I have a very basics question. I have a question about absorption in molecular bands. When looking at spectra in the infrared region and comparing it with simulated spectra, I can see that some lines get partially absorbed and then grows to fully absorbed in the "center" of the absorption feature. Also some lines to be fully absorbed and some lines seem to be only partially absorbed in H20 spectra. What determines if a spectral line is fully absorbed or only partially? Has it to do with the temperature and how much is occupied in a certain level in the atom or molecule?