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The red line represents the weighting function for the longwave infrared window channel. Notice that there is little atmospheric contribution. Almost all of the energy originates from the earth's surface or from clouds when they are present.
The yellow line represents the weighting function for the "dirty" window channel. It also peaks at low levels, but at a somewhat higher elevation than the longwave infrared window. This is due to the absorption by low-level water vapor in the atmosphere. Notice how the low-level atmospheric contribution to the "dirty" window channel decreases as moisture is removed. In the absence of moisture, these two channels sense the same brightness temperature.