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Cloud to ground lightning originates near the base of the cloud, and is initiated by a small discharge that releases free electrons that move toward the ground forming a stepped leader.† The stepped leader is an invisible discharge, moving downward in discrete microsecond steps that are about 50 meters (165 feet) long.† An upward moving leader begins from the ground to meet the approaching downward moving negative stepped leader once it is within 100 meters (330 feet) or less from the ground.† This upward moving leader generally comes from objects that are taller than their surroundings, such as trees or buildings.†
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After the leaders have made contact, the visible lightning strike, called the return stroke, moves up from the ground along the stepped leaderís ionized path.† Multiple successive strokes occur along the original path, separated only by several tens of milliseconds, continuing until the charge on the lower part of the cloud is removed.† The entire process typically takes about half a second to complete, and is referred to as a flash. The flickering effect that is often seen occurs because the human eye is just able to discern the individual pulses of luminosity created by each stroke.
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