Progressive lightning. VII Directly-correlated photographic and electrical studies of lightning from near thunderstorms

Abstract

An analysis has been made of the direct correlation of luminous and electrical field-changes produced by thirty-seven lightning discharges to ground. These discharges comprised 199 separate strokes, most of which were within a distance of 6 km. A new form of drum camera and a camera shutter operated by the first leader were em ­ ployed, and many records were obtained in daylight. When compared with results previously reported for flashes at distances of 10 km. or more, the following major differences are found: negative field-changes from near leaders, absence of electrical effects of first leader steps and hook-shaped field-changes during the period immediately following the return stroke. The hook field-changes are shown to be associated with components in the continuing luminosity of the channel of the type investigated by Malan & Collens and to be due to subsidiary discharges in this channel. These ' M ’ components carry negative charge to ground in quantities between one-fifth and one-hundredth of those carried by main strokes. The height of the lower negative charge in these thunderclouds is found to be from 4 to 9 km. During a discharge to ground, no rapid discharges occur within the cloud or elsewhere than in the cloud-ground channel. A fairly slow discharge of the upper positive charge in the thundercloud appears to occur to the upper air immediately after the lower negative pole has discharged to ground.