Lightning is an electrical discharge occurring in the atmosphere accompanied by a vivid flash of light. During a thunderstorm, a positive charge builds in the upper part of a cloud and a negative charge builds in the lower part of the cloud. The difference between the positive and negative charges increases, generating an electrical field, until the electrical charge jumps from one area to another. Lightning may travel from cloud to ground or cloud to air or cloud to cloud or stay within a cloud. The main types of lightning are:

• Streak lightning: A single or multiple zigzagging line from cloud to ground.
• Forked lightning: Lightning that forms two branches simultaneously.
• Sheet lightning: A shapeless flash covering a broad area.
• Ribbon lightning: Streak lightning blown sideways by the wind to make it appear like parallel successive strokes.
• Bead or chain lightning: A stroke interrupted or broken into evenly spaced segments or beads.
• Ball lightning: A rare form of lightning in which a persistent and moving luminous white or colored sphere is seen. It can last from a few seconds to several minutes, and it travels at about a walking pace. Spheres have been reported to vanish harmlessly, or to pass into or out of rooms—leaving, in some cases, signs of their passage, such as a hole in a window pane. Sphere dimensions vary but are most commonly from 4 to 8 inches (10 to 20 centimeters) in diameter.
• Heat lightning: Lightning seen along the horizon during hot weather and believed to be a reflection of lightning occurring beyond the horizon.