EUMeTrain: Water Vapour CAL Module - Water Vapour Channels

Water vapour channels' sensitivity range - Weighting functions

Because in the 6,2 µm channel water vapour absorbs radiation strongly, and the lower, warmer layers of the troposphere are more humid, only signals from the upper atmosphere will reach the satellite in this band. In the 7,2 µm channel radiation is less absorbed, so radiation from lower levels can reach the satellite. Neither channel sees to the surface, though.

In the following image there are the weighting functions that show the relative vertical contribution of radiation emitted by atmospheric substances in different IR channels. The two water vapour bands dominate the upper and middle troposphere.

You can study further the influence of temperature and humidity on the contribution functions here: http://satreponline.org/weighting/applet1.html.

The sensitivity of a channel indicates its ability to detect differences in humidity at different altitudes:

Channel	Large sensitivity/hPa	Largest sensitivit/hPa	Lower threshold/hPa
6.2 µm	200 - 600	400	750
7.3 µm	400 - 740	600	950