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Lightning strike probability
I once did a calculation of lightning strike probability for a lone
sailboat and unfortunately I lost it and am too lazy to repeat it. It
was based on the number of lightning strikes per surface area per year
in a given area times the cone of protection area produced by a mast
of certain height and taking into account how much you sailed. What I
got was a number expressing the probability of a lone sailboat under a
storm being hit by lightning, it was surprisingly high. I had a long
argument about this in a sailing NG but eventually I found the
insurance company stats and they were remarkably close to my number.
What I do remember is that your probability of being struck went up
with the SQUARE of mast height so a mast or tuna tower twice as high
as another quadrupled your probability of being struck.
This number said nothing about the probability of a sailboat with
others being in a marina being struck.
So, after this calculation, I am very careful about sailing in
thunderstormy weather. |
Frogwatch
Jul 2
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| Uh.... is this some sort of justification of a low transom..... |
Gene
Jul 2
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| No, its my way of avoiding work.
OK, I cannot help myself. I looked up the strike density.year for
where I am, roughly 10 km2/yr multiplied by protected cone area of my
mast gives a roughly 1/1000 chance of being hit IF THE BOAT WAS ALL
ALONE. This is NOT the same as the probabiluity of being hit if you
sail under a storm. That probability is much higher. |
Frogwatch
Jul 2
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| Man, are we feeling lazy today or what? If things suck that bad, I'm
guessing you could get a couple more distant places of PI before
lunch! |
Gene
Jul 2
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| Assume a storm is 10 km X 10 km which is 10^8 m2. If you sail under
said storm and stay, your (my) sailboat is then 10^-5 of the storm
area. We have to guess the number of cloud to ground strikes for a
storm and multiply by it to get our probability for getting struck if
you insist on sailing into a thunderstorm. I think this number will
come out to anywhere from .001 to .01. CLOSE ENOUGH, now I can go back to work. |
Frogwatch
Jul 2
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| That's all assuming some things, of course. Lightning will take the
path of least resistance (most of the time!), and that path may alter
the odds. |
Loogypicker
Jul 2
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| That's certainly true, but I think it is a fair assumption that
sticking that 50' aluminum conductor into the air just made you the
number one candidate for "... path of least resistance...." |
Gene
Jul 2
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| Eisboch had the real answer - it's all about differing potentials for
current flow. For that, you need to analyze more than just the number
of strikes per sq/ft. like ground conductivity, saline content, heavy
metals, etc. Additionally, in particular water, there's surface
temperature which is a part of the equation also. You also have to
account for ionization of the air particles, partial pressures of the
various components of gases in the atmosphere - yada, yada, yada. Admittedly, sticking a 50 foot piece of aluminum into the air helps
the probability a little, but if it were the only variable, why do
sail boats in marinas with short masts (by comparison) get hit as
often as larger boats with taller masts? I know when I had my towers, the number of strikes went down
dramatically when the heavy duty ground system was installed - a
copper wire grid that used bare #10 stranded welding wire - about a
mile of it I think. A good grounding system is a must to allow for equalization of
potential. |
Wizard
Jul 2
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| I thought the "cone of protection" term was used to describe a "safe
zone" when lightning hit, and no connection to strike probability.
???? --Vic |
Vic
Jul 2
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