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Lightning Explained, Part 1
The leader moves in straight-line bursts or steps with lengths ranging from 30m to 100m. Within each step is an extraordinarily complex ballet of electrical activity that is only beginning to be understood. In recent years, high-speed photography of lightning has produced some spectacular images but the frame rates available are still too slow to capture these details. However, we believe the basic process can be described as follows.
Within each step, as the leader starts its forward momentum (assume negative polarity) it injects electrons ahead of its path through a large zone of streamers at its tip. These electrons quickly combine with air molecules and form negative ions (space charge). Since both the leader and the newly produced space charge are of the same polarity (negative in this case) the space charge opposes the leader's forward propagation. The leader slows down and eventually comes to a complete stop, suspended in mid air. The flow of charges stops and the voltage at the leader tip increases during this stationary phase. Suddenly huge bursts of streamers reach out in a new direction, avoiding the negative space charge, which had previously stood in the leader's path. The leader follows in the new direction and takes another step.
Sometimes, to avoid the space charge in its path, the leader will branch out in two different directions to go around the electrostatic obstacle in its way. These processes produce the zigzag patterns and branching that we see in the sky.
As the leader propagates it creates a sheath of negative space charge that can be traced all the way back to the localized volume of charge in the sky that created it and through all the various steps and branches that were taken. As the leader extends farther and father away from the source volume of charge, the peak voltages that appear at the leader tip decline and the steps become increasingly smaller and the leader moves increasingly more slowly, even as it approaches the ground.
It is widely thought that the negative stepped leader zig zags because it is attracted to rising pockets of positive space charge coming from the ground, but this fails to explain the fact that the same zig zag pattern is observed when the stepped leaders move in between the clouds. Furthermore we see the same types of tortuous patterns in all electric discharges in the lab: They're never perfectly straight lines. The reasons for the more pronounced effect in lightning, is due to the sheer size of the charge volumes involved and the kind of voltage waveforms that result. But by using voltage rise times above the critical front for a particular laboratory gap, we can make leaders stop and start in the lab as well.
So once one of these stepped leaders is born, the parameters involved in dictating its path are complex and the particular path it takes will seem quite chaotic.
Lightning never actually comes down and touches any grounded object. Before that happens, the negative descending stepped lightning leader induces so much charge of opposite polarity that grounded objects launch positive upward leaders that rise up to meet the descending leader. It is like the most perfect missile guidance system. The trajectory of the positive upward connecting leader is powered and guided by its target, the charges sheathed around the negative descending stepped leader and those ahead of its tip.
Imagine a typical thunderstorm over an urban area populated by tall buildings with heights in the 60m range (20 stories). The cloud base is 2km above the ground, the electric field rises and falls as charges develop and discharge in the clouds. With a measured electric field of 10kV/m on the ground, a negative stepped leader is born and its trajectory is turning toward the earth. It is a comparatively small strike, of say a prospective return stroke current of only 12kA. An average value would be about 30kA and the most extreme, largest examples could be hundreds of kA.
-- continued on page 3
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