We present information obtained from VHF radio pictures of 47 lightning flashes that struck ground 1430 m above mean sea level (msl) to the north of Johannesburg, over a period of 16 years. Radio pictures were obtained using a hyperbolic method, i.e., by taking differences between times at which radio noise from lightning arrived at five widely spaced stations. These data were supplemented by recordings of electric field change. We found that stepped leaders and intracloud streamers emitted pulses and progressed at speeds that averaged 1.6×105 ms−1. Stepped leaders ranged in length from 3 to 13 km. BIL (breakdown‐intermediate‐leader) waveforms of electric field change were caused by stepped leaders whose branched channels followed complicated paths. The electric field changes they recorded in an effective bandwidth of 3.5 kHz could be reproduced faithfully by calculations based on an assumption that the radio sources became charged at the expense of a region near the origin of each leader. Some return strokes radiated trains of noise, called Q noise, whose sources extended at speeds near 108 ms−1. Sources of this Q noise retraced leader channels before extending them at these high speeds. Interstroke processes radiated Q noise whose sources extended at speeds that averaged 8.7×107 ms−1. Most Q sources (98%) were directed vertically, but they pervaded regions that retrogressed at “speeds” that averaged 2.2×104 ms−1 in directions away from the starting points of the flashes. This retrogression was directed horizontally, but the vertical Q streamers caused J changes to have positive or negative slopes according to the relative positions of Q sources and the reversal cone. Sixty percent of our sample of ground flashes were endowed with portions that were cloud flashes or were themselves minor components of cloud flashes. A more detailed summary appears at the end of the paper.

Five ground‐based VHF receivers were used to trace paths followed by flashes of lightning. Every lightning flash radiates a succession of radio noise pulses, and three Cartesian coordinates of the positions of pulse sources were determined by measuring the times at which the noise pulses arrived at each of the spaced receivers. Lightning channels were mapped in space and time by locating a large number of radio noise sources for each flash, whose shape and position then became apparent despite the presence of intervening hydrometeors. A center frequency of 253 MHz was chosen for the receivers and their bandwidths were wide enough for time differences to be measured with rms errors of 140 ns. Hence two of the three spatial coordinates of a particular source could be determined to an accuracy of 25 m rms and the vertical coordinate could be found to an accuracy which was typically 140 m rms, but the actual height accuracy depended on source position. Some factors that affect the design of the receivers are discussed and case studies of five cloud flashes are presented. Cloud flashes could be classified into two types according to the rates at which they emitted pulses in the VHF part of the radio spectrum. One class radiated pulses at rates that approximated 103 pulses per second when received in bandwidths of 10 MHz. These pulses were often nearly rectangular in form, lasted approximately 1 ms on average, and occurred in synchronism with pulses received at HF and at UHF. The second class emitted much shorter pulses (median durations of 0.2 to 0.4 ms were measured) at higher rates typically 105 pulses per second, and pulses were generally not in synchronism with those received at other radio frequencies. Diameters of the channels occupied by radio sources varied from 100 m to several hundred meters, and were enlarged by the extents of the sources themselves. It was possible to measure the principal extents of many individual sources active during low‐pulse‐frequency cloud flashes. Average sizes near 300 m were measured for low‐pulse‐frequency flashes, and sizes near 60 m were estimated for sources active during high‐pulse‐frequency flashes. It was found that pulses originated in regions near streamer tips, and that pulses were associated with initial ionization. First streamers progressed at speeds that ranged from 0.9 × 105 m/s to 2.1 × 105 m/s except for one extensive, positive flash that moved at 5 × 105 m/s for the first few kilometers. We define a positive flash as being one which conducts excess positive charge in the same direction as that in which the streamer progresses. Subsequent discharges along paths that had been ionized previously seldom radiated much noise but noise was radiated by channels that were several tens of milliseconds old, and then we measured streamer speeds that were at least an order of magnitude higher than first streamer speeds. Relatively few cloud flashes were found to have been oriented vertically. Most were horizontal and often consisted of several streamers that extended from a ...

A hyperbolic system with five VHF receivers is used to obtain three‐dimensional fixes of sferics as a function of time. Data recorded during one complete flash and parts of several others have been reduced. The integrated picture of one lightning flash shows a ‘waist’ region of minimum cross section between 3.8 and 4.5 km high. Branching extends upward and downward from the waist. Downward branches do not always emerge from the cloud base. Lower regions of previously established channels tend not to radiate during high‐order strokes. By far the greater part of the activity is confined to the cloud. The flash pervaded a volume 3×4×6 km. The instrument is particularly useful for tracking interstroke processes, higher regions of a flash to ground, and cloud flashes, but the accuracy of fixes obtained below the cloud is degraded by poor height resolution in this region. Noise is sometimes emitted by clouds that are not cumulonimbus. The method used to calculate the positions of the sferic sources is described, and the accuracy of the system is discussed.

Regions where 773 flashes began during 13 thunderstorms were located by calculating centroids of the sources of the first six or 10 VHF pulses that were emitted by each flash. Sources were located by measuring differences in the times at which their pulses arrived at five widely spaced receivers stationed on the ground. We found that the distribution of origin heights was bimodal with peaks at 5.3 and 9.2 km above mean sea level (amsl). Standard errors in the coordinates of flash origins were estimated to be 20–80 m in X and Y and 160–240 m in the height coordinate Z. There were 431 flashes in the lower group and 342 in the higher. Flashes in the lower group were more numerous in 10 storms; in three storms, high flashes were in the majority. There was evidence to suggest that this condition depended partly on the phase of the host thunderstorm. Recorded E field changes produced by 165 flashes whose paths we had mapped convinced us that the vast majority of the 773 flashes, including the 342 high flashes, had also been negative. Flash origins tended to cluster in regions that were a few kilometers or less in horizontal diameter. Densities of flash origins in these regions ranged from 1.25 to 25 km−3. The origins of 658 flashes were mapped onto radar precipitation patterns of their host storms. We found that 66% of the flashes began within one picture element, approximately 270 m, of the contours for 20 dBZ; 27% began inside these contours, and most of these began at edges of high‐reflectivity cores. The remaining 7% began outside the 20‐dBZ contours. Similar results (73%: 19%: 8%) were obtained for 276 high flashes that began at heights above 7.4 km amsl, but 195 ground flashes scored 54%: 36%: 9% and showed a greater tendency to begin inside the 20‐dBZ contours. The distribution of distances between origins and their nearest 20‐dBZ contours showed a marked peak near zero. We concluded that charge‐density in thunderclouds was affected by the presence of heavy precipitation and that 20‐dBZ surfaces enclosed regions that carried excess negative charge.

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