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Do lightning bolts move at the speed of light? If not, how fast and if so why does it look as if you can see itís travels down to the ground?
- Vincent Romelo (age 18)
Las Vegas Nevada
The bottom tip of a lightning bolt traveling from a cloud to the ground does travel rather quickly, although it travels at much less than the speed of light. A lightning discharge consists of electrons which have been stripped from their molecules flying through the air. They are accelerated by a strong electric field, a consequence of the big voltage difference between the cloud and the ground. They crash into air molecules on their way down and free other electrons, making a tube of ionized air.
The "leader", the first stroke of a lightning discharge, actually proceeds in steps -- lengthening by about 30 meters at a time, taking about a microsecond (one millionth of a second) to do each step. There is a pause between steps of about 50 microseconds. The whole process may take a few milliseconds (one-thousanths of a second), providing enough time to perceive motion. Most of the charge flows after this leader makes electrical contact with the ground, however. A powerful "return stroke" releases much more energy. That's not the whole story, however -- a lightning flash may have only one return stroke or may have several tens of strokes using the same column of ionized air. It may seem to flicker.
(republished on 07/13/06)
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