Metal Objects Do Not Attract Lightning

I've thought hard about how to prove my assertion that "metal objects do not attract lightning". I've put together, I think, a persuasive case. I've written this for the person who is not already familiar with the physics of lightning formation. It seems to me that those who are already practiced in this field will be disinclined to believe the notion that metal attracts lightning.

Do metal objects "attract" lightning? Perhaps this can be most clearly answered with a question-and-answer format. I should point out that I am not a working professional in this field, but I have a substantial background in physics, including some work with the physics of air breakdown by high intensity photon (laser and microwave) beams.

(1) What determines where lightning strikes?
Cloud-to-ground lightning is initiated high above the ground in thunderheads, far from any influence of ground structures. The leaders make their way in steps from the cloud toward the ground, following the large-scale electric field between the cloud and the earth. Until the stepped leader reaches within roughly 50 meters from a ground object, its path is unaffected by structures on the ground. To first order, the location of a lightning strike is determined entirely by the dynamics of charge production in the cloud.

(2) OK, now the leader is 50 meters from the surface. What determines exactly what or where it hits?

The approaching leader develops an extremely large electric field between its end and the ground, on the order of one megavolt per meter. The electric field induces a corona discharge, that is, a region in which electrons accelerated by the field acquire sufficient energy to ionize air molecules, thus multiplying the number of free electrons and producing a column of electrically conductive air.

The smaller the distance between the endpoint of the leader and any object on the ground, the higher the field. Hence there is a tendency for this discharge to occur first from the highest points, simply because of the reduced distance to the leader. This is why standing upright in an open field during a thunderstorm is a really bad idea. Swinging a golf club out in an open course is also dangerous; not because the club shaft is conductive, but because the business end of the club may, at either end of the swing, become the tallest point in the field.

(3) Does it matter whether the object that is the source of the corona discharge is metallic or not?

The current flow associated with the corona discharge is not large -- nothing approaching the current flow of the lightning stroke, or even of the leader. If the current flow were high, then the voltage drop across near-insulating ground objects would be sufficient to quench the discharge, and tall but nonconducting objects would not be favored lightning targets. By observation, we know that nominally nonconducting objects, such as buildings of wood or brick, and trees, are very commonly hit by lightning. Perhaps the strongest example is the observation, from the days of wooden sailing ships, that the tallest masts were by far the most frequently struck points, even though the thoroughly dried wood of ship masts is an extremely poor conductor.

Thus high electrical conductivity is certainly not required for the corona discharge source; even the very low conductivity of dead, dried wood will do. Given that result, there is no reason to expect the very high conductivity of metals to make any difference.

(4) So why is it commonly thought that metal "attracts" lightning?

First, lightning rods -- air terminals -- are necessarily metallic, and very well grounded. This high conductance construction is not to "attract" lightning, but it is essential for the rod to handle the extremely high current of the lightning strike without self-destruction.

Second, a nearby lightning strike can induce large currents in conductors, and those currents can produce high voltages if these secondary conductors are ungrounded -- hence the caution to avoid using wireline telephones during electrical storms. This may give the impression that the metal has "attracted" lightning, even though this phenomenon is purely one of electromagnetically induced current.

Jack McKay, Ph.D. Physics, M.S.E.E.

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com