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This remarkable photograph was taken by Johnny Autery at 4a.m. on May 3, 1984. It captures the bottom 700 feet or so of a cloud-to-ground lightning flash striking a 65-foot sycamore tree. Perhaps the most interesting scientific features of the photograph are the two unconnected, upward-propagating discharges-one from the tower, the other from the tree. The lengths of the discharges are about 45 feet and 25 feet, respectively, if they are parallel to the plane of the photograph, longer if not. In a typical cloud-to-ground lightning flash, when the downward -propagating lightning stepped leader (initiated in the cloud at a height of 15,000 feet or so) gets within 100 to 200 feet of objects on the ground, the strong electric field on those objects produced by the leader charge causes them to initiate upward discharges. One of these upard-propagating discharges connects with the downward moving leader and thereby determines the lightning path to earth. In this case, that upward discharge was apparently initiated at the top center of the tree and was no more than about 55 feet long--as evidenced by the downward branch in the channel about 55 feet above the tree top, indicating that at that point there was still a downward-propagating leader. Note that the unconnected discharge from the left of the sycamore is branched upward, indicating upward propagation. The horizontal section of the lightning channel at the tree top is another interesting feature. Instead of following the main trunk of the tree to the ground via the upward discharge at the top center of the tree, the downward discharge apparently jumped to and then followed the major branch (from which the unconnected discharge emanated) until it reached the main trunk. A final interesting feature is the apparent gap in the light at the very bottom of the sycamore. It is likely the lightning current flowed through metal rods that were leaning against the tree, thus producing no light for the 3- or 4-foot length of the rods. Mr. Autery's remarkable photograph is one of only two published close-up pictures of direct lightning strikes to trees of which I am aware. The other, a photograph of a lightning flash that struck a 23-foot European ash near Lugano, Switzerland, was taken at a distance of 200 feet by Prof. Richard E. Orville of the State University of New York at Albany. It was published in his article, "Photograph of a Close Lightning Flash," in Science magazine (vol. 162, 666-667, 1968) and on the cover of my paperback book, All About Lightning (Dover Publications, New York, 1986). Until now, it was the best lightning picture I had ever seen. Surprisingly, neither Autery's sycamore nor Orville's European ash suffered significant damage when struck, and both trees are healthy today. Martin A. Uman, one of the world's leading lightning experts, is chairman of the Department of Electrical Engineering at the University of Florida. |
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