atmospheric structure

The lower limit of the atmosphere usually refers to the surface, but there are also a small amount of air in the soil, loose sediments and some rocks below the surface, which are underground parts of the atmosphere, and their depth is generally less than 3 km; ; There is no clear upper boundary in the atmosphere, and it is generally believed that the density of interplanetary dust will jump at a height of 2000~3000 km. The physical properties of the atmosphere vary greatly in the vertical direction. According to the physical properties and motion characteristics of the atmosphere such as temperature, composition and charge, the atmosphere can be divided into troposphere, stratosphere, mesosphere, warm layer and mesosphere from bottom to top (Figure 2- 1).

(1) Troposphere (troposphere)

Troposphere is the lowest layer of the atmosphere, and its thickness varies with latitude. Near the equator 17 ~ 18 km, the poles are only 8 ~ 9 km, and the average thickness is about113 km. Moreover, the thickness also changes with the seasons, generally larger in summer and smaller in winter. Compared with the total thickness of the atmosphere, although the troposphere is very thin, its mass accounts for 70% ~ 75% of the total mass of the atmosphere, and almost all the water vapor and dust in the atmosphere are concentrated. The main features of the troposphere are as follows: ① The temperature decreases with the increase of altitude, and the average temperature decreases by 6℃ per 1 km, which is called the atmospheric cooling rate. This is because the heat in the troposphere mainly depends on absorbing long-wave radiation from the ground, so the higher it is from the ground, the less heat it gets; (2) The air has a strong convection movement, which is due to the temperature difference and density difference of the atmosphere at different latitudes and heights caused by uneven ground heating, resulting in the mutual flow of the atmosphere. Air convection transports heat, water vapor and impurities from the ground to high altitude, thus causing a series of weather phenomena such as wind, snow, rain and clouds. ③ The horizontal distribution of meteorological elements is uneven. Because the troposphere is greatly influenced by the surface, the horizontal distribution of temperature, humidity and air pressure is uneven, which leads to a series of physical processes and complex weather phenomena. ④ The troposphere is most significantly affected by human activities, and most of the air pollutants emitted by human production activities are concentrated in the troposphere.

Figure 2- 1 Vertical stratification of the atmosphere

(Quoted from Liu Benpei and others, 2000)

(2) Stratosphere

The stratosphere is the atmosphere from the tropopause to 35 ~ 55 kilometers, and its mass accounts for about 20% of the total mass of the atmosphere. The most striking feature of the stratosphere is that the airflow mainly moves in the horizontal direction, hence the name. The stratosphere is basically free of water vapor and dust, and there are no various weather phenomena in the troposphere. The upper part of this layer (30 ~ 55km) has multiple ozone layers, which can absorb more than 99% ultraviolet rays harmful to life from the sun, so it is called the protective umbrella of the earth. At first, the temperature in the stratosphere remained unchanged or slightly increased with the increase of altitude, but when it rose above 30 km, the temperature rose rapidly because ozone absorbed a lot of ultraviolet rays, and the temperature at the top of the stratosphere rose to -3 ~ 17℃.

(3) Intermediate layer

The height from the stratosphere top to the atmosphere is about 85 kilometers. Because there is no ozone to absorb the ultraviolet radiation of the sun, the temperature drops rapidly with the increase of height, and the temperature at the top boundary of the middle layer drops to -83 ~- 1 13℃. The vertical movement of air appears again because it is hot under and cold under. Weak ionization has appeared at the top of this layer.

(4) Thermosphere

Also known as the ionosphere, it is about 800 kilometers from the top of the mesosphere. The air in this layer is very thin, accounting for only 0.5% of the total mass of the atmosphere. Under the action of solar radiation and cosmic high-energy particles, the temperature of air particles in this layer rises rapidly, and the temperature rises with height. According to satellite observation, the temperature below 500 km is as high as 120 1℃, and the temperature above 500 km has little change. At the same time, due to the action of ultraviolet rays and cosmic rays, oxygen and helium are decomposed into atoms and ionized. According to the degree of ionization, they can be divided into several ionospheres, and each layer can reflect radio waves with different wavelengths, so it is of great significance in long-distance short-wave radio communication.

(5) Outer escape layer

Also known as the escape layer, it is located at an altitude of more than 800 kilometers to 2000~3000 kilometers, and the air is extremely thin. This layer is the transition zone between the atmosphere and interstellar space, and its temperature also increases with the increase of altitude. Because it is too far from the ground, the gravity of the earth is weak, and the air particles move very fast, so the gas particles continue to spread outward.