Abstract. The change in the electric potential due to lightning is evaluated. The potential along the lightning channel is a constant which is the projection of the pre-flash potential along a piecewise harmonic eigenfunction which is constant along the lightning channel. The change in the potential outside the lightning channel is a harmonic function whose boundary conditions are expressed in terms of the pre-flash potential and the post-flash potential along the lightning channel. The expression for the lightning induced electric potential change is derived both for the continuous equations, and for a spatially discretized formulation of the continuous equations. The results for the continuous equations are based on the properties of the eigenvalues and eigenfunctions of the following generalized eigenproblem: Find u ∈ H 1 0 (Ω), u = 0, and λ ∈ R such that ∇u, ∇v L = λ ∇u, ∇v Ω for all v ∈ H 1 0 (Ω), where Ω ⊂ R n is a bounded domain (a box containing the thunderstorm), L is a subdomain (the lightning channel), and ·, · Ω is the inner product ∇u, ∇v Ω = Ω ∇u · ∇v dx.Mathematics Subject Classification. 35J25, 35Q60, 35A20, 35P10.