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PASSAGE 18 Volts from the Sky Lightning has caused awe and wonder since old times. Although Benjamin Franklin demonstrated lightning as enormous electrical discharge more than 200 years ago， many puzzles still surround this powerful phenomenon. Lightning is generated when electrical charges separate in rain clouds， though processes are still not fully understood. Typically， positive charges build at the cloud top， while the bottom becomes negatively charged. In most instances of cloud-to-ground lightning， the negatively charged lower portion of the cloud repels negatively charged particles on the ground s surfaces， making it become positively charged. The positive charge on the ground gathers at elevated points. A flow of electrons begins between the cloud and earth. When the voltage charge becomes large enough， it breaks through the insulating barrier of air， and electrons zigzag earthward. We see the discharge as lightning. Lightning can occur within a cloud， between clouds， or between clouds and the ground. The first variety， intra-cloud lightning， is the most frequent but is often hidden from our view. Cloud-to-ground lightning， making up about 20 percent of lightning discharges， is what we usually see. Lightning comes in several forms， including sheet， ribbon， and ball. Intra-cloud lightning can illuminate a cloud so it looks like a white sheet， hence its name. When cloud-to-ground lightning occurs during strong winds， they can shift the lightning channel sideways， so it looks like a ribbon. The average lightning strike is more than 3 miles long and can travel at a tenth of the speed of light. Ball lightning， the rarest and most mysterious form， derives its name from the small luminous ball that appears near the impact point， moves horizontally， and lasts for several seconds. | 
