[Rhodes22-list] lightning strike probability

brad haslett flybrad@yahoo.com
Thu, 21 Nov 2002 03:36:07 -0800 (PST)


Roger,

It looks like there’s still some life in this “dead
horse” so I’m giving it one more kick.

Thanks for the input.  The data you referred to have
been verified many times by several different sources
including a recent study by a Russian scientist
studying lightning strikes in Hong Kong.  The problem
is that the data isn’t 100% relevant to the sailboat
application.  Those studies were done in industrial
and commercial settings where at the 25-foot level
there were many other targets as opposed to a solitary
tall structure. Plus, all of this data comes from land
based studies. As you say, when you increase the
height of a structure, its probability of being struck
increases substantially.  However, if you did the
analysis on several square miles of densely placed
500-foot towers it would skew the data just as it does
in an industrial plant full of 25-foot structures. 
That being said, strike probability clearly increases
geometrically with height. A Rhodes 22 in your
driveway with its mast up is no more likely to be
struck by lightning because of its mast height than
the two-story home next to it.  When you park it in
the middle of an open body of water the odds change. 
To quote Dr. Marcus O Durham, Associate Professor at
the University of Tulsa and director of the Power
Applications Research Center, “The strike frequency of
lightning goes up with the square of the height above
average terrain.  This makes isolated structures
particularly vulnerable……..Sharp points increase the
likelihood of a strike. (vhf antenna, mast, etc. bh) 
The high electric field below a charged cloud will
create ions in the neighborhood of sharp points. 
These ions increase the air conductivity and
probability of a cloud discharge (strike).”  Which
brings me around to the original question; if you are
sleeping on your R22 in quiet cove and are awakened by
approaching thunder do you?
a) leave the cove for open water – b) stay in the cove
and compete with the trees c) throw out your mast
grounding system or – d) put in your cassette of
“Don’t Worry, Be Happy!” and play it at a volume
greater than the thunder?  One nice thing I have
discovered in all this research is that in Tennessee
I’m number five on the risk list behind (in order)
Florida, North Carolina, Texas, and New York.  New
York?  Yep, didn’t make it up.

Brad Haslett
“CoraShen”




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