CLOUD-TO-GROUND LIGHTNING FLASH CHARACTERISTICS IN THE CONTIGUOUS UNITED-STATES - 1989-1991

Wideband magnetic direction finders have been used to obtain a cloud-t o-ground lightning flash count for the contiguous United States, an ar ea of 7.7 x 10(6) km2, for the period 1989 through 1991. Over 46 milli on flashes to ground were recorded and are divided among the three yea rs, 13.4 million in 1989, 15.9 million in 1990, and 16.9 million in 19 91. Maximum flash densities occur in Florida and increase each year, f rom 9 flashes km-2 (1989) to 13 flashes km-2 (1991). The database cont ains 1.7 million positive flashes divided among the three years, 0.4 m illion in 1989, 0.6 million in 1990, and 0.7 million in 1991. In 1990 and 1991 the positive flash density maximum occurred in Florida but el sewhere in 1989. Secondary maximum positive flash densities occur thro ughout the Midwest. The annual mean percentage of positive flashes in the total lightning count is 3.7% for the period 1989-1991. The intera nnual variation is small, ranging from 3.1% (1989) to 4.0% (1991). How ever, in any given year the geographical variation of the percentage o f positive flashes is large. The percentage of positive flashes is 2% (1989) at the latitude of Florida but near 25% and higher at the latit udes of the upper Midwest, Maine, and Oregon.