The solar radiation that penetrates the earth’s atmosphere and reaches the surface differs in both amount and character from the radiation at the top of the atmosphere. Part of the radiation is reflected back in to the space, especially by clouds. Furthermore, the radiation entering the atmosphere is partly absorbed by molecules in the air. Oxygen and Ozone (o3), absorb nearly all the Ultraviolet radiation, and water vapour and carbon dioxide absorb some of the energy in the infrared range. In addition, part of the solar radiation is scattered (i.e. its direction has been changed) by droplets in clouds by atmosphere molecules, and by dust particles.
Solar Radiation that has not been absorbed or scattered and reaches the ground directly from the sun is called “Direct Radiation” or Beam Radiation. Diffuse radiation is that Solar Radiation received from the sun after its direction has been changed by reflection and scattering by the atmosphere. Because of the Solar Radiation is scattered in all directions in the atmosphere, diffuses radiation comes to the earth from all parts of the sky. The sum of the beam and diffuse radiation flux is referred to as total or global radiation.
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