Earth emits most of its radiation in the infrared wavelengths owing to its comparitively cool temperature, as explained by Planck's law. Taken together, the gases in the atmosphere can absorb a good deal of this infrared, or "terrestrial", radiation emitted by the surface of earth. Because these gases are reasonably good absorbers of infrared radiation, they are also good emitters, and so emission of infrared radiation by the atmospheric gases must be factored into earh-system science considerations. On the other hand, reflection of infrared radiation by atmospheric gases, earth's surface, and clouds is a process which is very small and which can be ignored in comparison with absorption, emission, and transmission.