Berger’s measurements of the peak current probability distributions for return
strokes preceded by negative stepped and dart leaders, and for positive lightning,
were of log-normal form and ranged from a few kA to above l00 kA with a median
value of about 30 kA. As pointed out by Berger himself, his 70 m towers at 914 m
altitude often initiated negative lightning by upward positive leaders of at least 1 km
in length; positive lightning almost always began with an upward stepped (negative)
leader, so that the peak current values for his recorded strikes might differ from normal
downward flashes. In Diendorfer’s measurements, the median peak currents to the
tower were 12.3 kA, compared with 9.8 kA to the surrounding terrain (both values
obtained from radiated signal analysis). Direct measurement of lightning currents is
also feasible from rocket-triggered flashes. However, such lightning strikes are again
initiated by upward leaders and are thus atypical of natural downward flashes [33, 34].