Electromagnetic radiation is a central feature of crop environments; its energy determines soil and air temperatures, wind movements, evaporation, and photosynthesis. This chapter examines radiation sources and their roles in the macro- and microclimates of crops. When objects absorb radiation their temperature increases. That heat energy may remain in the object or it may be radiated as new long-wave radiation, transferred to another object, or dissipated in evaporation of water. All of these subjects are covered here. We begin with a review of several physical laws important in radiative transfers of energy among plants, soil, and atmosphere, as well as from the Sun.

Radiation concepts

Two types of electromagnetic radiation, distinguished by their sources and spectral distributions, are important in crop environments. Solar radiation from a very hot thermal radiator, the Sun, is termed short-wave radiation (SW) because most energy is received in relatively short wavelengths, 0.3 to 3 μm. In contrast, thermal radiation from objects on our planet, including soils, plants, and atmosphere occurs at longer wavelengths because these radiating bodies are at much lower temperatures. Such long-wave radiation (LW) is found mainly between 5 and 100 μm.

Thermal radiation

All objects with a temperature greater than 0 K are sources of a continuous spectrum of electromagnetic radiation that, because of its source, is termed thermal radiation. Intensity and spectral distribution of thermal radiation may be compared with those from a reference “black body”.