Bolts of cloud-to-ground lightning. Lightning occurs when a large electrical charge builds up in a cloud, probably due to the rapid movement of water droplets and ice particles in its turbulent interior. The charge is attracted to the oppositely-charged ground, and a few leader electrons travel from one to the other. When one makes contact, there is a huge backflow of energy along the path of the electron. This produces a bright flash of light and temperatures of up to 30,000 degrees Celsius. Photographed in Duisburg, Germany.