The effect of surface moisture on the electrostatic force of attraction between an uncharged particle and a ground plane is investigated. Describing a moist sphere with an effective complex permittivity allows the image force to be calculated using complex linear multipoles. The number of multipoles needed for an accurate image force calculation is determined as a function of particle-plane separation. Adsorbed moisture introduces a relaxation in the electrostatic image force. For particles exposed to a range of relative humidities, the model predicts relaxation times that vary by several orders of magnitude. In addition, at small particle-plane separations adsorbed moisture can increase the image force by several orders of magnitude. Finally, multipolar moments are used to calculate the electric field in the gap separating the particle and ground plane to determine the onset of breakdown or field emission.