Chapter 5 – Radiation Environment

Almost all the energy for physical and biological processes at the earth’s surface comes from the sun. This chapter reviews the quantity and quality of solar (short-wave) radiation reaching the earth and discusses the processes by which it is scattered and absorbed in the atmosphere. Sun-earth geometry, dependent on astronomical factors, determines the major features of radiation at the earth’s surface and their variation over millenia. The spectrum of solar radiation important for environmental physics ranges from the ultra-violet to the infra-red. Wavebands used for vision, photosynthesis, and aspects of plant development (phytochrome absorption) are reviewed. Solar radiation is scattered and absorbed in the atmosphere by molecules, cloud droplets, and particles (aerosol). Spectral aspects of radiation attenuation are discussed. At the ground, solar energy is received as direct or diffuse radiation. The importance of each term and its dependence on cloud and aerosol is reviewed. The ratio of visible to infra-red radiation depends on solar elevation and atmospheric properties. The chapter also discusses terrestrial (long-wave) radiation originating in the atmosphere and emitted from solid surfaces. Expressions describing the long-wave radiation incident at the surface from cloudless and cloudy skies are reviewed. Finally, the topic of the net radiation balance (long- and short-wave radiation) is introduced, and examples of net radiation variation at different time scales and over various natural surfaces are given.