Chapter Summary

The balance between absorbed solar radiation and outgoing longwave radiation at the top of the atmosphere is a key determinant of global climate. 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 spreads over a larger surface area, with 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. The geographic distribution of net radiation – the difference between solar radiation absorbed and longwave radiation emitted – is unequal. In general, there is an excess of solar radiation gain over longwave radiation loss in the tropics and a deficit at latitudes poleward of 35° to 40°.

Solar Geometry

Diurnal and seasonal variation in climate arises from the geometry of Earth's annual orbit around the Sun and its daily rotation on its axis. As Earth rotates over the course of a day, the Sun appears to sweep a broad arc through the sky. Two angles define the Sun's position (Figure 4.1): its altitude above the horizon; and its bearing on the horizon, which is called the azimuth angle. A third angle, the zenith angle, is often used instead of altitude. Whereas altitude is the angular distance above the horizon, zenith angle is the angular distance from a line perpendicular to the surface. Altitude is zero at sunrise and sunset and is greatest at solar noon. At solar noon, the Sun is due south on the horizon in the Northern Hemisphere and is due north in the Southern Hemisphere. In the morning, the Sun is east of south; it is west of south in the afternoon.