On the peak amplitude of lightning return stroke currents striking the sea

•Formation of ground lightning flashes is promoted by the presence of positive charge pockets in thunderclouds.•Conditions in maritime thunderstorms are unfavourable for the formation of positive charge pockets.•Thus, higher cloud potentials are required to initiate ground flashes in maritime storms.•Lightning flashes initiated under higher potentials harbour larger first return stroke currents.•This explains the reason why first return stroke currents in maritime thunderstorms are larger than their counterparts over land.

Data gathered from the US National Lightning Detection Network (TM) (NLDN) show that the peak currents of lightning flashes striking the sea are significantly higher than those of lightning flashes striking the land. We suggest that the unfavorable conditions for the formation of positive charge pockets in maritime clouds lead to lightning initiation at higher cloud potentials compared to their land counterparts, resulting in larger peak currents in negative lightning flashes striking the sea. As the positive charge pocket does not promote positive ground strokes, no such discontinuity should be expected in positive first return strokes to ground between land and sea.