A relation between near surface level specific humidity (q a ) and ERS-1/Along Track Scanning Radiometer's nadir and forward scan observations is proposed on the basis of radiative transfer simulations for tropical atmospheres. The simulation study shows that q a can be estimated within an r.m.s. error of 15 per cent when it is compared with the values derived from ship observations. As a case study analysis, the relation is applied to an actual ERS-1/ATSR data.