I had seen a project on MatchRockets.com that utilized ionization to detect ambient charge in the air. It involved burning a camping lantern (Coleman) mantle to harvest the Thorium content, and mixing the ash into an epoxy. The epoxy is then applied to the sharp edge of a razor blade and then mounted on a tall dowel rod/ broom handle as an antenna above your roofline. A ammeter is connected between this and earth ground. Supposedly, you can see the charge build up just prior to a strike. Also, Americium 241 can be removed from a smoke detector and used as well.