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Lightning over Tucson, Arizona, USA. Time-exposure image of lightning over Tucson city and vehicle tail lights on a road. The silhouettes of saguaro cacti (Carnegiea gigantea) are at far left and right. Lightning is an electrical discharge in the atmosphere that occurs during thunderstorms. A charge differential builds up between the clouds and the surface of the Earth. When the charge differential is high enough, the accumulated energy is discharged in a flash of lightning. The discharge takes a few milliseconds and temperatures reach as high as 30,000 degrees Celsius. (KEYSTONE/SCIENCE PHOTO LIBRARY/PETER MENZEL/SCIENCE PHOTO LIBRARY)