A numerical model of the radiation transfer through the atmosphere is used with a set of mid-latitude atmospheric profiles to simulate split-window measurements of the AVHRR/2 on the NOAA-7 satellite. These are used to quantify the degradation in accuracy which results in going from a sea surface temperature retrieval in which the effect of the atmosphere is estimated using measurements at 11 μm and 12 μm (channels 4 and 5), to one in which it is estimated using synthetic channel 5 data, as advocated by Singh et al (1985).