Lightning Ground Flash Measurements over the Contiguous United States: 1995–97

Abstract

Cloud-to-ground lightning data have been analyzed for the years 1995?97 for the contiguous United States for total flashes, positive flashes, the percentage of positive lightning, peak currents for negative and positive lightning, and for negative multiplicity. The authors examined a total of 75.8 million flashes divided among the three years, 22.7 million (1995), 26.2 million (1996), and 26.9 million (1997). The highest flash densities, uncorrected for detection efficiency, occur in Louisiana and Florida, typically exceeding 11 flashes km?2 on a grid scale of 0.2°. Positive flash densities exceed 1.1 flashes km?2 in Florida, Louisiana, and an area overlapped by the states of Tennessee, Mississippi, and Kentucky. The monthly percentage positive lightning ranges from 6.5% (July 1995) to 24.5% (January 1996). The annual percentage of positive lightning is 9.3% (1995), 10.2% (1996), and 10.1% (1997). Areas of positive lightning greater than 25% occur from the Canadian border as far south as Kansas, along the West Coast, as well as Maine. The median negative peak currents are approximately 20 kA from January through November, jumping to 24 kA in December. The median positive peak currents are highest in February (25 kA) and decrease to a minimum in July (15 kA). Median negative peak currents are high along continental coastal areas, particularly the West Coast. Mountainous regions appear to have lower median negative peak currents, on the order of 18 kA. Median positive peak currents exceed 40 kA in the upper Midwest, but are less than 10 kA in Louisiana and Florida. The mean flash multiplicity appears to increase with decreasing latitude in the eastern half of the United States.