Book excerpt: The solar radiant energy is in fact the only source of energy for the basic physical processes taking place in the atmosphere and on the earth's surface. When passing through the atmosphere and being reflected by the ground surface, solar radiation undergoes changes and conversions. Some of it is absorbed in the atmosphere and converted into other forms of energy, mainly into heat, and some is scattered by gases, by dust and by water vapour. Because of absorption and scattering in the atmosphere, solar radiation is changed by the time it reaches the earth's surface. That part of it which arrives as a beam of parallel rays is referred to as direct solar radiation, and that which is scattered in the atmosphere and reaches the earth's surface from all directions of the sky is called diffuse solar radiation. Both of them are reflected back into the atmosphere when they reach the earth's surface, and this third type of radiation is defined as reflected radiation. All of these radiations differ from solar radiation arriving at the upper level of the atmosphere in intensity as well as in spectral composition although they all fall within the spectral region of solar radiation. In atmospheric physics these types of radiation are known as short-wave radiation (SWR) as distinguished from long-wave or irifrared radiation (L WR) emitted by the atmosphere and the earth's surface.