How AIRS Works : (sounding)

Ref : http://airs.jpl.nasa.gov/technology/how_AIRS_works/

Atmospheric Infrared Sounder (AIRS)

Each infrared wavelength is sensitive to temperature and water vapor over a range of heights in the atmosphere, from the surface up into the stratosphere. 

By having multiple infrared detectors, each sensing a particular wavelength, 

a temperature profile, or sounding of the atmosphere, can be made. 

While prior space instruments had only 15 detectors, AIRS has 2378. This greatly improves the accuracy, making it comparable to measurements made by weather balloons.