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LIGHTNING STRIKE PROBABILITY
parray, you take back everything you ever said about anything.
Out of curiosity, I did a quick back of the envelope estimate of the probability of a lone sailboat getting hit by lightning in a thunderstorm. Assume LONE sailboat, Flat conducting ocean, assume the mast is well grounded, assume lightning equally distributed over the thunderstorm area. Now, I have forgotten how to do this calculation (this used to be my speciality) but I vaguely remember that a spike type conductor above a grounded plane produces minimal effects more than 3X its length away. This means that a mast produces an EFFECTIVE AREA (a) of pi*9*l^2 where l = mast length. BTW, I work in meters. As I write, a weak thunderstorm is passing over the bldg. lightning rate seems low, less than once/minute and most are prob NOT cloud to ground. However, I estimate wind at 40 kts. Based on a few storms I have seen on Apalachee bay, I think the 5.8% probability for those intense storms to be reasonable and I seriously avoid them. Strongest storms cover a larger area with greater wind. I have never been in one of those but have watched them. The sky doesnt just get dark, but turns a strange color. Intense strike rate for a few minutes followed by a sudden increase in rain rate followed by a taper off in wind and strike rate. I assume a thunderstorm diameter of 4 km based on driving through one two days ago for a total storm area of 12X10^6 m^2. I assume the lightning strike rate is about 3/minute and the storm lasts for 40 minutes for a total number of strikes/storm (n) to be 120. SO...the probability of getting hit is: P=n*a/A *L (L is your "luck factor", 0L1 means you are either optimistic or lucky, L=1 means you do not believe in Luck, L1 means you are pessimistic or unlucky, L=0 means you dont really sail). Top of my mast is 17 m above water for an effective area of 7803 m^2. So, P= 120*7803/16X10^6 *(L) = 585225X10^-6 * (L)= .058 * (L) or 5.8% * (L). Based on limited experience with games of chance, I estimate my L = 3 so my strike probability is 17.4%. Seriously, the 5.8% chance shows why sailboats do not get struck more often although they are the tallest things around. OK, OK, I take back anything I said about wimpy thunderstorms up north after viewing a series of websites with supercell storms over the great plains states and all the way up to MN. |
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