Therefore, water vapor strongly emits at these wavelengths as well (according to Kirchoff's Law). As you can tell from our familiar atmospheric absorption chart, these wavelengths are mostly absorbed by the atmosphere, and by water vapor in particular. In case you didn't catch it in the paragraph above, let me be clear: Water vapor imagery is another form of infrared imagery, but instead of using wavelengths that pass through the atmosphere with little absorption (like traditional infrared imagery), water vapor imagery makes use of slightly shorter wavelengths between about 6 and 7 microns. Now, what if we took the opposite approach? What if we looked at a portion of the infrared spectrum where atmospheric gases (namely water vapor) absorbed nearly all of the terrestrial radiation? What might we learn about the atmosphere? Water vapor imagery addresses that question. Meteorologists can use visible and infrared imagery to look at the structure and movement of clouds because these types of images are created using wavelengths at which the atmosphere absorbs very little radiation (so radiation reflected or emitted from clouds passes through the clear air to the satellite without much absorption). Our look at visible and infrared imagery has hopefully shown you that using a variety of wavelengths in remote sensing is helpful because this approach gives us a more complete picture of the state of the atmosphere.
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