As briefly explained in the section 'Radiative transfer' of 'Radiation basics', some of the photons scattered on air molecules experience inelastic rotational Raman scattering and thereby change their wavelength by up to several nanometres. While only a few percent of all scattering processes are inelastic, the effects on the spectra measured — and thus the retrieval — are profound. The effects of Raman scattering show up in a number of ways that need to be allowed for in the retrieval process and which are discussed in the following sections:

• Rotational Raman scattering causing a reduction in the depth of Fraunhofer lines in scattered light spectra.
• Rotational Raman scattering causing a distortion of atmospheric absorption measurements.
• Although not part of the Ring effect, vibrational Raman scattering in liquid water can also cause measurement errors to arise over areas with clear surface water.
• Compensation procedures involving the computation and application of a pseudo-absorber.