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Q & A: The Speed of Lightning

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Most recent answer: 10/22/2007
Q:
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
A:
Hi Vincent,

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.

Tom

(published on 10/22/2007)

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