Alex,
I've seen St Elmo's fire just once, in thirty years of flying for a living. It was when I flew RAF Puma helicopters. It was raining, under a CB cloud, in transit at about 1500 feet, on a very dark night in Denmark in the early 1980s. We were taking part in a special forces exercise.
Myself and the pilot in the other seat first noticed small bluish-white dots moving left/right/left across our windscreens. These turned out to be electrical discharges from the nuts holding on the wiper blades assemblies to the wiper arms. The light dots spread out and got larger until they looked similar to the picture painted on a radar screen, but still fluorescent blue. We quickly realised it was something to do with the pattern of the water film on the glass as it was scraped by the wipers. It was bright, enough to affect our night vision.
Very frightening as we thought we were about to be struck by lightning.
Thanfully, we weren't.