The elevation of the sun above the horizon, or, conversely, the angle of the sun from the vertical (straight up, or zenith) determines what is called air mass. Air-mass values are higher when the sun is lower in the sky. For example, air mass is 1 when the sun is directly overhead and the angle of the sun from the zenith direction is 0° ;; air mass is 2 when the angle is 60° . The air-mass value at any particular time depends on the location (latitude), the time of day, and the day of the year.
When the sun is closer to the horizon, direct beam radiation must pass through a longer distance in the earth's atmosphere than when the sun is overhead. This longer path length results in both more scattering and more absorption of the solar radiation.
The atmosphere through which the solar radiation passes is also quite variable. Significant variables are atmospheric turbidity (haziness due to aerosols, such as dust), water vapor, and clouds. So, what exactly is the atmosphere's effect on solar radiation? It basically acts as a dynamic filter, absorbing and scattering solar radiation. It creates spatial (geographic), temporal (hourly, daily), and spectral (wavelength) variations in solar radiation that we must characterize or describe with respect to their effects on operating solar energy conversion systems.