Infrared window 1638070 221762243 2008-06-25T23:59:16Z SmackBot 433328 Date the maintenance tags or general fixes {{Merge|Atmospheric window|date=June 2008}} {{Unreferenced|date=February 2008}} [[Image:Atmosfaerisk_spredning.gif|thumb|right|An atmospheric transmission 'window' can be seen between 8-14 µm.]] The '''atmospheric window''' refers to those parts of the [[electromagnetic spectrum]] that are, with the [[earth's atmosphere]] in its natural state, ''not absorbed at all''. One atmospheric window lies approximately at [[wavelengths]] of [[infrared radiation]] between 8 and 14 [[micrometre]]s. ==Causes== The absorptions of the principal ''natural'' [[greenhouse gas]]es are concentrated in two belts. Gases such as [[carbon dioxide|CO<sub>2</sub>]] and [[methane|CH<sub>4</sub>]] (along with less abundant [[hydrocarbons]]) absorb at wavelengths longer than this window due to the presence of relatively long C-H and [[carbonyl]] [[chemical bond|bonds]]. The bonds of [[water|H<sub>2</sub>O]] and [[ammonia|NH<sub>3</sub>]] absorb at wavelengths shorter than 8 micrometres. Except for the bonds in [[ozone|O<sub>3</sub>]], no bonds between carbon, hydrogen, oxygen and nitrogen atoms absorb between these two ranges. This means that most of the radiation emitted by the earth's surface at wavelengths within in the atmospheric window pass through the Earth's atmosphere without heating it; instead, this radiation is emitted into space. Without this window, the Earth would become much too warm to support life, and possibly so warm that it would lose its water as [[Venus (planet)|Venus]] did early in [[solar system]] history. Thus, the existence of a window in the electromagnetic spectrum is critical to Earth remaining a [[Planetary habitability|habitable planet]]. ==Threats== In recent decades, the existence of the atmospheric window has become threatened by the development of highly unreactive gases containing bonds between [[fluorine]] and either [[carbon]] or [[sulfur]]. The "stretching frequencies" of bonds between fluorine and other light [[nonmetal]]s are such that strong absorption in the atmospheric window will always be characteristic of compounds containing such bonds. This absorption is strengthened because these bonds are highly polar because of the extreme [[electronegativity]] of the fluorine atom. Bonds to other [[halogens]] also absorb in the atmospheric window, though much less strongly. Moreover, the unreactive nature of such compounds that makes them so valuable for many industrial purposes means that they are not removable in the natural circulation of the Earth's atmosphere. It is estimated, for instance, that perfluorocarbons (CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>8</sub>) can stay in the atmosphere for over fifty thousand years, a figure which may be an underestimate given the absence of natural sources of these gases. This means that such compounds have an enormous global warming potential. One [[kilogram]] of [[sulfur hexafluoride]] will, for example, cause as much warming as ''23 tonnes of carbon dioxide'' over 100 years. Perfluorocarbons are similar in this respect, and even [[carbon tetrachloride]] (CCl<sub>4</sub>) has a global warming potential of 1800 compared to carbon dioxide. Efforts to find substitutes for these compounds are still going on and remain highly problematic. ==See also== *[[Astronomical window]] *[[Greenhouse effect]] *[[Greenhouse gas]] *[[Optical window]] *[[Ozone depletion]] *[[Radio window]] ==External links== *[http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irwindows.html IR Atmospheric Window] [[Category:Atmosphere|Atmospheric window]] [[Category:Climate change|Atmospheric window]] [[Category:Climate forcing|Atmospheric window]] [[Category:Atmospheric radiation]] [[Category:Satellite meteorology and remote sensing]] [[Category:Electromagnetic spectrum]] [[de:Atmosphärisches Fenster]] [[fr:Fenêtre atmosphérique]] [[ja:大気の窓]] [[no:Atmosfærisk vindu]] [[nn:Atmosfærisk vindauge]] [[pl:Okno atmosferyczne]]