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The infrared wavelengths used to construct the infrared satellite images pretty much pass right through the Earth's atmosphere, unobstructed by anything but clouds. But there are other infrared wavelengths which are absorbed (and emitted) by one or more gases in the atmosphere. A satellite instrument tuned to such a wavelength can determine the temperature of these gases which emit the radiation.
One such gas is water vapor. Near a wavelength of 6.5 microns, water vapor absorbs and emits radiation. Consider a column of air which contains water vapor. The radiation emitted by the water vapor near the bottom of the column ends up getting absorbed by all the water vapor above it. Meanwhile, very little radiation is emitted from near the top of the air column because there is very little water vapor there. Most of the radiation escaping from the atmosphere at that wavelength will have been emitted from somewhere in the middle of the layer of water vapor. If there's lots of water vapor, this level will be high in the atmosphere where the temperature is cold, so the radiation will be weak. If there's very little water vapor, the radiation will come from low down in the atmosphere, where the temperature is warm, so the radiation will be strong. The average source level for the radiation is about 400 mb.
Here's a water vapor image, for the same time as the other images. The image has been colored in the conventional manner, so that cold areas (with lots of water vapor) are light and warm areas (with little water vapor in the upper atmosphere) are dark. High clouds still show up, as especially bright areas. But the usefulness of water vapor imagery is in those areas where there are no clouds. For example, over Mexico there are several light and dark streaks visible. These correspond to bands of water vapor being carried from the subtropics toward the southeastern United States.
Because the water vapor imagery shows structures even where there are no clouds, it is useful for deducing upper-level wind patterns. Features in the water vapor images tend to move at the same speed as features in the upper-level jet streams. Also, the water vapor pattern themselves are used to help forecast the locations of thunderstorm outbreaks and the potential for heavy rains.
Keep in mind that the water vapor image only shows upper-level moisture. For low-level and surface humidity, you must use surface measurements
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