Lightning. Computer artwork of a lightning strike over water. 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 with the ground, 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.