The propagation speed of a positive lightning return stroke

The first direct determination of the propagation speed of a lightning return stroke lowering positive charge to ground has been made. This stroke was the third of eight otherwise negative strokes in a triggered lightning flash initiated at the Kennedy Space Center, Florida. Two independent optical systems, one photographic and the other photoelectric, yielded common recordings for the third and fourth strokes; the respective two‐dimensional return stroke propagation speeds were 1.0 vs. 0.93 ×108 m/s for the positive (third) stroke and 1.0 vs. 1.0 ×108 m/s for the fourth stroke. Using fast electric‐field data, we estimated the positive stroke peak current to be 21 kA. Photoelectric data only yielded propagation speeds of 1.4, 1.6, 1.2, 1.3, 1.0 and 0.90 × 108 m/s for the first, second and fifth through eighth return strokes, respectively. All propagation speeds were evaluated over 850 m of channel near ground and have an error estimate of 10‐15%. For this positive stroke, we found a return stroke propagation speed typical of negative strokes. Whether positive return strokes, in general, travel at typical negative return stroke speeds must await future measurements.