The atmosphere (Gr. atmos means water vapor, vapor and Gr. sphaira means sphere) is a gaseous shell surrounding the globe. It does not have a well-defined shape and its volume is difficult to determine. The atmosphere is subject to the Earth’s attraction. The mixture of gases rising in the atmosphere is atmospheric air. A feature of the atmosphere is the constant variation in time and space of its physical parameters. Atmospheric phenomena and processes take place under the influence of solar radiation and the nature of the Earth’s surface.
They have a decisive influence on the landscape diversity of our planet, regulating the intensity of geomorphological processes, and hydrological processes in different regions of the world. The atmosphere, as already mentioned, is a mixture of gases and aerosols. Some of them are constant components of the atmosphere, as they do not change their proportions in the volume of air. These include nitrogen, oxygen, argon ,neon, helium, krypton). There are also variable components in the atmosphere, whose content varies over time and space. Changes in the content of these components can be related, for example, to the occurrence of volcanic eruptions. Variable components are mainly: water vapor, carbon dioxide, ozone, hydrogen sulfide, sulfur dioxide, ammonia, freons. Thus, the Earth’s atmosphere consists of the following components:
- Nitrogen N2 – 78.1%
- Oxygen O2 – 20.9%
- Argon Ar- 0.93 %
- Other: Carbon dioxide, water vapor, Neon, Helium, Cryptom, Hydrogen, Ozone
The first three components occupy 99.96% in a unit volume of air. Carbon dioxide content averages 0.03%. The remaining volume, which accounts for only 0.01%, is made up of a number of other gases found in trace amounts. Water vapor content varies near the earth’s surface from almost 0% in the polar regions to 4% in the equatorial zone. Water vapor 86% comes from ocean evaporation and 14% from the land surface. Water vapor exists in the atmosphere in varying amounts depending on the prevailing conditions.
It plays a very important role in the exchange of heat between the Earth’s surface and the atmosphere, as heat energy is required for the evaporation of water, which is then returned to the atmosphere during the condensation process. This heat is returned to the atmosphere during condensation, through the return of so-called latent heat. Condensation products such as clouds, scatter and reflect solar radiation and the Earth’s longwave radiation, and precipitation cleans the atmosphere to some extent.
Oxygen in the atmosphere is essential for the processes of respiration, burning and decay. The form of oxygen is ozone O3, Which is found in negligible amounts near the Earth’s surface, but in relatively large amounts at altitudes of 15- 55 km, with a maximum concentration in the 25- 30 km layer. The atmospheric ozone content is so small that if all atmospheric ozone could be concentrated in a single layer at the Earth’s surface, it would be only 3 mm thick at normal pressure. In the upper layers of the atmosphere it is formed when oxygen is transformed under the influence of ultraviolet molecules, while in the lower layers it is formed by electrical discharges. The gas also largely absorbs the sun’s ultraviolet radiation, which is harmful to biological life. Nitrogen triggers combustion processes, and metabolism, and is also used by plants to make protein. The carbon dioxide content of the atmosphere fluctuates widely. There is less of it during the day than at night. The amount of carbon dioxide is also higher in large cities.
The components of the atmosphere also include airborne liquid and solid particles called atmospheric aerosols. These are dusts of organic (bacteria, pollen), or inorganic (smoke particles, soot, ash, salt particles) origin. The smallest number of aerosol particles is recorded in high mountains, while the largest number is recorded over the sea, and in large industrial centers. The composition of the atmosphere up to an altitude of several tens of kilometers practically does not change. Only its density decreases. The atmosphere can be divided into two layers. The first, which extends from the Earth’s surface to a height of about 100 kilometers, is the homosphere.
The gases it contains have a constant composition. The homosphere consists of electrically inert molecules. The second layer, extending above an altitude of 100 km, is characterized by a changing chemical composition at different altitudes and is therefore referred to as the heterosphere. This layer has a varying composition because there is no mechanical mixing of gases in it. It contains ionized molecules, i.e. having some electrical charge. Ions are formed due to short-wave solar radiation at high altitudes. Up to an altitude of 200 km, the content is dominated by nitrogen, above that the proportion of oxygen increases. In the uppermost layers the main components are hydrogen and helium.
The upper limit of the atmosphere is undeterminable. Studies conducted from the Earth’s surface reach 30 km. We learn about the existence of the atmosphere above this height mainly through the occurrence of optical phenomena. Currently, it is assumed that the Earth’s atmosphere reaches up to an altitude of 000 km.2, although its traces were noticed at altitudes above 000 km20. however, it is difficult to clearly determine at what heights it passes into space. As altitude increases, it becomes more rarefied, and its physical characteristics such as mass, density, pressure and temperature change.
Structure of the atmosphere
As altitude increases, the air temperature, pressure, and density of the atmosphere changes. The vertical differentiation of the atmosphere became the basis for distinguishing its five main layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere. This division was adopted mainly on the basis of the thermal structure of the atmosphere. Transitional layers between them were also delineated and named: tropopause, stratopause and mesopause.
Troposphere
It is the lowest and thinnest layer of the atmosphere (Gr. tropos = turn, rotation) and accounts for about 80% of its total mass. It observes continuous movement of air, its mixing, and turbulent movements. Its upper limit varies depending on the season and latitude. Over the poles it reaches up to 7 km in winter and up to 9 km in summer.
In temperate latitudes from 10 km in winter to 13 km in summer. Over the equator, the range of the troposphere varies from 15 to km17 throughout the year. The varying thickness of the troposphere is mainly due to differences in the heating of areas lying at different latitudes. There is a decrease in air temperature in the troposphere with an increase in altitude of 0.6°C per 100 meters.
As altitude increases, humidity also decreases. This is caused by a decrease in atmospheric pressure by an average of 11.5 hPa per 100 m. The troposphere contains almost all the water vapor content of the atmosphere, so the processes that shape weather, and climate, occur in this layer. In the middle troposphere at an altitude of 1- 6 km, horizontal air movements prevail, and most clouds are formed at this altitude. In the upper troposphere ( above 6 km), westerly winds blowing at high speeds prevail. These winds are particularly strong at the tropopause boundary and are called jet currents. The temperature in the upper troposphere is, depending on the season and latitude, from about -45°C to ca. -80°C. The layer where the temperature drop ends with an increase in altitude is called the tropopause.
Stratosphere
Another layer of the atmosphere reaches up to 50- 55 km. In its lower part up to 25 km we observe a constant temperature of -55°C. In the upper part of the stratosphere (25- 50 km) the temperature increases reaching positive values. The increase in temperature at this altitude is due to the presence of ozone in this layer, which is formed by the action of ultraviolet radiation on oxygen molecules. Most of the incoming radiation is absorbed by this layer and converted to heat.
The increase in temperature in the stratosphere is due to the presence of ozone, which is almost entirely (90%) contained in this layer of the atmosphere. The processes of ozone formation and decay are accompanied by an increase in temperature. The area of maximum ozone concentration is called the ozonosphere. Ozonosphere is a protective layer very important for life on Earth. It protects against ultraviolet radiation, which is harmful to living organisms. The part of the ozonosphere with significantly reduced ozone content is the ozone hole.
It is formed as a result of freons entering the stratosphere. Due to the contact of freons with ultraviolet radiation, chlorine is released, which destroys ozone molecules by reacting with oxygen. The ozone hole phenomenon is most pronounced over Antarctica, as stratospheric winds push ozone-enriched air from above the equator toward the poles. The south pole receives less ozone-containing air. The stratosphere at an altitude of 50 to 55 km passes into a boundary layer called the stratopause.
Mesosphere
Mesosphere (Gr. mesos = intermediate, middle). It reaches up to 80 km above the Earth’s surface. It is characterized by a constant temperature in its lower part, and then a strong decrease in temperature (on average 2.3°C/1 km) to -90°C. This is the layer where the lowest temperatures in the entire atmosphere are observed. As a result of the intense vertical temperature drop, turbulence (turbulent flow of particles) takes place in the mesosphere. The mesosphere ends with a transition layer – the mesopause, located at an altitude of 80- 85 km.
Thermosphere
The thermosphere (Gr. thermós means warm, hot) is distinguished by high and steadily rising temperatures. In this part of the atmosphere, the gas is very diluted and strongly heated by the solar radiation that reaches here unhindered. At an altitude of 120 km, the temperature reaches about 100°C, while at an altitude of several hundred kilometers- 1000°C. In the thermosphere, two layers are sometimes distinguished: the ionosphere, lying at an altitude of 800- 1000 km, and the exosphere, located higher up. The exosphere passes into space without a clear boundary. In the ionosphere there is the phenomenon of ionization of atoms (atoms become endowed with a positive or negative electric charge), and absorption of solar radiation by ionized oxygen, nitrogen, helium, hydrogen. In the ionosphere, positive, or negative ions group together to form layers:
- Layer D (70-80 km from the ground)
- Layer E (120- 150 km from the ground)
- Layer F (250- 2600 km from the ground)
Of particular importance to humans are the so-called E layer with a predominance of positive ions, and the F layer with a predominance of negative ions. These layers are characterized by their ability to reflect radio waves. Thus, they enable radio communications even over very great distances. In the ionosphere there is also the phenomenon of aurora borealis caused by the influence of the Sun. The aurora borealis is produced by electrons and protons ejected by the Sun, which, encountering the Earth’s atmosphere, excite nitrogen and hydrogen, and the reaction results in a luminous phenomenon. Most often auroras occur at an altitude of 80-400 km, less often at altitudes higher than 1000 km. In the northern hemisphere, auroras most often occur between 65° and 75° north latitude and are relatively frequent phenomena
Exosphere
It is the uppermost layer of the atmosphere extending from about 800 km to an undefined boundary. (Gr. exo = outside). It is characterized by very strong dilution, and very high velocity of gas molecules. The exosphere is considered a transition zone between the atmosphere and space. The main components of this layer are hydrogen and oxygen. The exosphere is characterized by a very low density. Its particles collide with each other extremely rarely, which makes it possible for them to reach considerable speeds. It also becomes possible for them to escape from the area of Earth’s attraction. The molecules can also break free from the Earth’s atmosphere and penetrate interplanetary space. However, the gases penetrate from the lower layers of the atmosphere, which offsets their loss in the exosphere. Helium is provided by the radioactive decay of heavy atoms of elements such as uranium, thorium and radium found in the Earth’s crust. The source of hydrogen is water vapor and methane, which are mainly found in the stratosphere. Hydrogen and helium, as a result of their light weight, can also penetrate to considerable heights in the atmosphere. The temperature that prevails in this layer exceeds 800°C, while the pressure is less than 10-6 Pa.
