Ken Legace of Stewartstown, Pa., writes: "I thought lightning happened when positive-charged clouds and negative-charged earth connected electrically with a huge ZAP. Then what causes cloud-to-cloud lightning?" While the tops of thunderclouds become positively charged, the bottoms go negative, which induces a positive charge in the ground. Wherever the difference reaches 15 million volts per mile, it discharges. Bolts fly twice as often within clouds as between clouds and ground.