Some of the infrared radiation leaving the atmosphere originates near the earth’s surface and is transmitted relatively unimpeded through the atmosphere; this is the radiation from areas where there are no clouds and which is present in the part of the spectrum known as the atmospheric ‘‘window’’ (Fig. 2). The bulk of the radiation, however, is intercepted and reemitted both up and down. The emissions to space occur either from the tops of clouds at different atmospheric levels (which are almost always colder than the surface) or by gases present in the atmosphere that absorb and emit infrared radiation. Most of the atmosphere consists of nitrogen and oxygen (99% of dry air), which are transparent to infrared radiation. It is the water vapor, which varies in amount from 0 to approximately 3%, carbon dioxide, and some other minor gases present in the atmosphere in much smaller quantities that absorb some of the thermal radiation leaving the surface and reemit radiation from much higher and colder levels out to space. These radiatively active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effects of a greenhouse. Note that while a real greenhouse does work this way, the main heat retention in a greenhouse actually comes through protection from the wind. In the current climate, water vapor is estimated to account for approximately 60% of the greenhouse effect, carbon
dioxide 26%, ozone 8%, and other gases 6% for clear skies.