Cloud‐to‐ground lightning flash characteristics from June 1984 through May 1985

Abstract

A magnetic direction‐finding network for the detection of lightning cloud‐to‐ground strikes has been installed along the east coast of the United States. Time, location, flash polarity, stroke count, and peak signal amplitude are recorded in real time. Results are presented from nine direction finders for 1 year, June 1, 1984, through May 31, 1985. The data were recorded from Maine to North Carolina and as far west as Ohio; analyses were restricted to flashes within 300 km of a direction finder. Measurements of peak signal strength have been obtained from 720,284 first return strokes lowering negative charge. These return strokes are assumed to have a median peak current of 30 kA. It is also assumed that the return stroke speed is constant. The resulting distribution indicates that few negative strokes have peak currents exceeding 100 kA. Measurements have also been obtained of peak signal strength from 17,694 first return strokes lowering positive charge. These strokes have a median peak current of 45 kA, with some peak currents reaching 300–400 kA. The same assumptions are made for positive strokes as for negative strokes. The median peak signal strength, and we believe the peak current, doubles from summer to winter for both negative and positive first return strokes. The polarity of ground flashes is observed to be less than 5% positive throughout the summer and early fall, then increases to over 50% during the winter, and returns to less than 10% in early spring. The percent of positive flashes with one stroke is observed to be approximately 90% throughout the year. The percent of negative flashes with one stroke is observed to increase from 40% in the summer to approximately 80% in January, returning to less than 50% in the spring.