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Jagged
Wed May 26, 2004
Listen in RealAudio 
Hi, I'm Bryan Yeaton for The Weather Notebook. If you've seen a spark of static
electricity jump from an object to your finger, you may have noted that this mini-bolt
follows a pretty straight path. So why are lightning bolts in the sky so jagged?
The difference lies in the length of the two sparks. The finger to doorknob path is less
than an inch long while a lightning bolt generally reaches across three miles or
more.
Air does not conduct electricity very well so when charged areas build in clouds, large
voltage differentials, millions of volts, form between the cloud and ground below. To
relieve this electrical pressure, the cloud sends out a weak, negatively-charged
streamer that searches for the path of highest conductivity to an area of positive
charge. A streamer advances in discrete steps about 150 feet long to create an ionized
channel called a "stepped leader." But even within that channel, the streamer searches
millimetre by millimetre for the easiest route.
The leader moves across the sky connecting those regions of least resistance in an
irregular path, much the same as running through the forest would not follow a straight
path. Forest obstacles are easier to go around than straight through, and so are
randomly strewn segments of highly resistant air. Should the leader find two paths of
similar conductivity, the leader splits into a fork, exploring each for the best
route.
When a leader connects with an area of positive charge at the ground, a lightning bolt
races back up the leader channel to relieve the electrical pressure, illuminating the
jagged leader pathway. Usually, several bolts are required to relieve all the potential
during a typical flash.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is funded by Subaru of America and the National Science Foundation. We are a
program of the Mount Washington Observatory.
Today's Links
How Lightning Selects a Target
http://www.psihq.com/iread/whatlite.htm
What Causes Lightning?
http://www.voltnet.com/lightning_atmos/index.shtml
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