Abstract
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.
