Yeah that's interesting. Except for patches of Sweden that get an average 1 event a year, and most of Denmark (1, some 2 in the south), it's all <1 a year, like here on the West coast and like in Iceland, where scholars have noted the infrequency of lightning despite the importance of Thor. This must have changed over the last millennium, at least in Norway where most of the non-thrall Icelandic settlers came from. In fact I can see it's changed in recent decades since the map has Britain as also getting <1 event, except for a tiny spot or two, and I remember thunderstorms as common when I was growing up. Maybe one problem, since by events they seem to mean strikes, is that the scale isn't granular enough at the low end to capture the difference between one thunderstorm with visible lightning every 4 or 5 years in San Jose and 10–12 every year in London? Maybe 5 or 10 km2 would reveal more variation? Still nothing like the middle of the US :-)