ABSTRACT
This chapter is concerned especially with visible light and radiant heat. Light has wavelengths of less than a millionth of a metre. Radiation from bodies at the much lower Earth temperatures has wavelengths about twenty times as great, the difference reflecting the ratio of the temperatures of the Sun and Earth, effectively 5,770 and nominally 288 degrees Kelvin, respectively. The amount of radiation received by the Earth from the Sun is governed by the geometry of the Earth’s motions. Its orbit around the Sun is not quite circular and the Sun is offset from the centre. The amount of radiant energy received or emitted every second at a surface of one square metre is properly known as the radiance, though we lazily refer to it simply as the ‘radiation’. The air’s molecules and aerosols scatter the Sun’s rays, which increases their paths within the atmosphere and hence their absorption.
