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Multiple cloud-to-ground and cloud-to-cloud lightning strokes during night-time. Credit: NOAA
how lightning is formed
Lightning occurs when electrical charge separation occurs in thunderstorms, creating some regions with overall positive charge (+) and others with predominantly negative charge (-). Six types of lightning are indicated on the diagram, numbered (1) the most frequent to (6) the least common. Air currents in clouds are indicated by black arrows
A discharge of atmospheric electricity resulting in a flash of light in the sky. Most lightning occurs between two parts of a single cloud, some between cloud and ground, and a small amount between one cloud and another. Flashes range from a few kilometers to about 150km in length, and typically have an energy of around 300kWh and an electromotive force around 100MV.

A typical discharge consists of several lightning strokes, initiated by leaders that follow an irregular path of least resistance – the lightning channel. Intense heating by the discharge expands the channel rapidly up to a diameter of 13–25cm (5–10in), creating the sound waves of thunder.

Cloud-to-ground lightning usually appears as forked lightning. A relatively faint light moves towards the ground at about 125km/s in steps, often branching or forking. As this first pulse (leader stroke) nears the ground, electrical discharges (streamers) arise from terrestrial objects; where a streamer meets the leader stroke a brilliant, high-current flash (return stroke) travels up along the ionized path created by the leader stroke at about 100 million m/s (nearly half the speed of light). Several exchanges along the same path may occur. If strong wind moves the ionized path, ribbon lightning results.

Sheet lightning occurs when a cloud either is illuminated from within or reflects a flash from outside, in the latter case being called heat lightning (often seen on the horizon at the end of a hot day). Ball lightning, a small luminous ball near the ground, often vanishing with an explosion, and bead lightning, the appearance of luminous beads along the channel of a stroke, are rare.

Lightning results from a buildup of opposed electric charge in, usually a cumulonimbus clouds, negative near the ground and positive on high. There are several theories which purport to explain this buildup.

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