Radiation: Solar and Terrestrial

Electromagnetic Radiation

Radiation is a form of energy emitted by everything that has a temperature above absolute zero. Formally it was known as “electromagnetic radiation” since it involves electromagnetic forces with the components of an atom. Our current understanding of this form of energy gives it a dual nature. First, it can be conceived as a series of small bundles of energy, called photons, which travel at the speed of light and vibrate as they travel; the greater the number of photons, the greater the amount of energy. Because of this vibration and forward movement, photons appear to be propagated as a series of waves, so our second concept is of radiation as a series of moving waves. The two concepts are closely related, and the fundamental laws governing radiation indicate that the higher the temperature of the [Page 2347]emitting body, the shorter the wavelength and the higher the energy of the radiation.

In practical terms, we can conveniently divide the “bodies” doing the emitting into two classes: (1) the sun, which is very hot and emits great amounts of energy at a range of short wavelengths, and (2) Earth and everything on its surface and in its atmosphere, which are much cooler and emit much less energy and at a range of much longer wavelengths. Within the range of wavelengths emitted by the sun—the solar spectrum—there is a small portion that stimulates our eyes and that we sense as light. The shortest of these we sense as blue light, the longest as red light. So we can divide the solar spectrum into three regions: ultraviolet, visible, and (near) infrared. We do not sense the energy emitted by Earth and its atmosphere in any similar way. But it is sometimes, confusingly, called “infrared” energy, particularly when photographs using special films that are sensitive to these longer wavelengths are involved.

The emission of energy is a cooling process for the body doing the emission, while absorption leads to heating. So, in general, the absorption of solar energy by our planet warms the planet. As the temperature increases, the amount of terrestrial radiation the planet emits increases. In a static universe, planet Earth would reach a constant temperature where the amount of solar energy absorbed would be exactly balanced by the amount emitted to space. However, we do not have a static situation. Fundamentally, the changing position of the sun in the sky gives a changing solar input. Meanwhile changes, natural or anthropogenic, in the atmosphere and on the surface influence the fate of that solar energy. We can think of the planet as constantly striving to achieve this equilibrium temperature. This entry describes the changes that are primarily a response to the changing solar input.

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