The Kelvin equation relates the vapor pressure of a volatile liquid to the curvature of the liquid surface. It describes phenomena such as capillary condensation, capillary adhesion, nucleation, and the adsorption of vapors into porous media. Here we propose an extension of the Kelvin equation, which takes into account changes of the vapor pressure due to electric fields. The presence of electric fields reduces the saturation vapor pressure and leads to field-induced condensation. Field-induced condensation can explain the presence of water bridges in scanning probe nanolithographic methods such as anodic oxidation.