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Richard E. Orville and Daniel W. Spencer


Lightning flashes recorded in photographs by two satellites in the Defense Meteorological Satellite Program (DMSP) are used to calculate flash frequencies for land and ocean regions in 10° latitude belts between 60°S and 60°N for dusk and midnight local times. Data are analyzed for the first week of each month between March 1974 and February 1975. We find that the annual land-ocean ratio of global lightning at dusk ranges from 8 to approximately 20, depending on whether lightning near the coast is judged to be from thunderstorms produced predominantly by ocean effects or land effects. A similar analysis for data obtained near midnight yields ratios which range from 4 to 8. The global land-ocean lightning ratio is significantly higher during the northern summer than during the southern summer. The dusk lightning flash frequency as a function of latitude peaks at 10–20°N during the northern summer and 0–10°N during the southern summer. The midnight flash frequency peaks at 0–10°N throughout the year but has a broad equatorial maximum from 10°S–10°N during the southern summer. We estimate an annual global lightning flash frequency from the dusk satellite data to be 123 s−1 with average values of 142 s−1 for the northern summer and 100 s−1 for the southern summer. A similar analysis for the midnight satellite data yields an annual lightning frequency of 96 s−1 with average values of 110 s−1 in the northern summer and 79 s−1 in the southern summer. These flash frequency estimates may be in error by a factor of 2. The ratio of global lightning flash frequency during the northern summer to that in the southern summer is 1.4 for both the dusk and midnight satellite data. A comparison between the global lightning flash frequency as a function of season and the published values for the annual variation of the earth's electric field shows an inverse relation.

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