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 temperature distribution in the soil and the air layers adjacent to it largely depends on its value. Depend on him physical properties masses of air 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.

Virtually the only source of energy for everyone physical processes developing in the atmosphere is solar radiation. main feature radiation regime of the atmosphere so-called. greenhouse effect: the atmosphere weakly absorbs short-wave solar radiation (most of it reaches the earth's surface), but delays long-wave (all infrared) radiation thermal radiation 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 fluctuations in the density of the atmosphere. 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.

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 and in different ways receives and loses heat. Through the earth's surface, heat is transferred up to the atmosphere and down to 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. are used to heat the upper layers of soil and water. At the same time, the earth's surface itself radiates and thereby loses heat.

Secondly, heat comes to the earth's surface from above, from the atmosphere, through turbulent heat 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 loses heat when water evaporates from it. In the first case, it stands out latent heat, in the second heat goes into a latent state.

We will not dwell on less important processes (for example, the expenditure of heat for the melting of snow lying on the surface, or the propagation of heat into the depths of the soil along with precipitation water).

Let us consider the earth's surface as an idealized geometric surface without thickness, the heat capacity of which, therefore, is equal to zero. Then it is clear that in any period of time the same amount of heat will go up and down from the earth's surface as it receives from above and below during the same time. Naturally, if we consider not the surface, but some layer of the earth's surface, then there may not be equality of incoming and outgoing heat fluxes. In this case, the excess of incoming heat flows over outgoing flows, in accordance with the law of conservation of energy, will be used to heat this layer, and in the opposite case, to cool it.

So, the algebraic sum of all heat inflows and outflows on the earth's surface must be equal to zero - this is the equation heat balance earth's surface. To write the heat balance equation, we combine the absorbed radiation and the effective radiation into the radiation balance:

B = (S sin h + D)(1 – A) – E s .

The arrival of heat from the air or its release into the air by thermal conduction is denoted by the letter R. The same income or consumption by heat exchange with deeper layers of soil or water will be denoted by G. The loss of heat during evaporation or its arrival during condensation on the earth's surface will be denoted LE, where L – specific heat evaporation and E is the mass of evaporated or condensed water. Let us recall one more component - the energy spent on photosynthetic processes - PAR, however, is very small in comparison with the others, therefore, in most cases it is not indicated in the equation. Then the equation for the heat balance of the earth's surface takes the form

AT+ R+ G + LE + Q PAR = 0 or AT+ R+ G + LE = 0

It can also be noted that the meaning of the equation is that the radiative balance on the earth's surface is balanced by non-radiative heat transfer.

The heat balance equation is valid for any time, including a multi-year period.

The fact that the heat balance of the earth's surface is zero does not mean that the surface temperature does not change. If the heat transfer is directed downwards, then 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, consequently, the temperature of the earth's surface increases as well. When heat is transferred through the earth's surface from bottom to top, into the atmosphere, heat escapes, first of all, from the active layer, as a result of which the surface temperature drops.

From day to day and from year to year average temperature the active layer and the earth's surface in any place changes little. This means that during the day, as much heat enters the depths of the soil or water during the day as it leaves it at night. Since during the summer day more heat goes down than comes from below, the layers of soil and water and their surface heat up day by day. In winter, the reverse process occurs. Seasonal changes in heat input and output 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.

There are drastic differences in heating and thermal characteristics surface layers soil and upper layers of water basins. In soil, heat propagates vertically by molecular heat conduction, and in lightly moving water, also by turbulent mixing of water layers, which is much more efficient. Turbulence in water bodies is primarily due to waves and currents. At night and in the cold season, thermal convection joins this kind of turbulence: water cooled on the surface sinks down due to increased density and is replaced by warmer water from the lower layers. In the oceans and seas, evaporation also plays a role in the mixing of layers and in the heat transfer associated with it. With significant evaporation from the sea surface, the upper layer of water becomes more saline and therefore denser, as a result of which the water sinks from the surface to the depths. In addition, radiation penetrates deeper into water compared to soil. Finally, the heat capacity of water is greater than that of soil, and the same amount of heat heats a mass of water to a lower temperature than the same mass of soil.

As a result, daily temperature fluctuations in water extend to a depth of about tens of meters, and in soil - less than one meter. Annual temperature fluctuations in water extend to a depth of hundreds of meters, and in soil - only 10–20 m.

So, the heat that comes to the surface of the water during the day and summer penetrates to a considerable depth and heats up a large thickness of the water. The temperature of the upper layer and the surface of the water itself rises little at the same time. In the soil, the incoming heat is distributed in a thin top layer, which is very hot. Member G in the heat balance equation for water is much greater than for soil, and P correspondingly less.

At night and in winter, water loses heat from the surface layer, but instead of it comes the accumulated heat from the underlying layers. Therefore, the temperature at the surface of the water decreases slowly. On the soil surface, the temperature drops rapidly during heat transfer: the heat accumulated in the thin upper layer quickly leaves it and leaves without being replenished from below.

As a result, during the day and summer, the temperature on the soil surface is higher than the temperature on the water surface; lower at night and in winter. This means that the daily and annual temperature fluctuations on the soil surface are greater, and much greater than on the water surface.

Due to these differences in the distribution of heat, the water basin accumulates a large amount of heat in a sufficiently thick layer of water during the warm season, which is released into the atmosphere during the cold season. The soil during the warm season gives off at night most of the heat that it receives during the day, and accumulates little of it by winter. As a result, the air temperature over the sea is lower in summer and higher in winter than over land.

Let us consider, along with the atmosphere, the thermal regime of the Earth's active layer. The active layer is such a layer of soil or water, the temperature of which experiences daily and annual fluctuations. Observations show that on land, daily fluctuations propagate to a depth of 1 - 2 m, annual fluctuations - to a layer of several tens of meters. In the seas and oceans, the thickness of the active layer is ten times greater than on land. The connection between the thermal regimes of the atmosphere and the active layer of the Earth is carried out using the so-called heat balance equation of the earth's surface. This equation was first used in 1941 to construct the theory of the daily variation of air temperature by A.A. Dorodnitsyn. In subsequent years, the heat balance equation was widely used by many researchers to study various properties of the surface layer of the atmosphere, up to assessing the changes that will occur under the influence of active influences, for example, on the ice cover of the Arctic. Let us dwell on the derivation of the equation for the heat balance of the earth's surface. Solar radiation that has arrived at the earth's surface is absorbed on land in a thin layer, the thickness of which will be denoted by (Fig. 1). In addition to the flow of solar radiation, the earth's surface receives heat in the form of a flow of infrared radiation from the atmosphere, it loses heat through its own radiation.

Rice. one.

In the soil, each of these streams undergoes a change. If in an elementary layer with a thickness (- the depth counted from the surface into the depth of the soil) the flux Ф has changed by dФ, then we can write

where a is the absorption coefficient, is the density of the soil. Integrating the last relation in the range from to, we obtain

where is the depth at which the flow decreases by a factor of e compared to the flow Ф(0) at. Along with radiation, heat transfer is carried out by turbulent exchange of the soil surface with the atmosphere and molecular exchange with the underlying soil layers. Under the influence of turbulent exchange, the soil loses or receives an amount of heat equal to

In addition, water evaporates from the soil surface (or water vapor condenses), which consumes the amount of heat

The molecular flow through the lower boundary of the layer is written as

where is the coefficient of thermal conductivity of the soil, is its specific heat, - coefficient of molecular thermal diffusivity.

Under the influence of the influx of heat, the temperature of the soil changes, and at temperatures close to 0, ice melts (or water freezes). Based on the law of conservation of energy in a vertical column of soil, we can write down the thickness.

In equation (19), the first term on the left side is the amount of heat spent on changing the heat content cm 3 of the soil per unit time, the second amount of heat used to melt ice (). On the right side, all heat fluxes that enter the soil layer through the upper and lower boundaries are taken with the “+” sign, and those that leave the layer are taken with the “-” sign. Equation (19) is the heat balance equation for the soil layer thickness. In such general view this equation is nothing more than the heat gain equation written for a layer of finite thickness. It is not possible to extract from it any additional information (compared to the heat influx equation) on the thermal regime of air and soil. However, several special cases of the heat balance equation can be indicated, when it can be used as independent of differential equations boundary condition. In this case, the heat balance equation makes it possible to determine the unknown temperature of the earth's surface. The following are such special cases. On land that is not covered with snow or ice, the value, as already indicated, is quite small. At the same time, the ratio to each of the quantities that are of the order of the molecular range is quite large. As a result, the equation for land in the absence of ice melting processes can be written with a sufficient degree of accuracy in the form:

The sum of the first three terms in equation (20) is nothing but the radiation balance R of the earth's surface. Thus, the equation for the heat balance of the land surface takes the form:

The heat balance equation in the form (21) is used as a boundary condition in the study of the thermal regime of the atmosphere and soil.

In order to correctly assess the degree of heating and cooling of various earth surfaces, calculate evaporation for , determine changes in the moisture content in the soil, develop methods for predicting freezing, and also evaluate the impact of reclamation work on the climatic conditions of the surface air layer, data on the heat balance of the earth's surface are needed.

The earth's surface continuously receives and loses heat as a result of exposure to a variety of flows of short-wave and long-wave radiation. Absorbing to a greater or lesser extent the total radiation and counter radiation, the earth's surface heats up and emits long-wave radiation, which means it loses heat. The value characterizing the loss of heat of the earth
surface is the effective radiation. It is equal to the difference between the own radiation of the earth's surface and the counter radiation of the atmosphere. Since the counter radiation of the atmosphere is always somewhat less than that of the earth, this difference is positive. In the daytime, the effective radiation is blocked by the absorbed short-wave radiation. At night, in the absence of short-wave solar radiation, effective radiation lowers the temperature of the earth's surface. In cloudy weather, due to the increase in the counter radiation of the atmosphere, the effective radiation is much less than in clear weather. Less and nightly cooling of the earth's surface. In middle latitudes, the earth's surface loses through effective radiation about half of the amount of heat that they receive from absorbed radiation.

The arrival and consumption of radiant energy is estimated by the value of the radiation balance of the earth's surface. It is equal to the difference between the absorbed and effective radiation, the thermal state of the earth's surface depends on it - its heating or cooling. During the day, it is positive almost all the time, i.e., the heat input exceeds the consumption. At night, the radiation balance is negative and equal to the effective radiation. The annual values ​​of the radiation balance of the earth's surface, with the exception of the highest latitudes, are everywhere positive. This excess heat is spent on heating the atmosphere by turbulent heat conduction, on evaporation, and on heat exchange with deeper layers of soil or water.

If we consider the temperature conditions for a long period (a year or better a number of years), then the earth's surface, the atmosphere separately and the "Earth-atmosphere" system are in a state of thermal equilibrium. Their average temperature varies little from year to year. In accordance with the law of conservation of energy, we can assume that the algebraic sum of heat fluxes coming to the earth's surface and leaving it is equal to zero. This is the equation for the heat balance of the earth's surface. Its meaning is that the radiation balance of the earth's surface is balanced by non-radiative heat transfer. The heat balance equation, as a rule, does not take into account (because of their smallness) such flows as heat transferred by precipitation, energy consumption for photosynthesis, heat gain from biomass oxidation, as well as heat consumption for melting ice or snow, heat gain from freezing water.

The thermal balance of the "Earth-atmosphere" system over a long period is also equal to zero, i.e., the Earth as a planet is in thermal equilibrium: the solar radiation arriving at the upper boundary of the atmosphere is balanced by the radiation leaving the atmosphere from the upper boundary of the atmosphere.

If we take the air coming to the upper boundary as 100%, then 32% of this amount is dissipated in the atmosphere. Of these, 6% goes back into the world space. Consequently, 26% comes to the earth's surface in the form of scattered radiation; 18% of radiation is absorbed by ozone, aerosols and is used to heat the atmosphere; 5% is absorbed by clouds; 21% of radiation escapes into space as a result of reflection from clouds. Thus, the radiation coming to the earth's surface is 50%, of which direct radiation accounts for 24%; 47% is absorbed by the earth's surface, and 3% of the incoming radiation is reflected back into space. As a result, 30% of solar radiation escapes from the upper boundary of the atmosphere into outer space. This value is called the planetary albedo of the Earth. For the Earth-atmosphere system, 30% of reflected and scattered solar radiation, 5% of terrestrial radiation and 65% of atmospheric radiation, i.e., only 100%, go back into space through the upper boundary of the atmosphere.

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.

The arrival of heat from the air or its release into the air by thermal conduction will be denoted by 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 components of the heat balance 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 is equal to the change the heat content of the reservoir for a given time interval and the transfer of heat 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, equal to changes 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 the tables, where the values ​​of the heat balance members 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 diagram is shown in Fig. A unit surface of the outer boundary of the atmosphere receives a solar radiation flux equal to an average of about 250 kcal / cm 2 per year, 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 state of aggregation 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 atmospheric pressure 760 mmHg is selected from the tables.

Heat transfer. Heat exchange 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 with 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 transformation processes internal energy matter into radiation energy, transfer of radiation energy and its absorption by matter. The course of radiant heat transfer processes is determined by mutual arrangement in the space of 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 Qinc 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 absorptivity of a body is equal to 1, and therefore its reflectivity is equal to 0, that is, the body absorbs all the energy incident on it, then it is called a completely 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 this 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 fraction 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 slope factors radiation.

Radiant heat transfer plays a significant role in heat transfer processes occurring at temperatures of about 1000 °C and above. It is widespread in various areas technology: in metallurgy, heat power engineering, nuclear power, rocket technology, chemical technology, drying technology, solar technology.