[Stockbild] Lightning. Forked cloud-to-ground lightning over an urban area. 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 in the turbulent interior of the cloud. Cloud-to-ground lightning occurs between a negatively charged cloud base and the positively charged ground. 'Leader electrons' leave the cloud and, when contact is made, there is a huge backflow of energy along the path of the electrons. This produces a bright flash of light and temperatures of up to 30,000 degrees Celsius. Photographed in Finland. (KEYSTONE/SCIENCE PHOTO LIBRARY/PEKKA PARVIAINEN) === ===