Since the surface of the Sun is very hot (6,000 K), it emits much of its energy at short wavelengths.
The cooler surface of Earth (300 K) emits most of its energy at longer, infrared wavelengths, wavelengths too long to be detected by the human eye.
Humans cannot see infrared wavelengths, but they can feel warmth on their skin when this energy is absorbed. Thus, it is sometimes casually referred to as “heat radiation.”
Energy across all wavelengths from the Sun must pass through the atmosphere before reaching Earth’s surface. The atmosphere filters, absorbs, and reflects incoming solar energy, as shown in Figure 22.6. On average, Earth as a whole reflects about 30 percent of the total energy back into space, with two-thirds of the reflection occurring from clouds. The amount reflected at any one time depends on the extent of cloud cover, the amount of dust in the atmosphere, and the extent of snow and vegetation on the surface. Substantial changes in any of these influencing variables could increase or decrease the Page 548reflectivity, leading to increased heating or cooling of the atmosphere.
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