Every body contains a certain amount of heat energy that determines its temperature. The temperature, and its constant changes, depend on the exchange of heat between the Earth’s surface and the atmosphere. These changes in the atmosphere itself, on the other hand, depend on the exchange between its layers vertically, and between its masses horizontally, as well as between the atmosphere and space. Heat exchange takes place by way of:
- Short-wave radiation from the Sun, absorbed by the atmosphere and the Earth’s surface.
- Thermal conductivity between the Earth’s surface and the ground
- Heat transfer between the Earth’s surface and the atmosphere, or between land and water surfaces due to turbulent motion.
Changes in air temperature also occur independently of heat exchange with the environment, with the expansion, or compression, of certain amounts of air.
This phenomenon occurs due to changes in pressure and volume mainly in the rising, or falling, of air. Such processes are called adiabatic processes, or adiabatic transformations. Such processes occur on a large scale in the atmosphere with vertical currents and determine thermal equilibrium states.
However, the main source of heat on Earth is solar radiation reaching the surface of our planet in the form of shortwave radiation. The largest amount of energy that reaches the Earth comes precisely from solar radiation. Other types of energy that turn into heat energy and warm the Earth’s surface are: geothermal energy, tidal energy, radioactive decay energy, and the energy of burning fossil fuels.
The amount of energy that arrives from the sun is called the solar constant. It is the total energy that solar radiation carries per unit time through a certain unit of area that is perpendicular to the radiation at the average distance of the Earth from the Sun. The average value of the solar constant is about 1366.1 W/m². The value of this unit varies because the Sun’s activity also varies. Direct solar radiation is absorbed and scattered in the atmosphere mainly by gas particles, and aerosols.
For this reason, measuring the solar constant at the Earth’s surface is difficult because it must take into account the influence of the Earth’s atmosphere. The Earth’s surface receives only part of the sun’s energy, because the atmosphere attenuates solar radiation by scattering rays, and absorbing them. The scattering of radiation is called dispersion, while absorption is called absorption. The visible band of solar radiation is split in the atmosphere into all colors of the spectrum, namely red, orange, yellow, green, blue and violet. Blue radiation is the most strongly scattered. For this reason, the sky takes on a blue hue during the day. Only at low positions of the solar disk above the horizon. That is, during sunrise and sunset, and when the water vapor content is high, yellow, orange and red light is more strongly scattered, tinting the sky into shades of these colors.
The Earth’s surface is the main source of long-wave radiation, and thermal radiation, for the atmosphere. The way in which the Earth accepts and gives back the energy supplied to it depends on the type of this surface, which is primarily water or land. Only about 50% of the short-wave radiation, which is at the upper boundary of the atmosphere, reaches the ground surface. The ground absorbs the greater part of the radiation, the darker its surface. The parameter that determines the reflectivity of a surface is albedo, which is the ratio of reflected to incident radiation. The average albedo of the Earth is about 0.3. This means that 30% of the sunlight reaching the Earth is reflected back into space. The amount of albedo depends on the type of ground, as well as cloud cover.
Clouds reflect more light back into space than a clear blue sky. The albedo of clouds depends on several factors namely: the height of the cloud, its size and the number and size of water droplets inside. Rain clouds reflect 80-90% of solar radiation, while high clouds reflect 50-60%. Due to the presence of ice sheets and sea ice, albedo is much higher in higher latitudes than in circumpolar latitudes.
Building materials have a low albedo compared to some natural surfaces, e.g. 5-20% asphalt, 10-35% concrete, 20-25% stones, 10-35% tile, 20-40% brick. Fresh snow reflects up to 95 % of the radiation. However, some natural surfaces can also have a low albedo, such as black earth 5-10 %, or deciduous forest, which reflects only 15-20 %. The albedo of water varies from a few to tens of percent and depends on the angle of the sun’s rays. The amount of energy absorbed in the city is 15-30% higher than in non-urban areas.
Water, as a result of its great mobility in the form of waves, and sea currents, is subject to constant turbulence. Turbulent movements carry heat to considerable depths and transfer it to the water. The result of this process is a rapid equalization of temperature, and relatively small changes with depth. Water is characterized by its low ability to reflect solar radiation. The albedo of the world ocean is estimated at an average of 5- 20%, so water absorbs more than 80% of the radiation energy delivered to its surface. Water has a very high heat capacity. As a result, it heats up slowly while transferring heat into the depths, but also cools down slowly, so it stores a heat reserve.
The land surface transmits the heat energy obtained from the sun by conduction into the ground, while by long-wave radiation, and convective and turbulent motions, this energy is delivered to the atmosphere. On the surface of the ground there is evaporation, and condensation of water vapor, which is also a source of loss, or gain of a certain amount of heat. The ground absorbs solar energy by a very thin layer and a thickness of 1 mm. As a result, the ground surface heats up stronger and faster during the day, and in summer. During the night, and in winter, the ground surface loses heat very quickly through radiation. In the near-ground air layer, heat transfer takes place partly through long-wave radiation of the Earth’s surface to the atmosphere, and the atmosphere to the Earth’s surface. Thermal conductivity plays an insignificant role, as air is a poor conductor of heat.
Of greatest importance is the transfer of heat as a result of turbulence, i.e., the continuous, chaotic movement of small amounts of air, both with vertical convective motions and with the horizontal movement of air. A distinction can be made between dynamic turbulence, which is caused by the friction of air particles against uneven rough surfaces, and thermal turbulence, which is the result of non-uniform heating of fragments of the ground surface. These differences are mainly caused by non-uniform moisture content, different color of the land cover, or different land exposures. It is thermal turbulence that causes the vibration of the air, which can be observed on a sunny day over an asphalt highway or over a cornfield.
Absorbed short-wave radiation is radiated by thermal radiation. It is consumed in the evaporation of water, direct heating of the ground level of the atmosphere, and indirect heating of higher layers. A part of the heat is radiated into space. Solar energy reaches the Earth’s surface only during the day. Its amount decreases as the sun’s height above the horizon decreases. At night there is a loss of heat. The natural surface heat balance is the difference between the energy gained and the energy lost at the Earth’s surface. This balance can be expressed in terms of the following equation:
Q + H + L + G = 0
Where:
Q – radiation balance
H – turbulent heat felt, absorbed or transferred in the air or soil during a temperature change
L – turbulent latent heat, released or absorbed when water changes state of matter (e.g., evaporation = energy input, freezing = energy output)
G – heat transferred by conduction in the ground
Since the amount of solar energy supplied, and the direction of heat flow is not the same throughout the day, during the day the air is heated, so the heat balance is as follows:
Q – H – L – G = 0
At night, on the other hand, we do not observe the inflow of solar energy to the Earth’s surface, heat is transferred from the atmosphere to the ground. Usually at night the ground cools more than the air, because the air cools more slowly and so the night balance takes the form:
– Q + H + L + G = 0
Because of the above-discussed differences in inflow, and energy absorption between natural surfaces and buildings, the heat balance equation of urban areas is slightly different. It additionally includes the value of Qp, which represents heat exchange by conduction in the ground, streets, and walls of buildings, and Qf, which is anthropogenic heat supplied to the atmosphere mainly by burning fossil fuels. The heat balance of built-up areas taking into account these two values is as follows:
Q + Qp + H + L + Qf = 0
The heat balance is closely dependent on the radiation balance, which can be expressed by the following equation:
Q = (1-A) (I – sin h + i) + (Ez – Ea)
Where:
Q – radiation balance
A – albedo (in decimal numbers, not percent); (1-A) is the value of shortwave radiation absorbed by the substrate
(I – sin h) – intensity of direct radiation reaching the horizontal surface, where h is the height of the sun above the horizon, i – intensity of scattered radiation
Ez – longwave radiation of the Earth. Heat emitted by the ground into the atmosphere – the atmosphere absorbs about 96% of Ez, only a negligible part of it enters the interplanetary space, this amount depends on the content of water vapor and greenhouse gases in the air.
Ea – long-wave radiation of the atmosphere, otherwise known as return radiation (heat emitted by the atmosphere to the ground); (Ez – Ea) – this is the effective radiation and therefore the heat that the active surface loses.
A positive value of the heat balance indicates that more energy reaches the ground than is lost. A negative value of the above equation indicates a loss of energy, that is, a situation in which more energy is lost than reaches the ground.
When the average amount of energy reaching the planet does not change over time, and the energy input is equal to the energy output. Then the average temperature of the Earth practically does not change and so there is a state of thermal equilibrium. The Earth is in a state of thermal equilibrium, but the observed increase in the temperature of its surface, the melting of glaciers and the increase in the temperature of the oceans may indicate that our planet receives more energy than it radiates.
The intertropical zone has a positive thermal balance. In the temperate zone, on the other hand, this balance is positive in summer and negative in winter. The polar zones are characterized by a negative heat balance. Mainly due to the circulation of air, and sea currents during the year there is an exchange of heat between the zones, and a balance of heat balance across the Earth.
The Earth’s heat balance is affected by the so-called greenhouse effect. This phenomenon is caused by the ability of the atmosphere to transmit a large part of the sun’s radiation and retain the Earth’s radiation. Gases that make up the Earth’s atmosphere, such as CO2 , methane, and CFCs, cause the heat provided by the sun’s rays and also that reflected from the Earth’s surface to be retained in the atmosphere. The greenhouse effect is a prerequisite for life on Earth.
However, if the proportion of greenhouse gases is disturbed, then the amount of heat in the atmosphere will increase markedly and the average temperature on Earth will rise as a result. In recent years, as a result of human activity, there has been an increase in the previous presence of CO2 , and CFCs in the atmosphere. Climate warming as a result of more greenhouse gases in the atmosphere may lead to an increase in water levels, caused by the melting of glaciers. Further negative consequences of a warming climate may also be an intense course of desertification. However, some scientists do not connect the phenomenon of climate warming with the increase in greenhouse gases, while others deny claims of an increase in these components. There are also hypotheses that deny the phenomenon of warming of the Earth’s climate at all.
