When early mariners needed to know the depth of the water they were sailing in, they would use a weighted line to plumb the depths. They called this practice “sounding.” From space, AIRS “sounds” the atmosphere at multiple depths for temperature, humidity, trace gases and clouds.
The AIRS instrument looks down at Earth and measures the infrared brightness. Different infrared wavelengths are sensitive to the infrared brightness emitted from Earth's surface, to gases at a particular height in the atmosphere, or to clouds. AIRS has 2378 infrared detectors with each detector sensing a particular wavelength, so it breaks the light into 2378 different wavelengths for each scene.
Each set of 2378 values is known as a spectrum. The information in each AIRS spectrum tells us something about a temperature profile (from the surface through the troposphere), a water vapor profile, the amounts of some gases, the heights and amounts of clouds, and surface temperature: a sounding of the atmosphere.
SPECTRA, SOUNDINGS AND GLOBAL COVERAGE
AIRS is a continuously operating cross-track scanning sounder, consisting of a telescope directed into an echelle spectrometer that divides the infrared light into the separate wavelengths comprising a spectrum. During each scan, the rotating external mirror views the underlying Earth scene from 49° on one side of the nadir (straight downward) to 49° on the other side, in 90 steps with each step providing a spectrum. The spatial footprint of the infrared channels is 1.1° in diameter, which corresponds to about 15 by 15 km in the nadir.
Each day the AIRS instrument samples 2,916,00 spectra, about 34 each second. Nine of these spectra are combined to make a single sounding of atmospheric temperature, water vapor, trace gases, and of surface temperature. AIRS makes up to 324,000 soundings per day, sampling nearly the entire planet twice daily at a spacing of about 45 miles. While prior space instruments had only 15 detectors, AIRS has 2378. This greatly increases the information in AIRS soundings, making them comparable to measurements made by weather balloons but with coverage worldwide.