An atmospheric sounder measures how the physical properties of a column of air vary with altitude. The measurement as a function of altitude is sometimes called a “profile”, a “sounding”, or a “retrieval”. The term "sounder" refers to measuring how the temperature and salinity are similarly measured in the ocean using sound waves. “Retrieval” refers to using a computer algorithm to extract the profile from the measured data.
The AIRS Project Instrument Suite measures the upwelling spectrum in the infrared emitted from the Earth's surface and absorbed and emitted from the atmosphere's constituents. Each infrared wavelength, or channel in the AIRS instrument, is sensitive to different atmospheric constituents corresponding to a range of heights in the atmosphere depending on the degree of absorption of that constituent. Temperature profiles are produced by measuring CO2 absorption features with varying degrees of absorption; channels with little absorption see closer to the surface, while channels with high absorption see higher in the atmosphere. Water vapor profiles use H2O absorption features in a similar way. The AIRS suite uses a well-defined set of channels to create a profile with altitude, or sounding of the atmosphere. This is called a “retrieval.”
The figure below shows where 13 channels of AIRS are sensitive to CO2 at different altitudes, called “weighting functions”. If the atmosphere is warmer at a given altitude, then channels with a larger weighting function from the plot will have larger increases in radiance. By using a set of simultaneous wavelength radiances and the weighting functions, a computer algorithm can determine the temperature as a function of altitude.
AMSU-A also makes a similar temperature sounding, but using O2 instead of CO2.
Similarly, HSB could find the water vapor as a function or altitude by using multiple channels. The lower atmosphere sounding is aided by AMSU-A data.