Lightning and tall structures

The mechanism by which lightning is attracted to an earthed structure of low height is well established, and the distances over which strokes of different intensities are attracted (the so-called striking distances) have been determined. It appears that the numerical values so established do not apply to structures exceeding, say, 20m in height. In the electric field under a thundercloud, tall structures produce point-discharge currents. Field observations have shown that the resulting space charges can assume the same shape as the visible plume of smoke from a chimney. When a normal negative leader approaches such a positively charged `plume?, it will be attracted to it, and unusually long `striking distances? can result. A plume of positive space charge may be broken up by the action of the violent updrafts and downdrafts under an active thundercloud. The resulting pockets of space charge in the atmosphere are held responsible for the branching of earth flashes. They may also be responsible for strikes to points well below the tips of tall structures standing on open ground. On the other hand, it is suggested that structures surrounded by other tall structures or trees are subject to roughly the same striking distances as low structures. It is furthermore suggested that present knowledge is inadequate to determine striking distances for the less frequent positive discharges.