For purposes of keeping a budget of energy in the earth-atmosphere system, it is customary to break the electromagnetic spectrum into two broad categories. The shorter, visible wavelengths represent energy from the sun. This energy may interact with the atmosphere through the processes of scattering, reflection from bright surfaces, absorption, and transmission. From the point of view of energy involved, very little energy is emitted by substances at the normal temperature of the earth-atmosphere system. Longer, infrared wavelengths (sometimes referred to as "thermal wavelengths" or "terrestrial wavelengths") embody that portion of the electromagnetic spectrum where substances with temperature common to the earth-atmosphere system will emit radiant energy. The principal source of this thermal radiation is earth's surface, since it is generally the warmest part of the earth-atmosphere system. The atmospheric greenhouse gases are efficient absorbers of infrared energy, so a good deal of earth's radiant emission is absorbed by these gases, and then those air layers themselves will emit infrared radiation according to their temperature. Additionally, water in the liquid state (for example, cloud droplets) is a very strong absorber of infrared radiation, so clouds have an important role in the energy budget of the atmospheric greenhouse.
