On the Accuracy of Wait's Formula Along a Mixed Propagation Path Within 1 km from the Lightning Channel

We have examined the accuracy of Wait´s formula for evaluating lightning vertical electric field and azimuthal magnetic field over a mixed propagation path (vertically stratified) by using the finite-difference time-domain (FDTD) technique at near (200 m) and intermediate (1000 m) distance range from the lightning channel. For the first return stroke, Wait´s formula has a reasonable accuracy for predicting lightning electromagnetic fields within distances of 1000 m. However, for the subsequent return stroke, although Wait´s formula predicts the nearly same field peaks as that by FDTD no matter how the ground conductivity is vertically stratified, some waveform differences can be observed at the early-time response within a few microseconds. Therefore, Wait´s formula may cause some errors for predicting the field derivates within tens or hundreds of meters.