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[S 7213] Lightning. Night-time view of multiple bolts of lightning striking the ground in a city with the moon in the sky. Lightning occurs when a large electrical charge builds up in a cloud. This is probably due to the rapid movement of water droplets and ice particles within the cloud. Large thunderclouds generally have very turbulent interiors. The charge induces an opposite charge on the ground, and a few 'leader electrons' travel to the ground. When one makes contact, there is a huge backflow of energy up the path of the electron. This produces a bright flash of light and temperatures of up to 30,000 degrees celsius. Photographed in Tucson, Arizona, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/KEITH KENT)