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 labeled as Upward IR Heat Flux, in units of Watts / meter2.
Example of terrestrial radiation spectrum obtained by the Nimbus 3 IRIS instrument for clear sky conditions (from Hanel et al., 1972).
Here is a technical description.