Most lightning flashes are a result of negatively-charged leaders, called stepped leaders. These leaders develop downward in quick steps. Each step is typically about 50 meters (150 ft) in length. Stepped leaders tend to branch out as they seek a connection with the positive charge on the ground. When a branch of the stepped leader reaches within about 50 meters of the ground or some object on the ground, it connects with an upward-developing positive charge, often referred to as an upward streamer.
Upward streamers tend to develop from the taller objects beneath one or more branches of the stepped leader. When the downward-developing negative stepped leader makes contact with an upward-developing positive streamer, referred to as the attachment process, a conductive path is established for the rapid discharge of electricity that we see as a bright flash.
In general, stepped leaders travel at about 200,000 miles per hour, although speeds vary considerably. The highly visible return stroke moves upward through the leader channel at about 200 million miles per hour. The combination of the stepped leader and return stroke happens in just a fraction of a second. While both leaders and return strokes produce visible light (leaders produce a faint light that is more visible at night), they happen so quickly that the human eye cannot distinguish the two; however, high-speed cameras are able to capture the movement of leaders as they move toward ground.
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