Back in the early 1960s I experimented with high voltages, at one point in time I constructed one of those "pinwheel" type devices that work on the action reaction principle caused by the ionization (corona discharge) of the air molecules at the sharp points of the device.
It was late at night when I finished constructing it but I waned to test the device, I placed everything on the table, hooked it up to a 10 kv., ignition transformer and found out that it worked ok, so I unblugged the device and went to bed.

Next morning when I went down to the basement where the experiment was, I got a shock of the nonelectrical type, I saw the "pinwheel" spinning like grazy.

No one else had been in the basement so I thought that perhaps I forgot to unblug the thing, but looking at it from a closer range I saw clearly that the blug was on the cement floor.
I watched for a while and wonered why the thing was spinning, there was no apparent power going to the device, then I heard thunder from outside and everything made more sense.

Apperently the thunderstorm created enough of a electronic potential differece between the cement floor and the table top to cause ionization of the air at the sharp points of the "pinwheel".

Some history of corona or point discharge or St. Elmo's Fire: http://www.islandnet.com/~see/weather/elements/stelmo.htm

Also here: http://en.wikipedia.org/wiki/St._Elmo's_fire