Earth, like the other inner planets, receives virtually all of its energy from space in the form of solar electromagnetic radiation. Its total heat content does not vary significantly with time, indicating a close overall balance between absorbed solar radiation and the diffuse stream of low-temperature, thermal radiation emitted by the planet. The transformation of the incident solar radiation into scattered and thermal radiation, and the thermodynamic consequences for the earth’s gaseous envelope, are the subjects of this book. The scope must be narrowed, however, because in its broadest interpretation our title could include atmospheric photochemistry and many other topics usually treated in books dealing with the upper atmosphere. By restricting attention to the thermodynamic aspects, this problem of selection usually resolves itself. For example, the absorption of energy accompanying photodissociation or photoionization will be considered if the energy involved is comparable to that of other sources or sinks, but not otherwise. Similarly, the oxygen airglow has some thermodynamic consequences in the upper atmosphere, but the important topic of the airglow will be mentioned only in this limited context. The irradiance at mean solar distance—the solar constant—is slightly less than 1400 Wm-2, giving an average flux of solar energy per unit area of the earth’s surface equal to 350 W m-2 (the factor 4 is the ratio of surface area to cross section for a sphere). Of this energy, approximately 31% is scattered back into space, 43% is absorbed at the earth's surface, and 26% is absorbed by the atmosphere. The ratio of outward to inward flux of solar radiation is known as the albedo. We may speak of the albedo of the entire earth or of individual surfaces with reference either to monochromatic radiation or to a weighted average whole is about 0.31, and an average of 224 Wm-2 is available for heating, directly and indirectly, the earth and its atmosphere. The redistribution of this absorbed solar energy by dynamic and radiative processes and its ultimate return to space as low-temperature planetary or terrestrial radiation are the most important topics of this book.
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