In this paper we extend the Cooray‐Rubinstein (C‐R) formula into a mixed propagation path (vertically stratified conductivity) and estimate the lightning horizontal electric fields over the mixed path, and we have examined its accuracy at distances of 100 m to 1000 m from the lightning channel by using finite difference time domain (FDTD). When the ground conductivity near the observation point is less than that near the strike point, it is noted that the C‐R formula predicts a satisfactory accuracy in the initial several microseconds after the beginning of the return stroke. However, when the ground conductivity near the observation point is larger than that near the strike point, the initial negative excursion predicted by the C‐R formula is underestimated, and the error increases with the increase of the difference between the two sections conductivity.