Visibility, ultraviolet (UV) radiation intensity, and optical phenomena are affected by gases, aerosol particles, and hydrometeor particles interacting with solar radiation. In clean air, gases and particles affect how far we can see along the horizon and the colors of the sky, clouds, and rainbows. In polluted air, gases and aerosol particles affect visibility, optical phenomena, and UV radiation intensity. In this chapter, visibility, optics, and UV transmission in clean and polluted atmospheres are discussed. An understanding of these phenomena requires a study of the interaction of solar radiation with gases, aerosol particles, and hydrometeor particles through several optical processes, including reflection, refraction, diffraction, dispersion, scattering, absorption, and transmission. These processes are described next.

PROCESSES AFFECTING SOLAR RADIATION IN THE ATMOSPHERE

The solar spectrum is divided into UV (0.01 to 0.38 μm), visible (0.38 to 0.75 μm), and near-infrared (IR) (0.75 to 4.0 μm) wavelength ranges (Section 2.2).

In 1666, Sir Isaac Newton (1642–1727; Fig. 7.1), an English physicist and mathematician, showed that when white, visible light passed through a glass prism, each wavelength of the light bent to a different degree, resulting in the separation of the white light into a variety of colors that he called the light spectrum. Although the spectrum is continuous (the eye can distinguish 10 million colors), Newton discretized the spectrum into seven colors: red, orange, yellow, green, blue, indigo, and violet, to correspond to the seven notes on a musical scale. When the colors of the spectrum were recombined, they reproduced white light.