The ground-based GPS measurements in principle only provide information on the vertically integrated water vapour in the atmosphere above the GPS stations. It was shown by Kuo et al. (1996) in an observing system simulation study that more information on the vertical distribution of the moisture could be retrieved by adding humidity observations from surface stations. This possibility to improve the utilisation of ground-based GPS measurements will be investigated by running a 3D-Var data assimilation and forecast experiment over one month with and without 2-meter relative humidity observations.Deliverable. The results of the impact studies with surface humidity are included in the reports on the other impact studies carried out at INM. They are all described in the reports INM assimilation results (text) and Figures to INM assimilation results. For further information contact Jana Sanchez-Arriola, Beatriz Navascues or J.A. Garcia-Moya at INM. Notice that also the previous deliverable, D31 contains impact results regarding use of surface humidity observations.