The infrared atmospheric window refers to a region of the Infrared spectrum where there is relatively little absorption of terrestrial thermal radiation by atmospheric gases. The window plays an important role in the atmospheric greenhouse effect by maintaining the balance between incoming solar radiation and outgoing IR to space. In the Earth's atmosphere this window is roughly the region between 8 and 14 μm although it can be narrowed or closed at times and places of high humidity because of the strong absorption in the water vapor continuum or because of blocking by clouds. It covers a substantial part of the spectrum from surface thermal emission which starts at roughly 5 μm. Principally it is a large gap in the absorption spectrum of water vapor. Carbon dioxide plays an important role in setting the boundary at the long wavelength end. Ozone partly blocks transmission in the middle of the window. The importance of the infrared atmospheric window in the atmospheric energy balance was discovered by George Simpson in 1928, based on G. Hettner's 1918 laboratory studies of the gap in the absorption spectrum of water vapor. In those days, computers were not available, and Simpson notes that he used approximations; he writes about the need for this in order to calculate outgoing IR radiation: "There is no hope of getting an exact solution; but by making suitable simplifying assumptions . . . ." Nowadays, accurate line-by-line computations are possible, and careful studies of the spectroscopy of infrared atmospheric gases have been published. Mechanisms in the infrared atmospheric window The principal natural greenhouse gases in order of their importance are water vapor , carbon dioxide , ozone , methane and nitrous oxide . The concentration of the least common of these, , is about 400 ppbV. Other gases which contribute to the greenhouse effect are present at pptV levels. These include the chlorofluorocarbons (CFCs) and hydrofluororcarbons (HFC and HCFCs). As discussed below, a major reason that they are so effective as greenhouse gases is that they have strong vibrational bands that fall in the infrared atmospheric window. IR absorption by at 14.7 μm sets the long wavelength limit of the infrared atmospheric window together with absorption by rotational transitions of at slightly longer wavelengths. The short wavelength boundary of the atmospheric IR window is set by absorption in the lowest frequency vibrational bands of water vapor. There is a strong band