H/T to ‘intrepid Wanders‘ for this repost from the Uni of Reading meteorology section. No settled science here, and lab model derived from far IR wavebands used in climate models and energy budget diagrams rests on a bunch of assumptions. Who knew? Obviously not Trenberth, who has no error bounds on his energy budget. So along with cloud microphysics getting the predicted absorption of energy by clouds wrong by a large margin, we have big uncertainty in the spectral absorption lines of water vapour. Ho hum. Business-as-usual in climate science land.
Water vapour continuum
In addition to the spectral lines, it has long been recognized that water vapour possesses a continuum absorption which varies relatively slowly with wavelength and pervades the entire IR and microwave spectral region. This has a marked impact on the Earth’s radiation balance with consequences for understanding present day weather and climate and predicting climate change. It is also important for remote sensing of the Earth and its atmosphere.
Discovered by Hettner (1918) as a low-frequency component of water vapour absorption in atmospheric transparency window 8-14 mcr, this phenomenon remained unexplained for 20 years, until Elsasser (1938) suggested that the continuum is an accumulated far-wingcontribution of strong water vapour spectral lines from neighbour bands. This hypothesis was generally accepted until the end of 70th years when the strong quadratic pressure dependence of the continuum absorption (which could not be explained by Lorentz (1906) line profile) as well as the strong negative temperature dependence have been detected (Bignell et al.,1963;Penner and Varanasi,1967). In this connection Penner and Varanasi (1967) and Varanasi et al. (1968) suggested that the main contribution to the self-continuum could be caused not by far wings of water monomer lines but rather by water dimers. Similar assumption was made also by Viktorova and Zhevakin (1967) for microwave spectral region.