This is the first of some pages that will explain the details of emission spectra and their significance in the understanding of the greenhouse effect
This is an emission spectrum of a part of the Earth taken by satellite and it shows most of the main features of the spectra of the greenhouse gases. The units of radiance are milliwatts per square metre per steradian per reciprocal centimetre! The spectrum was recorded in 1974 and the machinery on board the satellite was not capable of recording radiance below 400 cm-1. For this reason further spectra of this kind on the website are simulated by using the MODTRAN facility provided by the University of Chicago. MODTRAN is the moderate resolution transmission programme which is linked to a database [HITRAN] of very detailed absorption spectra of all the greenhouse gases. It allows the calculation of the emission intensity of the chosen gaseous mixture. [It was used to generate the absorption spectrum of CO2 given on a previous page.]
As an aid to understanding the spectra there are several blackbody emission curves superimposed on the diagram. These are for different temperatures and allow conclusions to be made about the regions of the atmosphere from which the various sections of the spectra originate. More details are discussed on subsequent pages.
As an example of the temperature interpretation, the region from 800-1000 cm-1 shows that the emission comes from a region with a temperature about 295 K or 22° C. That spectral region forms part of the infrared 'window' where none of the greenhouse gases show appreciable absorption and that radiation originates at the Earth's surface. It may be assumed that 22° C is the temperature of the surface covered by the satellite when recording the spectrum.