The microwave sensor SSM/I (Special Sensor Microwave/Imager) on board of the DMSP satellite can be used to develop a retrieval method for the water vapour content in the atmospheric boundary layer close to the sea surface, by means of radiative transfer calculations. It is found that the SSM/ I measurements are sufficiently sensitive to the water vapour w1in the lowermost 500 m of the atmosphere to allow a retrieval of w, from the 19, 22, 37 GHz vertical and 19 GHz horizontal polarization measurements with an accuracy of 0-06 g cm−The technique is validated with globally distributed radiosonde measurements located together with satellite soundings during the months July and August 1987. A linear relationship is established statistically to determine the near-surface specific humidity from w, with an accuracy of l-2gKg−1