American in Spain


September 26, 2007

Last weekend, we had a thunderstorm come through. They are way more rare here than in my native North Carolina. We also have two new cranes that have been put up next to our house to build even more housing. Intrigued by the idea of two visible metal cranes towering into the sky and a coming electrical storm, I set up my camera to record out the window. I left it recording for a full hour. That's 1 GiB of video! Then I isolated the lightning strikes. Then I reduced it down to the only two worth showing here. Stay for the end of the video to see them in slow motion. The first clips are shown in realtime to demonstrate the delay between lightning and thunder. And the very first clip is some slightly sped up crane movement before the workers stopped. Notice the "re-strike" effect in the slow motion version.


There's something I'm confused about here. From what I understood, thunder was caused by the sonic boom from the rapidly expanding air around the lightning bolt. From Wikipedia:

The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke

This make sense to me. But what doesn't make sense to me is what I've recorded in the video, particularly the first strike. You can clearly hear the first strike several seconds before the thunder! This seems to clearly violate the thunder explanation. The only way that I could see where it might fit the theory is if this bottom part of the bolt wasn't hot enough to cause a shock wave, but the top part, several sound-seconds up, was. Either that or the thunder came from another bolt somewhere else.

I'm a little confused. The comments are open for postulation by other arm chair scientists.