Atmosphere Fundamentals

Structure and Main Parameters of the Atmosphere

The atmosphere is a gaseous envelope that surrounds the Earth from the ground surface up to several hundred kilometers. The atmosphere consists of different kinds of gaseous, liquid, and crystal structures, including gas molecules and atoms, aerosols, cloud, fog, rain, hail, dew, rime, glaze, and snow [1—16]. Except for the first two, the other are usually called in the literature as hydrometeors [9—15]. Furthermore, due to irregular and sporadic air streams and motions, that is, irregular wind motions, the chaotic structures defined as atmospheric turbulence are also present in the atmosphere [17—21].

Based mostly on temperature variations, the Earth’s atmosphere is divided into four primary layers [1,8]: (1) the troposphere that surrounds the Earth from the ground surface up to 10—12 km, with the tropopause region as the isothermal layer above troposphere up to ^20 km, which spreads upward to the stratosphere; (2) the stratosphere with the stratopause from 20 km up to ^50 km altitude; (3) the mesosphere with the mesopause up to ^90 km; and (4) the thermosphere (up to ^600 km), which contains the multilayered plasma structure usually called ionosphere (70-400 km).

In our further discussions, we focus on the effects of the troposphere on optical wave propagation starting with a definition of the troposphere as a natural layered air medium consisting of different gaseous, liquid, and crystal structures.

The physical properties of the atmosphere are characterized by main parameters such as temperature, T (in Kelvin), pressure, P (in millibars, Pascals or in mmHg), and density, p (in kg m-3). All these parameters significantly change with altitude, seasonal and latitudinal variability, and strongly depend on environmental conditions occurring in the troposphere [22].

Over 98% of the troposphere is comprised of the elements nitrogen and oxygen. The number density of nitrogen molecules, pN(h), at a height h can be found in Reference 22. The temperature T(h) and pressure P(h), where the height is measured in meters, for the first 11 km of the troposphere can be determined from the following expressions [6,8]:

The temperature and pressure for h changing from 11 to 20 km in the atmosphere can be determined from

The number density of molecules can be found from [6]:

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