Solar Radiation in the Atmosphere

The atmosphere may seem to be completely transparent to solar radiation, but in fact there are dynamic interactions occurring constantly that result in a complex and delicately balanced system crucial to the continuation of present life forms on Earth. In this section we will discuss how solar radiation is absorbed and reflected by the atmosphere and the Earth--and two resulting phenomena that are crucial to the maintenance of life on Earth:

1. The atmosphere acts as a filter, absorbing and reflecting portions of the electromagnetic spectrum, such as the ultraviolet region, that are harmful to humans and other life forms.
   
2. The atmosphere provides a natural "greenhouse effect," maintaining the temperatures and climates in which life forms on Earth have evolved to survive.
   

The atmosphere controls the amount of solar radiation reaching the surface of the earth, and regulates the amount of radiation from the Earth escaping into space. Even seemingly slight changes in the concentrations of certain gases could upset the balance of reactions and be detrimental to life as we know it. We will now explain these two exchanges in detail, and later discuss their relevance to two major environmental problems: global climate change and stratospheric ozone depletion.

To demonstrate how the atmosphere affects incoming solar radiation, Figure 8 shows the solar radiation spectrum first at the top of the atmosphere (A, in red), and again at sea level (B, in blue). The absorption of the molecules shown in Figure 8 is discussed in some detail because it is critical to maintaining some of the most important conditions for our viability on Earth. Note that the solar spectrum shown here is the same as a part of Figure 4, enlarged to show the details of absorption.

Figure 8: Solar spectrum (A) above the atmosphere and (B) near the Earth's surface, with some of the radiation having been absorbed by molecules in the atmosphere. Also depicted is a curve of the spectrum detectable by the eye (C).

This figure shows that ozone (O₃) absorbs ultraviolet. O₂ absorbs ultraviolet as well as some visible and infrared. The ultraviolet absorption properties of O₃ and O₂ are central to the protective ozone layer described later. Water vapor (H₂O) absorbs highly in the range of 0.4 and 0.9 µm and again above 1.2 µm and also in the microwave region. The absorption property of H₂O in the microwave region is, of course, the basis for the efficient cooking of foods by microwave ovens. CO₂ has high absorption around 1.4 µm and above.

When molecules absorb energy, the absorbed energy may go into causing a chemical change (as in cooking food in a microwave oven), or it may be re-emitted. Often molecules re-emit energy at wavelengths longer than that at which it was absorbed. Thus when molecules such as H₂O and CO₂ absorb visible or infrared light, they often re-emit it as longer wavelength infrared. This has great importance in our climate as described later.

The small black curve labeled C in Figure 8 approximates the sensitivity spectrum of our vision, or which wavelengths the eye can detect, with our maximum sensitivity by the green and yellow. This is why yellow light is used for markings on the roads and as the warning light color in traffic lights.

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