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 The simplest climate model is the energy balance model, which is concerned with the amount of energy received and emitted by the Earth. All matter emits electromagnetic radiation, and the wavelength (λ) of that emitted radiation straightforwardly varies with the temperature of the object. The Sun, a relatively hot object, emits E/M radiation across a very wide spectrum, from very short-wave gamma radiation (λ > 10$-12$ m) to very long-wave microwave and radio radiation (λ > 10$2$ m). Some of the radiation emitted by the Sun, of course, is in the very narrow range of the E/M spectrum that is visible to the naked human eye (λ = ~.4-.8 x 10$-6$ m). The surface temperature of the sun is approximately 5,778K; this means that the sun’s peak E/M emission—that is, the area of the E/M spectrum with the most intense emission—falls into this visible spectrum, at somewhere around λ = .5-.6 x 10$-6$ m. This corresponds to light that normal humans perceive as yellowish-green (the sun appears primarily yellow from Earth because of atmospheric scattering of light at the blue end of the visible spectrum). Similarly, the Earth emits electromagnetic radiation. However, the Earth is (thankfully) much cooler than the sun, so it radiates energy at a significantly different wavelength. Peak E/M emission wavelength is inversely proportional to the temperature of the radiator (this is why, for instance, the color of a heating element in a toaster progresses from red, to orange, to yellow as it heats up), and the Earth is sufficiently cold so that its peak E/M emission is somewhere around λ = 20 x 10$-6$ m. This means that the Earth’s emission is mostly in the infrared portion of the spectrum, a fact which plays a very significant role in the dynamics of the greenhouse effect (see Section 4.1.3).

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