* The ground is a little warmer than the atmosphere, so that factor will mean some more photons going up than down (but
since the back
radiation is mostly from low layers, the atmosphere emitting the back
radiation will not be that much cooler than the land so the effect from temperature will not be TOO great) * The ground is close to a
black body for IR (emissivity = 1 for all IR frequencies), but the atmosphere has bands where it does not emit or absorb well (emissivity ~ 0) and other bands where it does emit or absorb well (emissivity ~ 1).