A relation between near surface level specific humidity (qa) 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 qacan 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, we have applied the relation to an actual ERS-1/ATSR data dated 3 March 1992. The computed near-surface level specific humidity compared well with values derived from simultaneous ship observations and the r.m.s. error was found to be within 15 per cent.