As electromagnetic waves pass through a gas, certain wavelengths of light are absorbed by the gas due to the energy levels present in the gas molecules. An absorption spectrum will look like a full spectrum with small gaps at specific wavelengths. Different elements and molecules have different absorption spectra. An example of an absorption spectrum is to the left.

When observing electromagnetic radiation emitted from Earth, understanding absorption spectra is important. As the radiation moves up through the atmosphere, some of it is absorbed and understanding which wavelengths are absorbed by the gases in the atmosphere is important. In the far infrared that I am observing, water vapor is the greatest absorber, and when in high enough concentrations it can make the ground opaque to a space-based observer. This has both upsides and downsides. The upside is that it can be used to observe the concentrations of certain molecules in the atmosphere such as water vapor or CO2. However, it can make ground observations quite difficult a high concentrations. In the picture to the right you can see that as the wavelength moves the mid and far infrared the absorbtion of water vapor increases.