#Emission spectra free

The top of the Atmosphere curve is mostly due to the lapse rate, where higher levels of the Atmosphere tend to be cooler.

#Emission spectra free

Radiation in the ~10μ region has taken a pretty-much free pass through the Atmosphere. H2O has taken its toll in the ~7μ and ~15μ regions, while CO2 has taken its bite in its special part of the ~15μ region. Well, if a piece of blueberry pie has gone missing, and little Johnny has blueberry juice dripping from his mouth and chin, and that is pretty good circumstantial evidence of who took it.Ĭlearly, the GHGs in the Atmosphere are responsible. What Does This All Mean in Plain Language? Note how this bite corresponds roughly with the CO2 absorption spectrum. Part of it, from about 17μ and up approximates a 260ºK or 270ºK curve, but the region from about 14μ to 17μ has had quite a big bite taken out of it. Note that much of the ~10μ region approximates a 295ºK curve while the ~7μ region approximates a cooler 260ºK curve. The dashed line represents a “blackbody” curve characteristic of 300ºK. Top of the Atmosphere: The upper violet/purple curve (adapted from figure 6.6 in Petty and based on satellite measurements from the Tropical Pacific looking DOWN) indicates how the top of the Atmosphere passes certain portions of radiation from the surface of the Earth out to Space and re-emits selected portions up towards Space. Since gases tend to re-emit most strongly at the same wavelength region where they absorb, the ~7μ and ~15μ are well-represented, while the ~10μ region is weaker. Neither H2O nor CO2 absorb strongly in the ~10μ region. CO2 absorbs at up to 100% in the ~15μ region. H2O also absorbs strongly in the ~15μ region, particularly above 20μ, where it reaches 100%. Note that H2O absorbs at up to 100% in the ~7μ region. (I’ve included multiple H2O’s because water vapor, particularly in the tropical latitudes, is many times more prevalent than carbon dioxide.) “Greenhouse Gases”: The reason for the shape of the downwelling radiation curve is clear when we look at the absorption spectra for the most important GHGs: H2O, H2O, H2O, … H2O, and CO2. Note how the ~7μ and ~15μ regions approximate that curve, while much of the ~10μ region is not re-emitted downward. The dashed line represents a “blackbody” curve characteristic of 300ºK (equivalent to 27✬ or 80✯). The lower violet/purple curve (adapted from figure 8.1 in Petty and based on measurements from the Tropical Pacific looking UP) indicates how the bottom of the Atmosphere re-emits selected portions back down towards the surface of the Earth. Most of the ~10μ radiation is allowed to pass through. This radiation approximates a smooth “blackbody” curve that peaks at the wavelength corresponding to the surface temperature.īottom of the Atmosphere: On its way out to Space, the radiation encounters the Atmosphere, in particular the GHGs, which absorb and re-emit radiation in the ~7μ and ~15μ regions in all directions. Many thanks to Dave Springer and Jim Folkerts who, in comments to my previous posting Atmospheric Windows, provided links to emission graphs and a textbook “A First Course in Atmospheric Radiation” by Grant Petty, Sundog Publishing Company.ĭescription of graphic (from bottom to top):Įarth Surface: Warmed by shortwave (~1/2μ) radiation from the Sun, the surface emits upward radiation in the ~7μ, ~10μ, and ~15μ regions of the longwave band.

A real greenhouse primarily restricts heat escape by preventing convection while the “greenhouse effect” heats the Earth because GHGs absorb outgoing radiative energy and re-emit some of it back towards Earth. Perhaps a better analogy would be an electric blanket that, in addition to its insulating properties, also emits thermal radiation both down and up.

The Atmospheric “greenhouse effect” has been analogized to a blanket that insulates the Sun-warmed Earth and slows the rate of heat transmission, thus increasing mean temperatures above what they would be absent “greenhouse gases” (GHGs).