A technique is presented for calculating all the forces, with the exception of aerodynamic forces, acting on a raindrop falling at terminal velocity in air and subjected to a vertical electrical field such as might occur in the base of a thundercloud. The ultimate aim is a prediction of the onset of hydrodynamic instability of the drop under electric stress. Since as yet no model of aerodynamic flow around the drops is available, this method of calculating the forces and the deduced aerodynamic pressure profiles is presented. These profiles are compared to previous work and are shown to be reasonable. The assumption of a model of aerodynamic flow would allow the prediction of raindrop instability and corona, believed to be responsible for the initiation of lightning strokes.