Agricultural applications using surface temperatures obtained from NOAA AVHRR thermal infrared data involve the difficulty of obtaining the actual surface temperature of the crop, due to the atmospheric and crop's emissivity effects. The split-window method is often used to correct atmospheric effects, but generally is valid only for sea surface temperature. Two original methods to obtain the split-window equation are presented: the first one uses ground measurements, the second one uses atmospheric information contained in NOAA data itself (i.e. the difference between brightness temperatures of AVHRR's bands 4 and 5). Both lead to a mean error of 2 deg K on the equivalent black body temperature. An original method to obtain directly from NOAA data the difference between emissivities of the surface in bands 4 and 5 is presented. The error on the actual surface temperature obtained is I deg K. for an error of 1 per cent on the mean emissivity. Finally the surface temperature can be estimated from NOAA data with an error of 3 deg K.