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Greenhouse gases feature in many courses in applied chemistry as well as in courses on environmental science, atmospheric chemistry, and so on. We hear a great deal about such matters from the media. A more classical topic which first-year chemistry students will learn about in the physical chemistry component of their program is the black body concept and the Stefan–Boltzmann law. Recently in teaching about greenhouse gases to MSc students I have thought of a link between the topical and classical matters that is pedagogically interesting and might be of value to others teaching in this subject area.
A “gray body” is of course not simply a non-black one: it is one that has a lower emissivity than a black body and has that same emissivity for all wavelengths of interest. A body that is “non-gray” has different emissivities at different wavelengths. The part of the atmosphere containing greenhouse gases is transparent to radiation characteristic of the surface temperature of the sun—about 5000 K—and opaque to radiation characteristic of the temperature of the earth’s surface. Is it therefore unreasonable or unsound to describe that part of the atmosphere as being non-gray? It should be remembered that a black body can be entirely gaseous: many hydrocarbon/air flames approximate to black body behavior.
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