Infrared window

As the main part of the 'window' spectrum, a clear electromagnetic spectral transmission 'window' can be seen between 8 and 14 μm. A fragmented part of the 'window' spectrum (one might say a louvred part of the 'window') can also be seen in the visible to mid-wavelength infrared between 0.2 and 5.5 μm.

The infrared atmospheric window is an atmospheric window in the infrared spectrum where there is relatively little absorption of terrestrial thermal radiation by atmospheric gases.[1] 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.[2][3][4][5][6] 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[7] 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 ... ."[8] Nowadays, accurate line-by-line computations are possible, and careful studies of the spectroscopy of infrared atmospheric gases have been published.

  1. ^ "American Meteorological Society Meteorology Glossary".
  2. ^ Paltridge, G.W.; Platt, C.M.R. (1976). Radiative Processes in Meteorology and Climatology. Elsevier. pp. 139–140, 144–7, 161–4. ISBN 0-444-41444-4.
  3. ^ Goody, R.M.; Yung, Y.L. (1989). Atmospheric Radiation. Theoretical Basis (2nd ed.). Oxford University Press. pp. 201–4. ISBN 0-19-505134-3.
  4. ^ Liou, K.N. (2002). An Introduction to Atmospheric Radiation (2nd ed.). Academic. p. 119. ISBN 0-12-451451-0.
  5. ^ Stull, R. (2000). Meteorology, for Scientists and Engineers. Delmont CA: Brooks/Cole. p. 402. ISBN 978-0-534-37214-9.
  6. ^ Houghton, J.T. (2002). The Physics of Atmospheres (3rd ed.). Cambridge University Press. pp. 50, 208. ISBN 0-521-80456-6.
  7. ^ Hettner, G. (1918). "Über das ultrarote Absorptionsspektrum des Wasserdampfes". Annalen der Physik. 4. 55 (6): 476–497 including foldout figure. Bibcode:1918AnP...360..476H. doi:10.1002/andp.19183600603. hdl:2027/uc1.b2596204.
  8. ^ "Archived copy". Archived from the original on 2008-04-22. Retrieved 2009-06-26.{{cite web}}: CS1 maint: archived copy as title (link) Simpson, G.C. (1928). "Further Studies in Terrestrial Radiation". Memoirs of the Royal Meteorological Society. 3 (21): 1–26.