Ken Lagace of Baltimore recently heard someone mention heat lightning. "I haven't heard that in over 30 years. What happened to heat lightning?" he asks. Lots of us remember sitting outside on hot summer nights, watching the horizon flicker, silently and mysteriously. We called it "heat lightning" because it was oddly silent, and we saw it mostly on hot nights. But it's not a special kind of lightning, merely lightning flashes reflected from distant thunderstorms. A clap of thunder just doesn't travel much more than 10 miles.