Question

Path length of radiation through the atmosphere to the length of path when the sun is at zenith is called declination air mass azimuth solar constant

Answer 1

Air mass is a measure of how much atmosphere the sun's rays have to pass through on their way to the surface of the earth. Since particles in the atmosphere absorb and scatter light rays, the more atmosphere solar radiation passes through on its way to us, the less solar energy we can expect to get.