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LIGHTNING STRIKE PROBABILITY
On 11 Jul 2004 10:05:27 -0700, (Parallax)
wrote:
... I found a site giving the strike rate for Malaysia and remember
that only two places have a higher strike rate than Central FL,
Equatorial Africa and SE Asia. Malaysia has a strike rate of
25/km^2/yr. I remember that C. FL rate isnt too much smaller than the
highest so I assume 10/km^2/yr and you get (using my effective area)
of a 7% chance/yr of a LONE sailboat getting hit. I find this to be
VERY believable based on experience.
///
I have found web sites calculating strike probability and they all use
a formula similar to mine where effective area goes as the height
squared. However, none i have found specify the factor by wich to
multiply the height to account for electric field modification, they
just qualitatively say it goes as height squared or use empiracaly
derived effective areas.
Here's a possible reason for some lightning estimators using height
squared.
Electric utilities use a cone of protection of 60 degrees included
angle beneath a ground wire.
If one used this 60 degree cone, the surface area "protected" by a
mast, in this case the area at risk, would be pi r squared,
where r is given by mast height X tan 30 degrees = 0.577 X m.h.
so pi r squared is pi X (0.577 X m.h) ^2 = 1.047 X m.h.^2
In other words, mast height squared is a reasonable area at risk
using the assumptions given above.
Then instead of a 7% per annum risk in Tstorms,
the risk is about 0.26% p.a.
Brian Whatcott Altus OK
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