How does a cloud-to-ground flash unfold?
Despite its confident appearance, a lightning flash develops in fits and starts. The path of a typical cloud-to-ground (CG) flash lowering negative charge to earth is carved by a series of stepped leaders, each moving a bundle of charge a distance on the order of a city block. Each step takes only 1 microsecond or so, but the pauses between steps are much longer–on the order of 50 microseconds. At each step, the bolt may shift direction toward a stronger electric field, thus creating its crooked appearance. As a CG flash approaches several regions of opposite charge on the ground, it often branches into several parts. Just before it reaches ground, the step leader induces a huge electric potential (some 10 million volts), enough to bring up surges of positive charge from sharp objects or irregularities near the ground. Once the impulses meet–a few tens of meters above earth–the connection is established and the return stroke zips upward at a rate much faster than the stepped leader’
