The following graph shows the absorption of long-wave radiation by the third 3-km layer of the Earth's atmosphere, from altitude 6 km to 9 km. The graph is typical of a clear day in April, at latitude 30° North. Water vapor content is 1,000 ppm (that's 0.1%), CO2 is 330 ppm, air pressure is 550 mbar, and air temperature is 250 K (that's −23°C). This line is the same as the 6-km line in our Earth's Atmosphere post.
As before, we say our 3-km layer is transparent at a particular wavelength when it absorbs less than 63% of that wavelength. The third 3-km layer is transparent from 5.0 to 5.4 μm, 7.3 to 13.8 μm, and 16.3 to 23.1 μm.
The transparency in the range 16.3 to 23.1 μm is due to the drop in water vapor concentration and pressure that occurs as we ascend through the atmosphere. Absorption in this band is due mostly to water vapor dimers instead of bonds within the water molecule. Absorption by dimers is proportional to the square of the pressure. Continued absorption in 5.4 to 7.3 μm is due to bonds within water vapor molecules, and does not drop with pressure as quickly as absorption by dimers. Continued absorption in the range 13.8 to 16.3 μm range is mostly due to CO2.