An axisymmetrical electrostatic model of charges in a thunderstorm cloud at the mature stage is used to initiate a bidirectional vertical leader that develops into either a cloud-to-ground or an intracloud flash from the regions of maximum electric field. The principal result of this study is the determination of the physical and quantitative relationships among cloud charges, potentials, and electric fields, and the induced charges, currents, and electric field changes of the lightning channel from the numerical solution of the Poisson equation for a cloud charge model with lightning. An important consequence of the model is that the upper part of the cloud-to-ground leader and the lower part of the intracloud leader terminate inside the cloud. ¿ 1998 American Geophysical Union