The number of direct lightning strikes N to an isolated structure on a flat earth surface depends directly on the thunderstorm intensity Ng in the considered region and on the equivalent area Ae from which the structure collects the strikes. The lightning density Ng is available, as mentioned already, either from the data on thunderstorm days or (more properly) directly from the data of lightning location and registration systems. The equivalent area AeD depends on the lightning striking distance Rd, and also indirectly on the lightning current I and structure height h. Different relations for Rd 1⁄4 f(I) and for the average horizontal distance rh 1⁄4 f(h) from which the lightning strikes the structure are proposed in References 10 and 11. The earth surface occupied by a structure and its surroundings to a distance of rh is considered as an equivalent area AeD. The general relation for Rd is given by