Fundamentals: Quantities, Definitions, And Units

Solar radiation is a generic term that refers to different magnitudes of the solar electromagnetic radiation. The quantification of solar radiation incident at the Earth's surface is of high interest in many disciplines (radiative transfer in the atmosphere, meteorology and climatology, remote sensing of the atmosphere, solar energy studies, etc.). This multidisciplinary aspect of solar radiation sometimes produces duplication of names, definitions, or units. Moreover, different application-specific conventions for variable naming or units exist, which can be confusing. The solar irradiance that reaches a point at the Earth's surface is basically dominated by (i) the geometric aspects of the Earth's orbit around the Sun, and the inclination of its rotation axis in the ecliptic plane that determines the incident angle of the Sun rays; and (ii) the interaction mechanisms of solar radiation with various types of atmospheric constituents. This chapter intends to give the reader an overview of the basic definitions of the main variables that are commonly found in solar energy, and hence in this book as well. In addition, some basic aspects of solar geometry are briefly presented, followed by a concise description of the fundamentals of radiation-transfer modeling in the atmosphere. Detailed information on these topics, which is out of the scope of this book, can be found in many textbooks and the abundant literature on solar radiation, radiative transfer and atmospheric physics, to which the avid reader is referred for additional insight.