * p = I0 p2; I3 = I0p3 etc.
The transparency of the atmosphere is inconsistent and uneven in different conditions... The ratio of the transparency of the real atmosphere to the transparency of the ideal atmosphere - the turbidity factor - is always greater than one. It depends on the content of water vapor and dust in the air. With increasing latitude, the turbidity factor decreases: at latitudes from 0 to 20 ° N. NS. it is equal on average to 4.6, at latitudes from 40 to 50 ° N. NS. - 3.5, at latitudes from 50 to 60 ° N. NS. - 2.8 and at latitudes from 60 to 80 ° N. NS. - 2.0. In temperate latitudes, the turbidity factor is less in winter than in summer, and less in the morning than in the afternoon. It decreases with height. The greater the turbidity factor, the greater the attenuation of solar radiation.
Distinguish solar radiation direct, scattered and total.
Some of the solar radiation that penetrates through the atmosphere to the earth's surface is direct radiation. Some of the radiation scattered by the atmosphere is converted to scattered radiation. All solar radiation entering the earth's surface, direct and scattered, is called total radiation.
The ratio between direct and scattered radiation varies considerably depending on cloudiness, dustiness of the atmosphere, and also on the height of the Sun. In a clear sky, the fraction of scattered radiation does not exceed 0.1%; in a cloudy sky, scattered radiation can be greater than direct.
At a low solar altitude, the total radiation is almost entirely scattered. At a height of the Sun of 50 ° and a clear sky, the fraction of scattered radiation does not exceed 10-20%.
Maps of the average annual and monthly values of the total radiation allow us to notice the main regularities in its geographical distribution... Annual values of total radiation are distributed mainly zonal. The largest annual amount of total radiation on Earth is received by the surface in tropical inland deserts (Eastern Sahara and central Arabia). A noticeable decrease in the total radiation at the equator is caused by high air humidity and large clouds. In the Arctic, the total radiation is 60-70 kcal / cm2 per year; in Antarctica, owing to the frequent recurrence of clear days and the greater transparency of the atmosphere, it is somewhat higher.
In June, the northern hemisphere receives the largest amounts of radiation, and especially the inland tropical and subtropical regions. The amounts of solar radiation received by the surface in the temperate and polar latitudes of the northern hemisphere differ little due mainly to the long duration of the day in the polar regions. Zoning in the distribution of total radiation over. continents in the northern hemisphere and in the tropical latitudes of the southern hemisphere is almost not expressed. It manifests itself better in the northern hemisphere over the Ocean and is clearly expressed in the extratropical latitudes of the southern hemisphere. At the southern polar circle, the total solar radiation is approaching 0.
In December, the largest amounts of radiation enter the southern hemisphere. The high-lying ice surface of Antarctica with high air transparency receives significantly more total radiation than the surface of the Arctic in June. There is a lot of heat in the deserts (Kalahari, Great Australian), but due to the greater oceanicity of the southern hemisphere (the influence of high air humidity and cloudiness), its sum is somewhat less here than in June at the same latitudes of the northern hemisphere. In the equatorial and tropical latitudes of the northern hemisphere, the total radiation changes relatively little, and the zoning in its distribution is clearly expressed only to the north of the northern tropic. With increasing latitude, the total radiation decreases rather quickly, its zero isoline extends somewhat north of the Arctic Circle.
The total solar radiation, falling on the surface of the Earth, is partially reflected back into the atmosphere. The ratio of the amount of radiation reflected from a surface to the amount of radiation falling on this surface is called albedo... Albedo characterizes the reflectivity of a surface.
The albedo of the earth's surface depends on its condition and properties: color, moisture, roughness, etc. Freshly fallen snow has the highest reflectivity (85-95%). A calm water surface reflects only 2-5% when the sun's rays fall steeply on it, and when the sun is low, almost all the rays falling on it (90%). Albedo of dry chernozem - 14%, wet - 8, forest - 10-20, meadow vegetation - 18-30, sandy desert surface - 29-35, sea ice surface - 30-40%.
The large albedo of the ice surface, especially that covered with freshly fallen snow (up to 95%), is the reason for low temperatures in the polar regions in the summer, when the arrival of solar radiation there is significant.
Radiation of the earth's surface and atmosphere. Any body with a temperature above absolute zero (more than minus 273 °) emits radiant energy. The total emissivity of an absolutely black body is proportional to the fourth power of its absolute temperature (T):
E = σ * T4 kcal / cm2 per minute (Stefan - Boltzmann law), where σ is a constant coefficient.
The higher the temperature of the emitting body, the shorter the wavelengths of the emitted nm rays. The incandescent sun sends into space shortwave radiation... The earth's surface, absorbing short-wave solar radiation, heats up and also becomes a source of radiation (terrestrial radiation). Ho since the temperature of the earth's surface does not exceed several tens of degrees, its long-wavelength radiation, invisible.
Earth's radiation is largely trapped by the atmosphere (water vapor, carbon dioxide, ozone), but rays with a wavelength of 9-12 microns freely leave the atmosphere, and therefore the Earth loses some of its heat.
The atmosphere, absorbing part of the solar radiation passing through it and more than half of the earth's radiation, itself radiates energy both into world space and to the earth's surface. Atmospheric radiation directed towards the earth's surface towards the earth is called counter radiation. This radiation, like terrestrial, long-wave, invisible.
In the atmosphere, there are two fluxes of long-wave radiation - radiation from the Earth's surface and radiation from the atmosphere. The difference between them, which determines the actual heat loss by the earth's surface, is called effective radiation. The higher the temperature of the emitting surface, the greater the effective radiation. Air humidity reduces effective radiation, and clouds greatly reduce it.
The highest value of the annual sums of effective radiation is observed in tropical deserts - 80 kcal / cm2 per year - due to high temperature surface, dry air and clear sky. At the equator, with high air humidity, effective radiation is only about 30 kcal / cm2 per year, and its value for land and for the Ocean is very little different. Least effective radiation in polar regions. In temperate latitudes, the earth's surface loses about half of the amount of heat that it receives from the absorption of total radiation.
The ability of the atmosphere to transmit short-wavelength radiation from the Sun (direct and scattered radiation) and to block long-wavelength radiation from the Earth is called the greenhouse (greenhouse) effect. Due to the greenhouse effect, the average temperature of the earth's surface is + 16 °, in the absence of the atmosphere it would be -22 ° (38 ° lower).
Radiation balance (residual radiation). The earth's surface simultaneously receives radiation and gives it away. The arrival of radiation is made up of the total solar radiation and the counter radiation of the atmosphere. Consumption is the reflection of the sun's rays from the surface (albedo) and the intrinsic radiation of the earth's surface. The difference between the arrival and consumption of radiation - radiation balance, or residual radiation. The value of the radiation balance is determined by the equation
R = Q * (1-α) - I,
where Q is the total solar radiation per unit surface; α - albedo (fraction); I - effective radiation.
If the input is greater than the flow rate, the radiation balance is positive; if the input is less than the flow rate, the balance is negative. At night, at all latitudes, the radiation balance is negative, in the afternoon until noon - positive everywhere, except for high latitudes in winter; afternoon - negative again. On average, the radiation balance per day can be both positive and negative (Table 11).
On the map of the annual sums of the radiation balance of the earth's surface, one can see abrupt change the positions of the isolines during their transition from land to the Ocean. As a rule, the radiation balance of the Ocean's surface exceeds the radiation balance of the land (the influence of albedo and effective radiation). The distribution of the radiation balance is generally zonal. On the Ocean in tropical latitudes, the annual values of the radiation balance reach 140 kcal / cm2 (Arabian Sea) and do not exceed 30 kcal / cm2 at the border of floating ice. Deviations from the zonal distribution of the radiation balance on the Ocean are insignificant and are caused by the distribution of cloudiness.
On land in equatorial and tropical latitudes, the annual values of the radiation balance vary from 60 to 90 kcal / cm2, depending on the moisture conditions. The greatest annual amounts radiation balance are noted in those regions where albedo and effective radiation are relatively low (humid rainforests, savannah). Their lowest value is found in very humid (large cloudiness) and in very dry (high effective radiation) regions. In temperate and high latitudes, the annual value of the radiation balance decreases with increasing latitude (the effect of a decrease in total radiation).
The annual sums of the radiation balance over the central regions of Antarctica are negative (several calories per 1 cm2). In the Arctic, these values are close to zero.
In July, the radiation balance of the earth's surface in a significant part of the southern hemisphere is negative. Line zero balance runs between 40 and 50 ° S. NS. The highest value of the radiation balance is reached on the surface of the Ocean in the tropical latitudes of the northern hemisphere and on the surface of some inland seas, for example, the Black Sea (14-16 kcal / cm2 per month).
In January, the zero balance line is located between 40 and 50 ° N. NS. (over the oceans, it rises somewhat to the north, over the continents, it descends to the south). A significant part of the northern hemisphere has a negative radiation balance. The largest values of the radiation balance are confined to the tropical latitudes of the southern hemisphere.
On average, the radiation balance of the earth's surface is positive per year. In this case, the surface temperature does not increase, but remains approximately constant, which can only be explained by the continuous consumption of excess heat.
The radiation balance of the atmosphere is made up of absorbed solar and terrestrial radiation, on the one hand, and atmospheric radiation, on the other. It is always negative, since the atmosphere absorbs only a small part of solar radiation, and radiates almost as much as the surface.
The radiation balance of the surface and atmosphere together, as a whole, for the entire Earth for a year is equal to zero on average, but at latitudes it can be both positive and negative.
The consequence of such a distribution of the radiation balance should be the transfer of heat in the direction from the equator to the poles.
Heat balance. The radiation balance is the most important component of the heat balance. The surface heat balance equation shows how the incoming solar radiation energy is converted on the earth's surface:
where R is the radiation balance; LE - heat consumption for evaporation (L - latent heat of vaporization, E - evaporation);
P - turbulent heat exchange between the surface and the atmosphere;
A - heat exchange between the surface and the underlying layers of soil or water.
The radiation balance of a surface is considered positive if the radiation absorbed by the surface exceeds the heat losses, and negative if it does not compensate for them. All other terms of the heat balance are considered positive if due to them there is a loss of heat by the surface (if they correspond to the heat consumption). Because. all the terms of the equation can change, the heat balance is constantly disturbed and restored again.
The above equation of the heat balance of the surface is approximate, since it does not take into account some minor ones, but in specific conditions factors that are gaining in importance, for example, the release of heat during freezing, its consumption for melting, etc.
The heat balance of the atmosphere consists of the radiation balance of the atmosphere Ra, heat coming from the surface, Pa, heat released in the atmosphere during condensation, LE, and horizontal heat transfer (advection) Aa. The radiation balance of the atmosphere is always negative. Heat inflow as a result of moisture condensation and turbulent heat exchange values are positive. Heat advection leads, on average, per year to its transfer from low latitudes to high latitudes: thus, it means heat consumption in low latitudes and arrival in high latitudes. In a long-term derivation, the heat balance of the atmosphere can be expressed by the equation Ra = Pa + LE.
The heat balance of the surface and the atmosphere together, as a whole, in the long-term average is equal to 0 (Fig. 35).
The value of solar radiation entering the atmosphere per year (250 kcal / cm2) is taken as 100%. Solar radiation, penetrating into the atmosphere, is partially reflected from the clouds and goes back out of the atmosphere - 38%, partially absorbed by the atmosphere - 14% and partially in the form of direct solar radiation reaches the earth's surface - 48%. Of the 48% that have reached the surface, 44% are absorbed by it, and 4% are reflected. Thus, the albedo of the Earth is 42% (38 + 4).
Radiation absorbed by the earth's surface is consumed as follows: 20% is lost through effective radiation, 18% is spent on evaporation from the surface, 6% is spent on heating the air during turbulent heat exchange (total 24%). The heat consumption by the surface balances its arrival. The heat received by the atmosphere (14% directly from the Sun, 24% from the earth's surface), together with the effective radiation of the Earth, is directed into space. The Earth's albedo (42%) and radiation (58%) balance the supply of solar radiation to the atmosphere.
The amount of direct solar radiation (S) arriving at the earth's surface in a cloudless sky depends on the height of the sun and transparency. The table for three latitudinal zones shows the distribution of monthly sums of direct radiation in a cloudless sky (possible sums) in the form of averaged values for the central months of the seasons and the year.
The increased arrival of direct radiation in the Asian part is due to the higher transparency of the atmosphere in this region. High values of direct radiation in summer in the northern regions of Russia are explained by a combination of high transparency of the atmosphere and long day length
Reduces the arrival of direct radiation and can significantly change its daily and annual course. However, under average cloud conditions, the astronomical factor is predominant and, therefore, the maximum direct radiation is observed at the highest sun altitude.
In most of the continental regions of Russia in the spring-summer months, direct radiation in the pre-noon hours is greater than in the afternoon. This is associated with the development of convective cloudiness in the afternoon hours and with a decrease in the transparency of the atmosphere at this time of the day as compared to the morning hours. In winter, the ratio of pre- and afternoon values of radiation is the opposite - the pre-noon values of direct radiation are lower due to the morning maximum cloud cover and its decrease in the second half of the day. The difference between the pre- and afternoon values of direct radiation can reach 25–35%.
In the annual course, the maximum of direct radiation falls on June-July, with the exception of the regions of the Far East, where it shifts to May, and in the south of Primorye in September, a secondary maximum is noted.
The maximum monthly amount of direct radiation on the territory of Russia is 45–65% of that possible with a cloudless sky, and even in the south of the European part it reaches only 70%. Minimum values celebrated in December and January.
The contribution of direct radiation to the total arrival under actual cloudiness conditions reaches its maximum in the summer months and averages 50–60%. An exception is the Primorsky Territory, where the largest contribution of direct radiation falls on the autumn and winter months.
The distribution of direct radiation under average (actual) cloudiness conditions over the territory of Russia largely depends on. This leads to a noticeable violation of the zonal distribution of radiation in certain months. This is especially evident in spring... So, in April, there are two maximums - one in the southern regions
If the atmosphere let all the sun's rays pass to the surface of the earth, then the climate of any point on the Earth would depend only on the geographical latitude. So it was believed in antiquity. However, when the sun's rays pass through the earth's atmosphere, as we have already seen, their weakening occurs due to the simultaneous processes of absorption and scattering. Water droplets and ice crystals, which make up clouds, absorb and scatter a lot.
That part of solar radiation that enters the earth's surface after scattering it by the atmosphere and clouds is called scattered radiation. That part of solar radiation that passes through the atmosphere without scattering is calleddirect radiation.
Radiation is scattered not only by clouds, but also in a clear sky - by molecules, gases and dust particles. The ratio between direct and scattered radiation varies widely. If, with a clear sky and vertical incidence of sunlight, the fraction of scattered radiation is 0.1% direct, then
in a cloudy sky, scattered radiation may be more direct.
In parts of the world where clear weather prevails, such as Central Asia, the main source of heating of the earth's surface is direct solar radiation. Where cloudy weather predominates, as, for example, in the north and northwest of the European territory of the USSR, diffuse solar radiation becomes essential. Tikhaya Bay, located in the north, receives scattered radiation almost one and a half times more than the straight one (Table 5). In Tashkent, on the contrary, scattered radiation is less than 1/3 of direct radiation. Direct solar radiation in Yakutsk is greater than in Leningrad. This is explained by the fact that in Leningrad there are more cloudy days and less transparency of the air.
Albedo of the earth's surface. The earth's surface has the ability to reflect rays falling on it. The amount of absorbed and reflected radiation depends on the properties of the earth's surface. The ratio of the amount of radiant energy reflected from the surface of the body to the amount of incident radiant energy is called albedo. Albedo characterizes the reflectivity of a body surface. When, for example, they say that the albedo of freshly fallen snow is 80-85%, this means that 80-85% of all radiation falling on the snow surface is reflected from it.
The albedo of snow and ice depends on their purity. V industrial cities due to the deposition of various impurities on the snow, mainly soot, the albedo is lower. On the contrary, in the arctic regions the snow albedo sometimes reaches 94%. Since the albedo of snow is the highest in comparison with the albedo of other types of the earth's surface, then with a snow cover, the heating of the earth's surface occurs weakly. The albedo of grass and sand is much less. The albedo of grass vegetation is 26%, and that of sand is 30%. This means that the grass absorbs 74% of the sun's energy and the sand 70%. The absorbed radiation is used for evaporation, plant growth and heating.
Water has the greatest absorption capacity. Seas and oceans absorb about 95% of the incoming solar energy on their surface, ie, the albedo of water is 5% (Fig. 9). True, the albedo of water depends on the angle of incidence of the sun's rays (V.V. Shuleikin). With a vertical incidence of rays from the surface pure water only 2% of the radiation is reflected, and when the sun is low, almost all.
