Look! Up in the sky! Light through the atmosphere

Figure 4-1 (earlier in this chapter) shows the difference between the spec­trum of sunlight that hits Earth’s upper atmosphere and the spectrum that reaches ground level. Under normal conditions, as much as 35 percent of solar radiation is reflected back into space or absorbed before it hits the sur­face of the Earth. In the presence of clouds or pollution, the reflection and absorption can be much greater.

Water vapor, carbon dioxide, and ozone absorb 10 to 15 percent of sunlight.

In the upper atmosphere, ozone (three oxygen atoms bonded together, or O3) filters out virtually all the ultraviolet radiation (short wavelength radiation — very energetic. Note in Figure 4-1 that plenty of ultraviolet light comes from the sun, but almost none of it actually reaches the Earth’s surface. If it were to reach the planet’s surface, life would drastically change.

A number of years ago, people were greatly concerned about the hole in the ozone layer. But this concern was partially allayed with the banning of certain aerosols and refrigerants that contained ozone-depleting compounds called chlorofluorocarbons (CFCs), commonly known as Freon. Here’s a good example of how environmental policy actually works!

image075In addition, atmospheric depth makes all the difference. At noon, when the sun is directly overhead, sunlight passes through much less atmosphere than at dawn or dusk (see Figure 4-2).

You can see why the middle of the day is hottest. It’s not because, as some people believe, the sun is closer. Rather, the sunlight passes through much less atmosphere. The Earth is tilted on its axis, -23° to be precise. Figure 4-3 shows what happens as Earth rotates.

The sun seems hotter in the summer than in the winter, but that’s not really true. The sun doesn’t change; it’s the air and the position of the Earth that change. Less sunlight energy reaches the Earth’s surface in the winter for the same reason that you get less solar energy at dawn and dusk.

Figure 4-2:

Подпись: AtmosphereПодпись:image078The amount of atmosphere light passes through at different times of day.






depth of the atmosphere.