Infrared window

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. The principal natural greenhouse gases in order of their importance are water vapor waterH2O, carbon dioxide carbon dioxideCO2, ozone ozoneO3, methane methaneCH4 and nitrous oxide Nitrous oxideN2O. The concentration of the least common of these, nitrous oxideN2O, 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).