Friday, August 3, 2012

Lightning in Slow Motion

Here's a video may change your understanding of lightning, http://vimeo.com/28457062
It shows a strike at 7,207 frames per second and begins with a "negative leader" forking its way down trying to find something to "ground" it. Note how the tendrils branch out like tree roots. Then one branch touches ground and something remarkable happens. All the other branches vanish as a positive return stroke follows the lightning path back up to the cloud. I was also surprised at how long the return strike lasted - I thought the movie was over but the frame counter at the bottom was still going.

Here's how Wikipedia describes a lightning strike:
In a typical cloud-to-ground strike, electrons descends from cloud base to ground. Just before the strike leader reaches the ground, the charge in the step leader induces a huge electric potential in objects connected to the ground (some 10's of million volts), that brings up spikes of positive charge flow from high sharp objects, lightning rods, people, trees, etc. connected to the ground. Once the descending and rising charge paths have met massive amounts of charge flows in the 1 cm thick ionized channel of air centered in a lightning bolt channel--this massive flow of charged particles heats the air and gives the brightest part of a lightning strike. The stepped leader of a bolt of lightning may take on average about 20 milliseconds to reach the ground. Occasionally much longer lightning strokes occur which take more time. Once the downward and upward current flow impulses meet--a few meters or tens of meters above earth—a much more conductive connection is established between the cloud and the ground and the front edge of the return stroke electrons zip from the cloud at about 0.3-0.5 times the speed of light, c, on the highly ionized lightning stroke path. Return currents may continue for several microseconds or even repeat.The return stroke with its much larger current flow produces the highly visible intense main lightning strike as it heats and ionizes the surrounding air in the lightning channel to about 30,000 degrees C (54,000 degrees F).

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