A statistical analysis on the risetime of lightning current pulses in negative upward flashes measured at Säntis tower

We present statistical data on the risetime of lightning current pulses in negative upward flashes measured on the Säntis tower in 2010 and 2011. Two different definitions were used to determine the current risetime. The first is the t_10-90 rise time usually adopted in lightning applications and lightning standards. The second one is the risetime associated with the maximum rate of rise t_mr given by the ratio between the current peak and the current maximum derivative. The obtained statistical values for t_mr are found to be substantially smaller than those for the classical t_10-90, the median and geometrical mean for t_10-90 (0.9 μs and 1.08 μs, respectively) being 3 times larger than those associated with t_mr (0.3 μs and 0.37 μs). The relation between the risetime and the maximum significant frequency of the measured pulses is also discussed. It is shown that, in general, the t_mr definition is more consistent with the maximum frequency content of the signals, compared with the t_10-90 risetime.