Fig. 3

Observation probability of negative potential gradient (PG) near or during the rain time at Kakioka, 150 km southwest of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1). The 1-min PG date in 10 min bins is used to classify according to a 10-min average, b minute values at negative peak, and c minute values at positive peak, into <\(-\,400\), \(-\,400\) to \(-\,200\), \(-\,200\) to \(-\,100\), \(+\,100\) to \(+\,200\), \(+\,200\) to \(+\,400\), and >\(+\,400\,\hbox {V/m}\) for different rain conditions: continuous rain (\(\ge \,0.5\,\hbox {mm}\)), either light rain (\(<\,0.5\,\hbox {mm}\)) or within 10 min before the rain, and no rain over 30 min before or after. Since the percentage is very low for no rain cases, the unit is changed in the graph (right axis). The rain record is obtained by an accumulation method every 10 min (see text for detail). The data are compared between immediately after the FNPP1 accident (13 March–30 April, 2011) and the same period (13 March–30 April) at different years (2006–2015). Total numbers of 10-min bins are given at the top of each bar