Ask A Scientist© Weather Archive

name        Jeff
status      educator
age         40s

Question -  Why when lightening strikes a tall pine tree it creates a clockwise
spiral around the outside of the tree. I think the the electrical
circuit between the cloud and the ground interacts with the earths
magnetic field causing the charge ions to spiral and this charge going
up and down the bark causing the disruption I see on the bark.


Jeff,

The earth's magnetic field is much too weak to affect
lightning. More likely, the pine tree has a natural twist
in the grain. This could cause the lightning to follow the
twist as it follows the lowest resistance path of sap to
the ground.

David Cook
lightning researcher
Argonne National Laboratory
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Dear Jeff-

   That is an intriging theory about the spiral track of lightning. I'm not
aware of any investigations into this area. Studies have shown that
lightning follows the path of least electrical resistance through the air,
and that magnetic field effects on ionized particles exist at much higher
altitudes than the region of convective weather occurrences.

Here are a couple of links that discuss lightning formation and its effects
in more detail This first one is a NASA link, and has some good graphics.

http://thunder.msfc.nasa.gov/primer/

This next one, from the publication "American Scientist," is more technical,
but describes how the electrical charge in clouds originates.

http://www.sigmaxi.org/amsci/articles/98articles/blackintro.html

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
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