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.