AWH 5-5

Chapter 5, Heat and Temperature 5-5 5.6.1.1 Solar and Terrestrial Radiation All objects emit radiation energy, including the Sun (solar radiation) and the Earth (terrestrial radiation). An object’s wavelength of maximum radiation is inversely related to its temperature; the hotter (colder) the object, the shorter (longer) the wavelength. The Sun’s wavelength of maximum radiation is relatively short and is centered in the visible spectrum. The Earth’s wavelength of maximum radiation is relatively long and is centered in the infrared spectrum. Figure 5-3. Temperature’s Effect on Radiation Wavelength Some of the solar radiation that reaches the Earth’s surface is radiated back into the atmosphere to become heat energy. Dark-colored objects such as asphalt absorb more of the radiant energy and warm faster than light-colored objects. Dark objects also radiate their energy faster than light-colored objects. 5.6.1.2 Solar Zenith Angle The intensity of incoming solar radiation that strikes the Earth’s surface (insolation) varies with solar zenith angle. Solar zenith angle is the angle measured from the Earth’s surface between the Sun and the zenith (i.e., directly overhead). Solar zenith angle varies with latitude, season, and the diurnal cycle (sunrise/sunset). Figure 5-4 illustrates this concept. Insolation is maximized when the solar zenith angle is zero degrees (0°), i.e., the Sun is directly overhead. With increasing solar zenith angle, the insolation is spread over an increasingly larger surface area (y is greater than x) so that the insolation becomes less intense. Also, with increasing solar zenith angle, the Sun’s rays must pass through more of the Earth’s atmosphere, where they can be scattered and absorbed before reaching the Earth’s surface. Thus, the Sun can heat the surface to a much higher temperature when it is high in the sky, rather than low on the horizon.