Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Kelvin force microscopy measurements on films of noncrystalline silica and aluminum phosphate particles reveal complex electrostatic potential patterns that change irreversibly as the relative humidity changes within an electrically shielded and grounded environment. Potential adjacent to the particle surfaces is always negative and potential gradients in excess of +/- 10 MV/m are found parallel to the film surface. These results verify the following hypothesis: the atmosphere is a source and sink of electrostatic charges in dielectrics, due to the partition of OH(-) and H(+) ions associated to water adsorption. Neither contact, tribochemical or electrochemical ion or electron injection are needed to change the charge state of the noncrystalline hydrophilic solids used in this work.131321138111386Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)