The Ozone Monitoring Instrument (OMI) uses reflected sunlight to retrieve concentrations of trace gases in the atmosphere, like O₃ and NO₂. These retrieved trace gas amounts must be corrected for the presence of clouds in the atmosphere. Cloud heights are derived from OMI observations of scattered light in the UV-visible range. However, satellite imagers like MODIS use thermal infra-red radiation, emitted by the clouds themselves. The two observation methods give a different view of clouds: OMI sees the middle of the cloud, whereas MODIS sees the top of the cloud. For more details see