The solar constant S is a quantity denoting the amount of total (i.e., covering the entire solar spectrum) solar energy reaching the top of the atmosphere. It is defined as the flux of solar energy (energy per unit time) across a surface of unit area normal to the solar beam at the mean distance between the sun and the earth. Solar insolation is defined as the flux of solar radiation per unit of horizontal area for a given locality. It depends primarily on the solar zenith angle and to some extent on the variable distance of the earth from the sun. It can be computed as a function of latitude and the time of year taking into account of the secular variations of Earth's orbit eccentricity e, the oblique angle ε, and the longitude of the perihelion relative to the vernal equinox ω. The daily insolation is the total solar energy received by a unit of area per one day. It may be calculated by integrating total insolation over the daylight hours. It is particularly important, together with information on cloud coverage, in order to plan and manage solar power systems. Yearly total insolation together with average cloud coverage are among the most important parameters to be considered for the choice of the best (i.e. the ones promising the higher energy production) location of solar power plants. Modeled daily solar insolation together with short/medium-term forecast of cloud coverage are also fundamental for the management (e.g. for planning the suspension of activities for maintenance) of solar energy production plants .
The temperature and pressure profiles determine the atmospheric structure. The latter consists of four basic levels, considering the vertical variability of the temperature. These main four levels are troposphere, stratosphere, mesosphere, and thermosphere. In the troposphere (0-12km), which is the lowest layer of the atmosphere, all the meteorological processes that affect our everyday life take place. The lowest part of the troposphere is known as the boundary layer (0-3km), where all the surface-atmosphere interactions and exchanges take place. The troposphere concentrates the water vapor and 90% of atmospheric mass, while the chemical composition of all atmospheric layers consists of nitrogen, oxygen, argon and trace gases. The main parameters that characterize the atmosphere structure are pressure, density, and temperature. All the aforementioned parameters are related to the atmospheric composition and vary with altitude, latitude, longitude and season. Additionally, the stratosphere, which is the layer above the troposphere, contains almost all of the ozone abundance (~90%) of the atmosphere in a region named as ozone layer and traced between 15 and 35km. The interaction of the incoming solar radiation with ozone in this layer causes the reduction of the incoming harmful UV radiation provoking the temperature increase in the stratospheric layer. The 99.9% of total atmospheric mass is concentrated in lower atmosphere (<50km) with Nitrogen (N2, 78.08%), Oxygen (O2, 20.95%) and argon (Ar, 0.93%) being the major constituents of the atmosphere. Water vapor (H2O) is considered as a significant factor, too. Despite the fact that it depicts a very small amount of total atmospheric mass, it’s one of the most important greenhouse gases, along with carbon dioxide (CO2) and methane (CH4), absorbing the Earth’s longwave (infrared) radiation, affecting the energy balance of Earth-Atmosphere system. Furthermore, water vapor plays a decisive role in the formation of clouds and precipitation. Together with the basic chemical (atoms, molecules, ions) constituents of a "standard" atmosphere, aerosols of natural and anthropogenic origin have to be considered too, as far as the interaction of e.m. radiation with atmosphere is concerned.