Chapter summary

The balance between absorbed solar radiation and outgoing longwave radiation at the top of the atmosphere is an important determinant of global climate. The electromagnetic and particle properties of radiation are covered in Chapter 3. This chapter further examines solar radiation, its geographic variation, and its annual cycle. The Sun's position in the sky, which varies over the course of a day and throughout the year from the geometry of Earth's annual orbit around the Sun and its daily rotation on its axis, determines the intensity of solar radiation. A surface receives the most solar radiation when it is oriented perpendicular to the Sun's rays. At other angles, the Sun's radiation is spread over a larger surface area leading to less radiation per unit area. As solar radiation passes through the atmosphere, some is absorbed and some is scattered, both upwards to space and downwards onto the surface. The downward scattered radiation is known as diffuse radiation and emanates from all directions of the sky. Direct beam radiation is not scattered and originates from the Sun's position in the sky. For the planet as a whole and averaged over the year, the solar radiation absorbed by Earth is equal to the longwave radiation emitted to space. That is, the net radiation absorbed by Earth is zero in the annually averaged planetary mean. This is the physical basis for a simple planetary energy balance model, which can be used to derive global mean planetary temperature.