The primary source of energy that drives all processes that take place in Earth's atmosphere and on the surface is the Sun. The Sun's energy comes to us mostly in the form of electromagnetic radiation, or light. Understanding climate, weather, biological processes, and the interactions among these requires understanding the nature of electromagnetic radiation and how it interacts with Earth's atmosphere, surface, and biota.

Electromagnetic radiation interacts with atoms and molecules in a wide variety of ways, both in the atmosphere and in the biosphere. The interactions of the Sun's light with the constituents of the atmosphere causes much of the higher energy radiaiton, which can be harmful to living organisms, to be absorbed so that little or none of this radiation reaches Earth's surface. Anything that alters the chemical composition of the atmosphere has the potential to affect the amount of light, or other electromagnetic radiation, that falls on Earth's surface. We can measure the radiation that is scattered into space and use those measurements to determine the concentrations of many important chemicals in Earth's atmosphere.

1.1 Chapter Objectives

In this chapter we will describe the essential features of light and the characteristics of its interaction with constituents of the atmosphere: atoms, molecules, and relatively large, semitransparent particles such as water droplets, ice crystals, and other aerosols. We will describe the light that comes from the Sun and reaches the top of the atmosphere, how light is changed by its interactions with constituents in the atmosphere (and how those constituents are changed in turn) and describe the light that falls on Earth's surface. We will also discuss the radiation that leaves Earth.

This chapter will address