Temperature and humidity profiles may vary significantly with time and location. The major advantage of satellites is their capacity for regular, long-term observation on a global scale.
Atmospheric sounding using spaceborne instruments basically works like this: the brightness temperatures measured in the spectrum lines of CO₂ absorption bands is strongly correlated to the vertical profile of the atmospheric temperature. Indeed, the radiant energy measured by IASI is the sum of thermal emissions from the different absorbing layers of the atmosphere.
By selecting wavelengths so that emittance (absorptance) is maximum in a given layer and weak in the other layers, and assuming that emittance is known at this level (this is possible for gases such as CO₂), we can use measurements at each wavelength to derive the temperature of the corresponding layer. The set of measurements provides the whole temperature profile.
Similarly, if the temperature profile is known, measurements in water vapour (or ozone) absorption bands allow us to derive the humidity profile (or ozone vertical distribution).
The NOAA/HIRS operational sounder (which offers a spectral resolution of about 10 cm^-1) currently provides an accuracy of around 2.5 Kelvin for temperature and 40% for humidity.

Better spectral resolution (0.3 to 0.5 cm^-1) is required to improve accuracy. The targets for IASI are one Kelvin and 10%. That kind of resolution would enable us to derive contents of chemical components present in small concentrations in the atmosphere but which play a major role in the greenhouse effect, i.e., ozone, methane, nitrous oxide and carbon monoxide.

In addition, IASI has the ability to measure the total column content of the main greenhouse gases. It will supply valuable data for scientific studies to achieve a closer understanding of climate processes and to represent them better in global models.

The methods to derive these products with the required accuracy are studied by the ISSWG in the framework of the Preparatory Program. The studies have shown that very accurate Radiative transfer models (Line by Line models), like the french model 4A/OP, include all the spectral features needed for IASI simulation. The accuracy of existing models has been compared to existing in situ (airborne) measurements allowing to quantify forward model errors needed in inversion techniques. It was also shown that IASI will enable some studies in the atmospheric chemistry domain.
(See Documentation on Science results)

Overall, the scientific studies have shown that mission requirements will be met and that for some parameters the accuracy, which could be achieved, is better than specified. Moreover, additional products of interest for atmospheric and climate studies can also be derived.