Modtran
This model simulates the emission and absorption of infrared radiation in the atmosphere. The smooth lines are theoretical emission spectra of blackbodies at different temperatures. The red line, labeled "Atm" is the spectrum of infrared light at the top of the atmosphere looking down. The total energy flux from all IR light is listed as Iout, in units of Watts / meter2.
Note. This is an old model (1990's), illustrative but not necessarily in strict quantitative agreement with current state of the art line-by-line models.
If you have trouble with the interactive version, try the version labeled non-Flash.