Lightning. Forked cloudtoground 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. Cloudtoground 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.