Lighting Ground Flash Density in the Contiguous United States: 1992–95

Cloud-to-ground lightning data for the years 1992–95 have been analyzed for geographical distribution of total flashes, positive flashes, and the percentage of flashes that lower positive charge to ground. In the contiguous United States the measured total cloud-to-ground lightning flash counts were 16.3 million (1992), 24.2 million (in both 1993 and 1994), and 22.3 million in 1995. The maximum flash densities occurred in Florida in 1992 (9–11 flashes per square kilometer) and in the Midwest in 1993 (11–13 flashes per square kilometer), coinciding with the storms and floods that dominated the summer of 1993 in the Midwest. In 1994, the area of maximum flash density was again in Florida (11–13 flashes per square kilometer). In 1995, the flash density maxima (9–11 km−2) were in southern Louisiana and near the Kentucky–Illinois border. Positive flash densities had maxima in the Midwest in all four years with values of 0.4 (1992), 1.0 (1993), 0.7 (1994), and 1.8 flashes per square kilometer (1995). The annual mean percentage of flashes that lowered positive charge to ground was between 4% and 5% for the three years, 1992–94, but increased to 9.3% in 1995. The monthly values of the percentage of positive flashes ranged from 3% (August 1992) to 25% (December 1993). The positive flash maxima in the Midwest appear to be near the geographical areas in which cloud-ionosphere discharges (sprites) have been reported.