Infrared (IR) atmospheric radiation (wavelength 4.0–100 µm) is a key term in the surface energy budget and is vitally important for climatological and meteorological studies and in applications such as agricultural meteorology and designing solar heat collectors and building heating/cooling systems. Thus, knowledge of atmospheric radiation at the Earth’s surface is of fundamental importance and has long been studied both empirically and theoretically. This paper is intended to discuss the nature of the atmospheric radiation and explains its origins and characteristics of equations which quantify it.