Solar radiation and heat balance. Thermal balance of the earth's surface and atmosphere The concept of the Earth's thermobaric field
The atmosphere, like the earth's surface, receives almost all of its heat from the Sun. Other sources of heating include heat coming from the bowels of the Earth, but it is only a fraction of a percent of the total amount of heat.
Although solar radiation is the only source of heat for the earth's surface, the thermal regime of the geographic envelope is not only a consequence of the radiation balance. Solar heat is converted and redistributed under the influence of terrestrial factors, and primarily transformed by air and ocean currents. They, in turn, are due to the uneven distribution of solar radiation over latitudes. This is one of the clearest examples of the close global connection and interaction of various components in nature.
For the living nature of the Earth, the redistribution of heat between different latitudes, as well as between oceans and continents, is important. Thanks to this process, a very complex spatial redistribution of heat occurs on the Earth's surface in accordance with the superior directions of movement of air and ocean currents. However, the total heat transfer is directed, as a rule, from low latitudes to high latitudes and from oceans to continents.
The distribution of heat in the atmosphere occurs by convection, heat conduction and radiation. Thermal convection manifests itself everywhere on the planet, winds, ascending and descending air currents are ubiquitous. Convection is especially pronounced in the tropics.
Thermal conductivity, that is, the transfer of heat during direct contact of the atmosphere with a warm or cold surface of the earth, is of relatively little importance, since air is a poor conductor of heat. It is this property that has found wide application in the manufacture of window frames with double glazing.
The inflows and outflows of heat in the lower atmosphere are not the same at different latitudes. North of 38°N sh. more heat is emitted than absorbed. This loss is compensated by warm oceanic and air currents directed to temperate latitudes.
The process of receipt and expenditure of solar energy, heating and cooling of the entire system of the Earth's atmosphere is characterized by a heat balance. If we take the annual input of solar energy to the upper boundary of the atmosphere as 100%, then the balance of solar energy will look like this: 42% is reflected from the Earth and returned back to outer space (this value characterizes the Earth's albedo), with 38% reflected by the atmosphere and 4% - the surface of the earth. The rest (58%) is absorbed: 14% - by the atmosphere and 44% - by the earth's surface. The heated surface of the Earth gives back all the energy absorbed by it. At the same time, the radiation of energy by the earth's surface is 20%, 24% is spent on heating the air and evaporating moisture (5.6% for heating the air and 18.4% for evaporating moisture).
Such general characteristics of the heat balance of the globe as a whole. In fact, for different latitudinal belts for different surfaces, the heat balance will be far from being the same. Thus, the heat balance of any territory is disturbed at sunrise and sunset, with the change of seasons, depending on atmospheric conditions (cloudiness, air humidity and dust content in it), the nature of the surface (water or land, forest or onion, snow cover or bare ground). ), altitude above sea level. Most heat is radiated at night, in winter, and through rarefied, clean, dry air at high altitudes. But in the end, the losses due to radiation are compensated by the heat coming from the Sun, and the state of dynamic equilibrium prevails on the Earth as a whole, otherwise it would warm up or, conversely, cool down.
Air temperature
The heating of the atmosphere occurs in a rather complicated way. Short wavelengths of sunlight ranging from visible red to ultraviolet light are converted at the Earth's surface into longer heat waves, which later, when emitted from the Earth's surface, heat the atmosphere. The lower layers of the atmosphere warm up faster than the upper ones, which is explained by the indicated thermal radiation of the earth's surface and the fact that they have a high density and are saturated with water vapor.
A characteristic feature of the vertical distribution of temperature in the troposphere is its decrease with height. The average vertical temperature gradient, that is, the average decrease calculated per 100 m of altitude, is 0.6 ° C. Cooling of moist air is accompanied by moisture condensation. In this case, a certain amount of heat is released, which was spent on the formation of steam. Therefore, when moist air rises, it cools almost twice as slowly as dry air. The geothermal coefficient of dry air in the troposphere is 1 °C on average.
The air that rises from the heated land surface and water bodies enters a zone of low pressure. This allows it to expand, and in connection with this, a certain amount of thermal energy is converted into kinetic energy. As a result of this process, the air is cooled. If at the same time it does not receive heat from anywhere and does not give it anywhere, then the entire described process is called adiabatic, or dynamic cooling. And vice versa, the air descends, enters the zone of high pressure, it is condensed by the air that surrounds it, and the mechanical energy is converted into thermal energy. Because of this, the air experiences adiabatic heating, which averages 1 °C for every 100 m of subsidence.
Sometimes the temperature rises with altitude. This phenomenon is called inversion. The causes of u "manifestations are varied: radiation from the Earth over ice sheets, the passage of strong currents of warm air over a cold surface. Inversions are especially characteristic of mountainous regions: heavy cold air flows into mountain hollows and stagnates there, displacing lighter warm air upwards.
Daily and annual changes in air temperature reflect the thermal state of the surface. In the surface layer of air, the daily maximum is set at 2-3 pm, and the minimum is observed after sunrise. The greatest daily amplitude takes place in subtropical latitudes (30 ° C), the smallest - in the polar (5 ° C). The annual course of temperature depends on the latitude, the nature of the underlying surface, the height of the place above the ocean level, the relief, and the distance from the ocean.
Certain geographic regularities have been revealed in the distribution of annual temperatures on the earth's surface.
1. In both hemispheres, average temperatures are decreasing towards the poles. However, the thermal equator - a warm parallel with an average annual temperature of 27°C - is located in the Northern Hemisphere at about 15-20° latitude. This is explained by the fact that land occupies a larger area here than at the geographic equator.
2. From the equator to the north and south, temperatures change unevenly. Between the equator and the 25th parallel, the decrease in temperature is very slow - less than two degrees for every ten degrees of latitude. Between 25° and 80° latitude in both hemispheres, temperatures drop very rapidly. In some places, this decrease exceeds 10 ° C. Further towards the poles, the rate of temperature decrease decreases again.
3. Average annual temperatures of all parallels of the Southern Hemisphere are less than the temperature of the corresponding parallels of the Northern Hemisphere. The average air temperature of the predominantly "continental" Northern Hemisphere is +8.6 ° С in January, +22.4 ° С in July; in the southern "oceanic" hemisphere, the average temperature in July is +11.3 ° С, in January - +17.5 ° С. The annual amplitude of air temperature fluctuations in the Northern Hemisphere is twice as large due to the peculiarities of the distribution of land and sea at the corresponding latitudes and the cooling effect of the grandiose ice dome Antarctica on the climate of the Southern Hemisphere.
Isotherm maps provide important characteristics of the distribution of air temperatures on Earth. Thus, based on the analysis of the distribution of the July isotherms on the earth's surface, the following main conclusions can be formulated.
1. In the extratropical regions of both hemispheres, the isotherms over the continents bend to the north relative to its position on the windows. In the Northern Hemisphere, this is due to the fact that the land is heated more than the sea, and in the South - the opposite ratio: at this time, the land is colder than the sea.
2. Over the oceans, the July isotherms reflect the influence of cold air temperature currents. This is especially noticeable along those western coasts of North America and Africa, which are washed by the cold correspondence of the California and Canary ocean currents. In the Southern Hemisphere, the isotherms are curved in the opposite direction to the north - also under the influence of cold currents.
3. The highest average temperatures in July are observed in the deserts located north of the equator. It is especially hot at this time in California, the Sahara, Arabia, Iran, and the interior of Asia.
The distribution of January isotherms also has its own characteristics.
1. The bends of the isotherms over the oceans to the north and over the land to the south become even more prominent, more contrasting. This is most pronounced in the Northern Hemisphere. The strong bends of the isotherms towards the North Pole reflect an increase in the thermal role of the Gulf Stream ocean currents in the Atlantic Ocean and the Kuro-Sio in the Pacific Ocean.
2. In the extratropical regions of both hemispheres, the isotherms over the continents are noticeably curved to the south. This is due to the fact that in the Northern Hemisphere the land is colder, and in the Southern Hemisphere it is warmer than the sea.
3. The highest average temperatures in January occur in the deserts of the tropical zone of the Southern Hemisphere.
4. The areas of greatest cooling on the planet in January, as in July, are Antarctica and Greenland.
In general, it can be stated that the isotherms of the Southern Hemisphere during all seasons of the year have a more rectilinear (latitudinal) strike pattern. The absence of significant anomalies in the course of isotherms here is explained by the significant predominance of the water surface over land. An analysis of the course of the isotherms indicates a close dependence of temperatures not only on the magnitude of solar radiation, but also on the redistribution of heat by oceanic and air currents.
Radiation balance is the difference between the inflow and outflow of radiant energy absorbed and emitted by the Earth's surface.
Radiation balance - the algebraic sum of radiation fluxes in a certain volume or on a certain surface. Speaking about the radiation balance of the atmosphere or the "Earth-atmosphere" system, most often they mean the radiation balance of the earth's surface, which determines heat transfer at the lower boundary of the atmosphere. It represents the difference between the absorbed total solar radiation and the effective radiation of the earth's surface.
The radiation balance is the difference between the incoming and outgoing radiant energy absorbed and emitted by the Earth's surface.
The radiation balance is the most important climatic factor, since the distribution of temperature in the soil and the air layers adjacent to it largely depends on its value. It determines the physical properties of air masses moving across the Earth, as well as the intensity of evaporation and melting of snow.
The distribution of annual values of the radiation balance on the surface of the globe is not the same: in tropical latitudes, these values reach up to 100 ... 120 kcal/(cm2-year), and the maximum (up to 140 kcal/(cm2-year)) are observed off the northwestern coast of Australia ). In desert and arid regions, the values of the radiation balance are lower compared to areas of sufficient and excessive moisture at the same latitudes. This is caused by an increase in albedo and an increase in effective radiation due to the high dryness of the air and low cloudiness. In temperate latitudes, the values of the radiation balance rapidly decrease with increasing latitude due to a decrease in total radiation.
On average, over the year, the sums of the radiation balance for the entire surface of the globe turn out to be positive, with the exception of areas with a permanent ice cover (Antarctica, the central part of Greenland, etc.).
The energy, measured by the value of the radiation balance, is partly spent on evaporation, partly transferred to the air, and, finally, a certain amount of energy goes into the soil and goes to heat it. Thus, the total heat input-output for the Earth's surface, called the heat balance, can be represented as the following equation:
Here B is the radiation balance, M is the heat flux between the Earth's surface and the atmosphere, V is the heat consumption for evaporation (or heat release during condensation), T is the heat exchange between the soil surface and the deep layers.
Figure 16 - The impact of solar radiation on the Earth's surface
On average, over the year, the soil practically gives off as much heat to the air as it receives, therefore, in the annual conclusions, the heat turnover in the soil is zero. Heat consumption for evaporation is distributed on the surface of the globe very unevenly. On the oceans, they depend on the amount of solar energy reaching the surface of the ocean, as well as on the nature of ocean currents. Warm currents increase the consumption of heat for evaporation, while cold ones reduce it. On the continents, the cost of heat for evaporation is determined not only by the amount of solar radiation, but also by the reserves of moisture contained in the soil. With a lack of moisture, causing a reduction in evaporation, the heat costs for evaporation are reduced. Therefore, in deserts and semi-deserts, they are significantly reduced.
Practically the only source of energy for all physical processes developing in the atmosphere is solar radiation. The main feature of the radiation regime of the atmosphere is the so-called. greenhouse effect: the atmosphere weakly absorbs short-wave solar radiation (most of it reaches the earth's surface), but delays long-wave (entirely infrared) thermal radiation of the earth's surface, which significantly reduces the heat transfer of the Earth into outer space and increases its temperature.
The solar radiation entering the atmosphere is partially absorbed in the atmosphere mainly by water vapor, carbon dioxide, ozone and aerosols and is scattered by aerosol particles and atmospheric density fluctuations. Due to the scattering of the radiant energy of the Sun in the atmosphere, not only direct solar, but also scattered radiation is observed, together they constitute the total radiation. Reaching the earth's surface, the total radiation is partially reflected from it. The amount of reflected radiation is determined by the reflectivity of the underlying surface, the so-called. albedo. Due to the absorbed radiation, the earth's surface heats up and becomes a source of its own long-wave radiation directed towards the atmosphere. In turn, the atmosphere also emits long-wave radiation directed towards the earth's surface (the so-called counter-radiation of the atmosphere) and outer space (the so-called outgoing radiation). Rational heat exchange between the earth's surface and the atmosphere is determined by effective radiation - the difference between the Earth's own surface radiation and the atmosphere's counter-radiation absorbed by it. The difference between the shortwave radiation absorbed by the earth's surface and the effective radiation is called the radiation balance.
Transformations of the energy of solar radiation after its absorption on the earth's surface and in the atmosphere constitute the heat balance of the Earth. The main source of heat for the atmosphere is the earth's surface, which absorbs the bulk of solar radiation. Since the absorption of solar radiation in the atmosphere is less than the loss of heat from the atmosphere to the world space by long-wave radiation, the radiative heat consumption is compensated by the influx of heat to the atmosphere from the earth's surface in the form of turbulent heat transfer and the arrival of heat as a result of condensation of water vapor in the atmosphere. Since the total amount of condensation in the entire atmosphere is equal to the amount of precipitation, as well as the amount of evaporation from the earth's surface, the influx of condensation heat in the atmosphere is numerically equal to the heat spent on evaporation on the Earth's surface.
THERMAL BALANCE OF THE EARTH'S SURFACE
THERMAL BALANCE OF THE EARTH'S SURFACE is the algebraic sum of heat fluxes coming to the earth's surface and leaving it. Expressed by the equation:
where R- radiation balance of the earth's surface; P- turbulent heat flow between the earth's surface and the atmosphere; LE- heat consumption for evaporation; AT- the flow of heat from the earth's surface into the depths of the soil or water or vice versa. The ratio of the balance components changes over time depending on the properties of the underlying surface and the geographical latitude of the place. The nature of the heat balance of the earth's surface and its energy level determine the features and intensity of most exogenous processes. Data on the heat balance of the earth's surface play an important role in the study of climate change, geographic zonation, and the thermal regime of organisms.
Ecological encyclopedic dictionary. - Chisinau: Main edition of the Moldavian Soviet Encyclopedia. I.I. Grandpa. 1989
- THERMAL RADIATION
- THERMAL BALANCE OF THE EARTH-ATMOSPHERE SYSTEM
See what "THE HEAT BALANCE OF THE EARTH'S SURFACE" is in other dictionaries:
thermal balance of the earth's surface- The algebraic sum of heat fluxes coming to the earth's surface and radiated by it ... Geography Dictionary
The heat balance of the Earth, the ratio of the input and output of energy (radiant and thermal) on the earth's surface, in the atmosphere and in the Earth's atmosphere system. The main source of energy for the vast majority of physical, chemical and biological ... ...
THERMAL BALANCE- the earth's surface is the algebraic sum of heat fluxes coming to the earth's surface and leaving it. Expressed by the equation: R + P + LE + B=0, where R is the radiation balance of the earth's surface; P turbulent heat flow between the earth ... ... Ecological dictionary
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Let us first consider the thermal conditions of the earth's surface and the uppermost layers of soil and water bodies. This is necessary because the lower layers of the atmosphere are heated and cooled most of all by radiative and non-radiative heat exchange with the upper layers of soil and water. Therefore, temperature changes in the lower layers of the atmosphere are primarily determined by changes in the temperature of the earth's surface and follow these changes.
The earth's surface, i.e., the surface of soil or water (as well as vegetation, snow, ice cover), continuously receives and loses heat in various ways. Through the earth's surface, heat is transferred upward - into the atmosphere and downward - into the soil or water.
First, the total radiation and the counter radiation of the atmosphere enter the earth's surface. They are absorbed to a greater or lesser extent by the surface, i.e., they go to heat the upper layers of soil and water. At the same time, the earth's surface itself radiates and loses heat in the process.
Secondly, heat comes to the earth's surface from above, from the atmosphere, by conduction. In the same way, heat escapes from the earth's surface into the atmosphere. By conduction, heat also leaves the earth's surface down into the soil and water, or comes to the earth's surface from the depths of the soil and water.
Thirdly, the earth's surface receives heat when water vapor condenses on it from the air or, on the contrary, loses heat when water evaporates from it. In the first case, latent heat is released, in the second case, heat passes into a latent state.
In any period of time, the same amount of heat goes up and down from the earth's surface as it receives from above and below during this time. If it were otherwise, the law of conservation of energy would not be fulfilled: it would be necessary to assume that energy arises or disappears on the earth's surface. However, it is possible that, for example, more heat may go up than came from above; in this case, the excess heat transfer should be covered by the arrival of heat to the surface from the depths of the soil or water.
So, the algebraic sum of all incomes and expenses of heat on the earth's surface should be equal to zero. This is expressed by the equation of the heat balance of the earth's surface.
To write this equation, first, we combine the absorbed radiation and the effective radiation into a radiation balance.
We will denote the arrival of heat from the air or its return to the air by thermal conductivity as P. The same income or consumption by heat exchange with deeper layers of soil or water will be called A. The loss of heat during evaporation or its arrival during condensation on the earth's surface will be denoted by LE, where L is the specific the heat of evaporation and E is the mass of evaporated or condensed water.
It can also be said that the meaning of the equation is that the radiative balance on the earth's surface is balanced by non-radiative heat transfer (Fig. 5.1).
Equation (1) is valid for any period of time, including for many years.
The fact that the heat balance of the earth's surface is zero does not mean that the surface temperature does not change. When the heat transfer is directed downward, the heat that comes to the surface from above and leaves it deep into it remains to a large extent in the uppermost layer of soil or water (in the so-called active layer). The temperature of this layer, and therefore the temperature of the earth's surface, increases as well. On the contrary, when heat is transferred through the earth's surface from the bottom up, into the atmosphere, the heat escapes primarily from the active layer, as a result of which the surface temperature drops.
From day to day and from year to year, the average temperature of the active layer and the earth's surface in any place varies little. This means that during the day, almost as much heat enters the depths of the soil or water during the day as it leaves it at night. But still, during the summer days, the heat goes down a little more than it comes from below. Therefore, the layers of soil and water, and therefore their surface, are heated day by day. In winter, the reverse process occurs. These seasonal changes in heat input - heat consumption in soil and water almost balance out over the year, and the average annual temperature of the earth's surface and the active layer varies little from year to year.
Heat balance of the Earth- the ratio of the income and consumption of energy (radiant and thermal) on the earth's surface, in the atmosphere and in the Earth-atmosphere system. The main source of energy for the vast majority of physical, chemical and biological processes in the atmosphere, hydrosphere and in the upper layers of the lithosphere is solar radiation, so the distribution and ratio of the heat balance components characterize its transformations in these shells.
The heat balance is a particular formulation of the law of conservation of energy and is compiled for a section of the Earth's surface (the heat balance of the earth's surface); for a vertical column passing through the atmosphere (heat balance of the atmosphere); for the same column passing through the atmosphere and the upper layers of the lithosphere or the hydrosphere (thermal balance of the Earth-atmosphere system).
The equation for the heat balance of the earth's surface:
R + P + F0 + LE = 0. (15)
represents the algebraic sum of energy flows between an element of the earth's surface and the surrounding space. In this formula:
R - radiation balance, the difference between the absorbed short-wave solar radiation and long-wave effective radiation from the earth's surface.
P is the heat flux that occurs between the underlying surface and the atmosphere;
F0 - heat flow is observed between the earth's surface and deeper layers of the lithosphere or hydrosphere;
LE - heat consumption for evaporation, which is defined as the product of the mass of evaporated water E and the heat of evaporation L heat balance
These streams include the Radiation balance (or residual radiation) R - the difference between the absorbed short-wave solar radiation and the long-wave effective radiation from the earth's surface. The positive or negative value of the radiation balance is compensated by several heat fluxes. Since the temperature of the earth's surface is usually not equal to the air temperature, a heat flux P arises between the underlying surface and the atmosphere. A similar heat flux F0 is observed between the earth's surface and deeper layers of the lithosphere or hydrosphere. In this case, the heat flux in the soil is determined by molecular thermal conductivity, while in water bodies, heat transfer, as a rule, has a turbulent character to a greater or lesser extent. The heat flux F0 between the surface of the reservoir and its deeper layers is numerically equal to the change in the heat content of the reservoir over a given time interval and the heat transfer by currents in the reservoir. In the heat balance of the earth's surface, the heat consumption for evaporation LE is usually of significant importance, which is defined as the product of the mass of evaporated water E and the heat of evaporation L. The value of LE depends on the moistening of the earth's surface, its temperature, air humidity and the intensity of turbulent heat transfer in the surface air layer, which determines the rate of transfer of water vapor from the earth's surface to the atmosphere.
The atmosphere heat balance equation has the form:
Ra + Lr + P + Fa = ΔW, (16)
where ΔW is the change in heat content inside the vertical wall of the atmospheric column.
The heat balance of the atmosphere is composed of its radiation balance Ra; heat input or output Lr during phase transformations of water in the atmosphere (r is the sum of precipitation); the arrival or consumption of heat P, due to the turbulent heat exchange of the atmosphere with the earth's surface; heat gain or loss Fa caused by heat exchange through the vertical walls of the column, which is associated with ordered atmospheric motions and macroturbulence. In addition, the equation for the heat balance of the atmosphere includes the term ΔW, which is equal to the change in heat content inside the column.
The heat balance equation for the Earth-atmosphere system corresponds to the algebraic sum of the terms of the equations for the heat balance of the earth's surface and atmosphere. The components of the heat balance of the earth's surface and atmosphere for various regions of the globe are determined by meteorological observations (at actinometric stations, at special heat balance stations, on meteorological satellites of the Earth) or by climatological calculations.
The average latitudinal values of the components of the heat balance of the earth's surface for the oceans, land and Earth and the heat balance of the atmosphere are given in tables, where the values of the terms of the heat balance are considered positive if they correspond to the arrival of heat. Since these tables refer to average annual conditions, they do not include terms characterizing changes in the heat content of the atmosphere and the upper layers of the lithosphere, since for these conditions they are close to zero.
For the Earth as a planet, together with the atmosphere, the heat balance scheme is shown in Fig. A solar radiation flux equal to an average of about 250 kcal / cm 2 per year per unit surface of the outer boundary of the atmosphere, of which about 1/3 is reflected into the world space, and 167 kcal / cm 2 per year is absorbed by the Earth
Heat exchange spontaneous irreversible process of heat transfer in space, due to a non-uniform temperature field. In the general case, heat transfer can also be caused by the inhomogeneity of the fields of other physical quantities, for example, the difference in concentrations (diffusion thermal effect). There are three types of heat transfer: thermal conductivity, convection and radiant heat transfer (in practice, heat transfer is usually carried out by all 3 types at once). Heat transfer determines or accompanies many processes in nature (for example, the evolution of stars and planets, meteorological processes on the surface of the Earth, etc.). in technology and everyday life. In many cases, for example, when studying the processes of drying, evaporative cooling, diffusion, heat transfer is considered together with mass transfer. Heat transfer between two coolants through a solid wall separating them or through the interface between them is called heat transfer.
Thermal conductivity one of the types of heat transfer (energy of thermal motion of microparticles) from more heated parts of the body to less heated ones, leading to temperature equalization. With thermal conductivity, the transfer of energy in the body is carried out as a result of the direct transfer of energy from particles (molecules, atoms, electrons) that have more energy to particles with less energy. If the relative change in the thermal conductivity temperature at a distance of the mean free path of particles l is small, then the basic law of thermal conductivity (Fourier law) is satisfied: the heat flux density q is proportional to the temperature gradient grad T, i.e. (17)
where λ is the thermal conductivity, or simply thermal conductivity, does not depend on grad T [λ depends on the aggregate state of the substance (see table), its atomic and molecular structure, temperature and pressure, composition (in the case of a mixture or solution).
The minus sign on the right side of the equation indicates that the direction of the heat flow and the temperature gradient are mutually opposite.
The ratio of the Q value to the cross-sectional area F is called the specific heat flux or heat load and is denoted by the letter q.
(18)
The values of the thermal conductivity coefficient λ for some gases, liquids and solids at an atmospheric pressure of 760 mm Hg is selected from the tables.
Heat transfer. Heat transfer between two coolants through a solid wall separating them or through the interface between them. Heat transfer includes heat transfer from a hotter fluid to the wall, thermal conductivity in the wall, heat transfer from the wall to a colder moving medium. The intensity of heat transfer during heat transfer is characterized by a heat transfer coefficient k, numerically equal to the amount of heat that is transferred through a unit of wall surface per unit time at a temperature difference between liquids of 1 K; dimension k - W/(m2․K) [kcal/m2․°С)]. The value R, the reciprocal of the heat transfer coefficient, is called the total thermal resistance heat transfer. For example, R of a single-layer wall
,
where α1 and α2 are the heat transfer coefficients from the hot liquid to the wall surface and from the wall surface to the cold liquid; δ - wall thickness; λ is the coefficient of thermal conductivity. In most cases encountered in practice, the heat transfer coefficient is determined empirically. In this case, the results obtained are processed by the similarity theory methods
Radiant heat transfer - radiative heat transfer is carried out as a result of the processes of transformation of the internal energy of matter into radiation energy, the transfer of radiation energy and its absorption by matter. The course of processes of radiant heat transfer is determined by the mutual arrangement in space of the bodies exchanging heat, the properties of the medium separating these bodies. The essential difference between radiant heat transfer and other types of heat transfer (thermal conduction, convective heat transfer) is that it can also occur in the absence of a material medium separating the heat transfer surfaces, since it is carried out as a result of the propagation of electromagnetic radiation.
The radiant energy incident in the process of radiant heat transfer onto the surface of an opaque body and characterized by the value of the incident radiation flux Qfall is partially absorbed by the body and partially reflected from its surface (see Fig.).
The flux of absorbed radiation Qabs is determined by the relation:
Qabs \u003d A Qpad, (20)
where A is the absorptive capacity of the body. Due to the fact that for an opaque body
Qfall \u003d Qab + Qotr, (21)
where Qotr is the flux of radiation reflected from the surface of the body, this last value is equal to:
Qotr \u003d (1 - A) Qpad, (22)
where 1 - A \u003d R is the reflectivity of the body. If the absorption capacity of a body is 1, and therefore its reflectivity is 0, that is, the body absorbs all the energy incident on it, then it is called an absolutely black body. Any body whose temperature is different from absolute zero emits energy due to the heating of the body. This radiation is called the body's own radiation and is characterized by the flux of its own radiation Qe. Self-radiation, related to the unit surface of the body, is called the flux density of its own radiation, or the emissivity of the body. The latter, in accordance with the Stefan-Boltzmann law of radiation, is proportional to the temperature of the body to the fourth power. The ratio of the emissivity of a body to the emissivity of a completely black body at the same temperature is called the degree of blackness. For all bodies, the degree of blackness is less than 1. If for some body it does not depend on the wavelength of radiation, then such a body is called gray. The nature of the distribution of radiation energy of a gray body over wavelengths is the same as that of an absolutely black body, that is, it is described by Planck's law of radiation. The degree of blackness of a gray body is equal to its absorption capacity.
The surface of any body entering the system emits fluxes of reflected radiation Qotr and its own radiation Qcob; the total amount of energy leaving the surface of the body is called the effective radiation flux Qeff and is determined by the relation:
Qeff \u003d Qotr + Qcob. (23)
Part of the energy absorbed by the body returns to the system in the form of its own radiation, so the result of radiant heat transfer can be represented as the difference between the fluxes of its own and absorbed radiation. Value
Qpez \u003d Qcob - Qabs (24)
is called the resulting radiation flux and shows how much energy the body receives or loses per unit time as a result of radiant heat transfer. The resulting radiation flux can also be expressed as
Qpez \u003d Qeff - Qpad, (25)
that is, as the difference between the total consumption and the total arrival of radiant energy on the surface of the body. Hence, given that
Qpad = (Qcob - Qpez) / A, (26)
we obtain an expression that is widely used in calculations of radiant heat transfer:
The task of calculating radiant heat transfer is, as a rule, to find the resulting radiation fluxes on all surfaces included in a given system, if the temperatures and optical characteristics of all these surfaces are known. To solve this problem, in addition to the last relation, it is necessary to find out the relationship between the flux Qinc on a given surface and the fluxes Qeff on all surfaces included in the radiant heat exchange system. To find this connection, the concept of the average angular coefficient of radiation is used, which shows what proportion of the hemispherical (that is, emitted in all directions within the hemisphere) radiation of a certain surface included in the radiant heat exchange system falls on this surface. Thus, the flux Qfall on any surfaces included in the radiative heat exchange system is defined as the sum of the products Qeff of all surfaces (including the given one, if it is concave) and the corresponding angular coefficients of radiation.
Radiant heat transfer plays a significant role in heat transfer processes occurring at temperatures of about 1000 °C and above. It is widely used in various fields of technology: in metallurgy, thermal power engineering, nuclear power engineering, rocket technology, chemical technology, drying technology, and solar technology.
By absorbing the radiant energy of the Sun, the Earth itself becomes a source of radiation. However, the radiation of the Sun and the radiation of the Earth are essentially different. Direct, scattered and reflected solar radiation has a wavelength ranging from 0.17 to 2-4 mk, and called shortwave radiation. The heated surface of the earth, in accordance with its temperature, emits radiation mainly in the wavelength range from 2-4 to 40 mk and called longwave. Generally speaking, both solar radiation and earth radiation have wavelengths of all wavelengths. But the bulk of the energy (99.9%) lies in the indicated wavelength range. The difference in the wavelengths of radiation from the Sun and the Earth plays a large role in the thermal regime of the earth's surface.
Thus, being heated by the rays of the Sun, our planet itself becomes a source of radiation. The long-wavelength, or thermal, rays emitted by the earth's surface, directed from the bottom up, depending on the wavelength, either freely leave through the atmosphere, or are delayed by it. It has been established that the radiation of waves with a length of 9-12 mk freely escapes into interstellar space, as a result of which the surface of the earth loses some of its heat.
To solve the problem of the heat balance of the earth's surface and atmosphere, it was necessary to determine how much solar energy enters various regions of the Earth and how much of this energy is converted into other forms.
Attempts to calculate the amount of incoming solar energy on the earth's surface belong to the middle XIXcentury after the first actinometric instruments were created. However, only in the 1940s XXcentury, a broad development of the problem of studying the heat balance began. This was facilitated by the extensive development of the actinometric network of stations in the postwar years, especially in the period of preparation for the International Geophysical Year. In the USSR alone, the number of actinometric stations reached 200 by the beginning of the IGY. At the same time, the scope of observations at these stations was significantly expanded. In addition to measuring the short-wave radiation of the Sun, the radiation balance of the earth's surface was determined, that is, the difference between the absorbed short-wave radiation and the long-wave effective radiation of the underlying surface. At a number of actinometric stations, observations were organized on the temperature and humidity of the air at heights. This made it possible to calculate the heat costs for evaporation and turbulent heat transfer.
In addition to systematic actinometric observations carried out on a network of ground-based actinometric stations under the same type of program, experimental work has been carried out in recent years to study radiation fluxes in the free atmosphere. To this end, systematic measurements of the balance of long-wave radiation at various heights in the troposphere are carried out at a number of stations using special radiosondes. These observations, as well as data on radiation fluxes in the free atmosphere, obtained with the help of free balloons, airplanes, geophysical rockets and artificial Earth satellites, made it possible to study the regime of the heat balance components.
Using the materials of experimental studies and widely applying computational methods, employees of the Main Geophysical Observatory named after. A. I. Voeikova T. G. Berlyand, N. A. Efimova, L. I. Zubenok, L. A. Strokina, K. Ya. Vinnikov and others under the leadership of M. I. Budyko in the early 50s for the first time a series of maps of heat balance components for the entire globe was constructed. This series of maps was first published in 1955. The published Atlas contained maps of the total distribution of solar radiation, radiation balance, heat consumption for evaporation and turbulent heat transfer on average for each month and year. In subsequent years, in connection with the receipt of new data, especially for the IGY period, the data on the components of the heat balance were refined and a new series of maps was built, which were published in 1963.
The heat balance of the earth's surface and the atmosphere, taking into account the inflow and release of heat for the Earth-atmosphere system, reflects the law of conservation of energy. To draw up an equation for the heat balance of the Earth - the atmosphere, one should take into account all the heat - received and consumed - on the one hand, by the whole Earth together with the atmosphere, and on the other hand, by the separately underlying surface of the earth (together with the hydrosphere and lithosphere) and the atmosphere. Absorbing the radiant energy of the Sun, the earth's surface loses part of this energy through radiation. The rest is spent on heating this surface and the lower layers of the atmosphere, as well as on evaporation. The heating of the underlying surface is accompanied by heat transfer to the soil, and if the soil is moist, then heat is simultaneously spent on the evaporation of soil moisture.
Thus, the heat balance of the Earth as a whole consists of four components.
Radiation balance ( R). It is determined by the difference between the amount of absorbed short-wave radiation from the Sun and long-wave effective radiation.
Heat transfer in the soil, characterizing the process of heat transfer between the surface and deeper layers of the soil (BUT). This heat transfer depends on the heat capacity and thermal conductivity of the soil.
Turbulent heat transfer between the earth's surface and atmosphere (R). It is determined by the amount of heat that the underlying surface receives or gives off to the atmosphere, depending on the ratio between the temperatures of the underlying surface and the atmosphere.
Heat spent on evaporation( LE). It is determined by the product of the latent heat of vaporization ( L) for evaporation (E).
These components of the heat balance are interconnected by the following relationship:
R= A+ P+ LE
Calculations of the components of the heat balance make it possible to determine how the incoming solar energy is converted on the surface of the earth and in the atmosphere. In middle and high latitudes, the influx of solar radiation is positive in summer and negative in winter. According to calculations south of 39 ° N. sh. The balance of radiant energy is positive throughout the year. At a latitude of about 50° on the European territory of the USSR, the balance is positive from March to November and negative during the three winter months. At a latitude of 80°, a positive radiation balance is observed only in the period May-August.
In accordance with calculations of the Earth's heat balance, the total solar radiation absorbed by the earth's surface as a whole is 43% of the solar radiation arriving at the outer boundary of the atmosphere. The effective radiation from the earth's surface is 15% of this value, the radiation balance is 28%, heat consumption for evaporation is 23%, and turbulent heat transfer is 5%.
Let us now consider some results of the calculation of the heat balance components for the Earth-atmosphere system. Here are four maps: total radiation for the year, radiation balance, heat costs for evaporation and heat costs for heating air by turbulent heat transfer, borrowed from the Atlas of the heat balance of the globe (edited by M. I. Budyko). From the map shown in Figure 10, it follows that the largest annual values of total radiation fall on the arid zones of the Earth. In particular, in the Sahara and Arabian deserts, the total annual radiation exceeds 200 kcal / cm 2, and in high latitudes of both hemispheres it does not exceed 60-80kcal / cm 2.
Figure 11 shows a map of the radiation balance. It is easy to see that at high and middle latitudes the radiation balance increases towards low latitudes, which is associated with an increase in total and absorbed radiation. It is interesting to note that, in contrast to the isolines of the total radiation, the isolines of the radiation balance break when moving from the oceans to the continents, which is associated with the difference in albedo and effective radiation. The latter are smaller for the water surface, so the radiation balance of the oceans exceeds the radiation balance of the continents.
The smallest annual amounts (about 60 kcal / cm 2) are characteristic of regions where cloudiness prevails, as well as in dry regions, where high values of albedo and effective radiation reduce the radiation balance. The largest annual sums of the radiation balance (80-90 kcal / cm 2) are characteristic of slightly cloudy, but relatively humid tropical forests and savannas, where the arrival of radiation, although significant, the albedo and effective radiation are greater than in the desert regions of the Earth.
The distribution of annual evaporation rates is shown in Figure 12. Heat consumption for evaporation, equal to the product of the evaporation rate and the latent heat of vaporization (LE), is determined mainly by the amount of evaporation, since the latent heat of vaporization under natural conditions varies within small limits and is on average equal to 600 feces per gram of evaporated water.
As follows from the above figure, evaporation from land mainly depends on heat and moisture reserves. Therefore, the maximum annual amounts of evaporation from the land surface (up to 1000 mm) take place in tropical latitudes, where significant thermal
resources are combined with great hydration. However, the oceans are the most important sources of evaporation. Its maximum values here reach 2500-3000 mm. At the same time, the greatest evaporation occurs in areas with relatively high temperatures of surface waters, in particular, in zones of warm currents (Gulf Stream, Kuro-Sivo, etc.). On the contrary, in the zones of cold currents, the evaporation values are small. In the middle latitudes there is an annual course of evaporation. At the same time, in contrast to land, the maximum evaporation on the oceans is observed in the cold season, when large vertical gradients of air humidity are combined with increased wind speeds.
The turbulent heat exchange of the underlying surface with the atmosphere depends on the radiation and moisture conditions. Therefore, the greatest turbulent heat transfer occurs in those areas of land where a large influx of radiation is combined with dry air. As can be seen from the map of annual values of turbulent heat transfer (Fig. 13), these are desert zones, where its value reaches 60 kcal / cm 2. The values of turbulent heat transfer are small in the high latitudes of both hemispheres, as well as in the oceans. The maximum annual values can be found in the zone of warm sea currents (more than 30 kcal / cm 2 year), where large temperature differences are created between water and air. Therefore, the greatest heat transfer on the oceans occurs in the cold part of the year.
The heat balance of the atmosphere is determined by the absorption of short-wave and corpuscular radiation from the Sun, long-wave radiation, radiant and turbulent heat transfer, heat advection, adiabatic processes, etc. Data on the arrival and consumption of solar heat are used by meteorologists to explain the complex circulation of the atmosphere and hydrosphere, heat and moisture circulation, and many other processes and phenomena occurring in the air and water shells of the Earth.
- Source-
Pogosyan, H.P. Atmosphere of the Earth / Kh.P. Poghosyan [and d.b.]. - M .: Education, 1970. - 318 p.
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