DETERMINATION OF THE STRIKING DISTANCE OF LIGHTNING TO EARTHED STRUCTURES

A theoretical determination based upon a critical charge criterion and ionization processes is presented of the striking distance r(s) at which the upward leader is initiated from a struck object. The striking distance is shown to be leader-current dependent in the form r(s) proportional to i(2/3). Comparisons are made with earlier, more numerically based models, which used an alternative equivalent-radius approach. The altitude dependence of the striking distance is also investigated. Since pressure, absolute humidity and temperature decrease with increasing altitude, the variation of streamer properties with altitude can be inferred from laboratory experiments. Calculations on this basis show that the striking distance increases significantly with increasing altitude, causing a corresponding increase in the risk of lightning strikes for given structures in mountain or aviation environments.