¶ General information about the atmosphere, its composition
Earth's atmosphere is an airy gaseous shell with solid and liquid particles contained in it, which is in constant motion relative to the earth's surface.
The lower boundary of the atmosphere is considered to be the underlying surface - the earth's surface interacting with the atmosphere.
Atmospheric air, unlike water, is compressible, and therefore its density decreases with height. The atmosphere does not have a sharp upper boundary, but gradually passes into interplanetary space, which contains about 100 particles (protons and electrons) in each cubic centimeter. Such a concentration of particles is observed approximately at altitudes of 20–30 thousand km above the earth's surface. Therefore, these heights are considered the upper limit of the atmosphere.
In the layer from the earth's surface to a height of about 200 km, the predominant gas of atmospheric air is molecular nitrogen. Above 200 km, the atmosphere is dominated by atomic oxygen. The upper limit of the nitrogen-oxygen atmosphere is an altitude of about 1000 km, above which the atmosphere consists mainly of helium and hydrogen in the atomic state.
Almost 50% of the total mass of the atmosphere is contained in its lower five-kilometer layer, 75% - up to a height of 10 km, 95% - up to a height of 20 km, 99.9% - up to a height of 100 km.
In the atmosphere, processes associated with air movements of various scales take place. Spatial and temporal characteristic scales of atmospheric processes, which are studied in the course of aviation meteorology, are presented in Table
Characteristic scales of atmospheric processes
| Scales | Horizontal dimension, km | Period of existence |
|---|---|---|
| synoptic scale | 100–1000 | 1–7 days |
| Mesoscale | 10–100 | 1-24 hours |
| Scale of convective clouds | 1–10 | 10 min - 1 h |
| micro scale | < 1 | < 10 min |
The small gas components of atmospheric air include ozone - triatomic oxygen that protects living organisms on Earth from harsh ultraviolet radiation. Ozone absorbs solar radiation with wavelengths from 0.15 to 0.29 microns. Due to the chemical reactions in which ozone is involved, by absorbing hard ultraviolet radiation, the upper layers of atmospheric air are heated.
The content of gas in the atmosphere can be characterized by the reduced thickness of the layer of this gas. If the ozone contained in the atmosphere is brought to a pressure of 1013 hPa and a temperature of 0 °C, then the thickness of the ozone layer will average about 3 mm. The reduced thickness of the oxygen layer is 1500 m, and the reduced thickness of the nitrogen layer is 6200 m.
Water vapor, carbon dioxide and ozone are called variable gas components of atmospheric air, since their content in the atmosphere varies significantly in different parts of the globe in time and space. These gases are greenhouse gases because they absorb radiation from the earth's surface, contributing to the warming of the atmosphere. Thus, water vapor, carbon dioxide and ozone play an important role in the radiation processes in the atmosphere,
determining its thermal state.
¶ The structure of the atmosphere
The atmosphere is vertically divided into layers according to the following criteria:
- distribution of air temperature in height;
- chemical composition of atmospheric air;
- content of ions and ozone in the atmosphere;
- interaction of the atmosphere with the underlying surface;
- the influence of the atmosphere on aircraft.
The distribution of atmospheric air temperature along the height is called the temperature stratification of the atmosphere. Depending on the temperature stratification in the atmosphere, five main layers and four transitional layers are distinguished.
The main layers of the atmosphere are the troposphere, stratosphere, mesosphere, thermosphere and exosphere. Transitional layers of the atmosphere - tropopause, stratopause, mesopause, thermopause. The heights of the transitional layers deviate significantly from the average values depending on the geographic latitude, season, and synoptic processes.
Basic and transitional layers of the atmosphere
| Base layer | Average height of the lower and upper boundaries of the main layer, km | Transition layer |
|---|---|---|
| Troposphere | 0–11 | Tropopause |
| Stratosphere | 11–50 | Stratopause |
| Mesosphere | 50–90 | Mesopause |
| Thermosphere | 90–450 | Thermopause |
| Exosphere | > 450 | - |
The troposphere extends from the underlying surface to the tropopause and is characterized by a decrease in air temperature with increasing altitude.
The tropopause is a transitional layer between the troposphere and the stratosphere with a thickness of several hundred meters to 2 km, in which the vertical air temperature gradient is less than 0.3 °C per 100 m.
The stratosphere is located above the tropopause and is characterized by an increase in air temperature with height. During winter stratospheric warming, the air temperature in the upper part of the stratosphere can rise to 20 C or more. At high latitudes, at altitudes of 22–24 km, stratospheric mother-of-pearl clouds are sometimes observed, which have an iridescent color and consist of supercooled water droplets.
In the mesosphere, the air temperature decreases with height and can reach very low values (down to –110 C). In the upper part of the mesosphere at altitudes of about 80 km, noctilucent clouds are sometimes observed, consisting of ice crystals.
Homosphere and heterosphere are distinguished depending on the chemical composition of atmospheric air. In the homosphere, which extends from the earth's surface to a height of about 100 km, the composition of the main gases and the molar mass of air change little in height due to the turbulent mixing of atmospheric air.
Above the homosphere is the heterosphere, where the molar mass of air decreases with height due to the gravitational separation of gases. The content of heavy gases decreases with height faster than the content of light gases.
The ozonosphere is the layer of the atmosphere where most of the ozone is concentrated. The ozonosphere roughly coincides with the stratosphere, although a small amount of ozone contained in the air of the troposphere.
The ionosphere is a layer of the atmosphere in which the content of charged particles (ions and electrons) increases sharply compared to the underlying layers of the atmosphere. The ionosphere is located above the mesosphere, encompassing the thermosphere and exosphere
Taking into account the features of the interaction of the atmosphere with the underlying surface, a friction layer (the boundary layer of the atmosphere) and a free atmosphere are distinguished. In the friction layer, the air movement is greatly influenced by the force of friction of the moving air against the underlying surface. In a free atmosphere, the friction force is small compared to other acting forces, and therefore this force is assumed to be zero.
The friction layer extends from the underlying surface to a height of approximately 1–1.5 km. In this layer, diurnal variations in meteorological values are well expressed. In the lower part of the friction layer, a surface layer of the atmosphere is distinguished, the height of which is several tens of meters. In the surface layer, meteorological quantities change especially sharply with height.
Depending on the impact on aircraft, the atmosphere is divided into dense layers of the atmosphere, located from the earth's surface to a height of about 150 km, and near-Earth space - above 150 km. Within the limits of the dense layers of the atmosphere, an aircraft with the engines turned off cannot make at least one revolution around the Earth: it will lose speed or burn out.
¶ International Standard Atmosphere (ISA)
As explained above, the atmosphere varies continuously (temperature, pressure, wind, chemical composition, ...). Therefore, engineers decided to create an International Standard Atmosphere (ISA).
The International Standard Atmosphere represents an average of the conditions in the whole atmosphere for all latitudes, seasons and altitudes. The ISA is a hypothetical vertical distribution of atmospheric temperature, pressure and density that is considered representative of the atmosphere for the purposes of calibration, calculations and design of aircraft, missiles etc.
| Property | Metric Units | Imperial Untis |
|---|---|---|
| Sea level pressure | 1013,25 hectopascals | 29.92 inches of mercury |
| Sea level temperature | 15 °C | 59 °F |
| Lapse rate of temperature in the troposphere | 6,5 °C/1 000 meters | 3,57 °F/1 000 feet |
| Temperature at the tropopause | 11 000 meters | 36 089 feet |
| Exosphere | -56,5 °C | -69,7 °F |
AERONAUTICAL METEOROLOGY T. V. Safonova
VID 582348 - Creation