The multi-spectral method for atmospheric corrections is investigated. The analysis shows that the non-linear effects of the atmospheric absorption can be accounted for by using three channels. The results clarify and lend theoretical support to previous works. A simple correction procedure is proposed. In the case where channel 3 of the NOAA AVHRR is not available or is contaminated, simultaneous METEOSAT infrared data can be used instead. The result tested with in situ measured temperatures shows a marked improvement when three channels are used. In this procedure, the detailed knowledge of the local atmospheric profiles is not required.