 Let's examine the wavelength spectrum of radiance observed by a satellite looking down at a location on Earth. Because the absorptivity and thus emissivity of different gases changes dramatically from 6 to 25 microns, the satellite is observing radiance from different types of matter at different wavelengths. The radiance depends on temperature, so once we know the radiance, we know the temperature of the object that is radiating. The Planck distribution function spectral radiance is plotted for curves of different temperatures from 200 Kelvin to 300 Kelvin. Thus the radiance gives us the object's temperature, and since we have a rough idea about the temperature profile of the atmosphere, we can make a pretty good guess at the height of the radiating object and what is actually radiating, whether it be Earth's surface or a gas like water vapor, carbon dioxide, or ozone. Between 8 and 13 microns, no infrared gas absorbs very well in the atmosphere, except for ozone around 9.6 microns. Note that the radiance in this window came from matter at a temperature near 300 Kelvin or 27 degrees C. From the satellite's position, this radiance is known to come from the ocean Pacific. At the edges of the strong water vapor absorption band at 6 microns, say at about 7.5 microns, note that the radiating temperature is about 260 Kelvin. This radiance must be coming from water vapor at 10 to 20,000 feet altitude. At 6 microns, the temperature is quite a bit lower, and so therefore this radiance comes from water vapor at a much greater altitude in the atmosphere. In the CO2 absorption band near 15 microns, the radiance is equivalent to a temperature of 220 Kelvin, which is from CO2 near the tropopause, since this is the lowest radiating temperature that we see. Note that little spike in the middle of the strongly absorbing CO2 band. It is coming from CO2 that is warmer than the tropopause, but we know that it must be coming from above the tropopause because the center of the CO2 band absorbs the strongest, and thus this radiance must be coming from CO2 higher than above the tropopause. It must be coming from the stratosphere. This makes sense. The stratosphere is warmer than the tropopause. So we can actually learn a lot about what is being observed simply by looking at the satellite thermal infrared spectrum like this one.