The infrared region of the spectrum that is used for atmospheric remote sensing ranges from around 600 to 2800 cm-1 (16.6 to 3.3 microns in wavelength). In this part of the spectrum we can use CO2 spectral bands at 15 and 4.3 microns to give us information on the temperature structure of the atmosphere. Information on water vapour content can be gained from a large number of H2O lines between 5 and 8 microns. The CO2 and H2O bands have been used for many years in satellite sounding applications. In addition, parts of the infrared spectrum are sensitive to ozone (8.9-10.1 micron band), and other trace gases such as CH4 and N2O.

The Infrared region offers a complementary data set to the microwave frequencies. Clouds are opaque in the infrared, meaning that to date it has not been possible to use these data in cloudy regions. However the new generation of infrared sounding instruments contain information at a greater vertical resolution than the microwave sounding instruments.

The High-resolution InfraRed Sounder (HIRS), a 20 channel radiometer, has been used operationally at the Met Office for many years to provide information on the temperature and water vapour structure of the atmosphere. Together with microwave instruments AMSU-A and AMSU-B, HIRS forms part of the ATOVS sounding system (on NOAA and Metop satellites), which is the backbone of satellite data assimilation for numerical weather prediction.

The Microwave Sounding page contains more information about the use of ATOVS data at the Met Office.

The next generation of infrared sounding instruments are now beginning to be launched. These instruments, such as the Atmospheric Infrared Sounder (AIRS) on EOS-Aqua and the Infrared Atmospheric Sounding Interferometer (IASI) on Metop, boast very high spectral resolution (0.4-2.1 cm-1 for AIRS; 0.25-0.5 cm-1 for IASI) and thousands of individual channels. EOS-Aqua was launched in 2002, providing us with AIRS data which has been assimilated to good effect since May 2004.