Variation in light intensity with height and time from subsequent lightning return strokes

Abstract

Relative light intensity has been measured photographically as a function of height and time for seven subsequent return strokes in two lightning flashes at ranges of 7.8 and 8.7 km. The film used was Kodak 5474 Shellburst, which has a roughly constant spectral response between 300 and 670 nm. The time resolution was about 1.0 μs, and the spatial resolution was about 4 m. The observed light signals consisted of a fast rise to peak, followed by a slower decrease to a relatively constant value. The amplitude of the initial light peak decreases exponentially with height with a decay constant of about 0.6 to 0.8 km. The 20% to 80% rise time of the initial light signal is between 1 and 4 μs near ground and increases by an additional 1 to 2 μs by the time the return stroke reaches the cloud base, a height between 1 and 2 km. The light intensity 30 μs after the initial peak is relatively constant with height and has an amplitude that is 15% to 30% of the initial peak near the ground and 50% to 100% of the initial peak at cloud base. The logarithm of the peak light intensity near the ground is roughly proportional to the initial peak electric field intensity, and this in turn implies that the current decrease with height may be much slower than the light decrease. The absolute light intensity has been estimated by integrating the photographic signals from individual channel segments to simulate the calibrated all‐sky photoelectric data of Guo and Krider (1982). Using this method, we find that the mean peak radiance near the ground is 8.3×105 W/m, with a total range from 1.4×105 to 3. ×106 W/m.