In September, the 2001 International Conference on Lightning and Static Electricity (ICOLSE), which is part of the Aerospace Congress and Exhibition (ACE), was held in Seattle, Wash. Although the conference has become the central forum for the aerospace lightning protection community, it also presents research reports on lightning phenomenology, ground facility protection, and other lightning safety issues.
One report, submitted by J. G. Dash and J. S. Wettlaufer from the University of Washington, Seattle, addresses one of the great mysteries of lightning: the microscopic processes leading to the enormous voltages that develop in clouds from charge separation. In general, we know that some electron or ion transfer involving small ice particles occurs, but a microscopic understanding had been lacking. The authors developed a microphysical model for the charge transfer produced in collisions between microscopic ice particles and hail. The model reproduces many of the observed electrification observations.
Another interesting report was presented by C. B. Moore, G. Aulich, and W. Rison, from the New Mexico Institute of Mining and Technology. The authors compare the effectiveness of various lightning rod tips in terminating lightning discharges. In an experiment designed as a competition, they installed an array of three rods (forming an equilateral triangle) on a lightning-prone mountaintop. Each rod had a different tip: a sharp one, a blunt (hemispherical) one, and one with an early-streamer emission (ESE) device. Periodically, the size of the blunt tip was changed using tips with diameter sizes of 9.5 mm, 12.7 mm, 19 mm, or 51 mm.
Over an 8-year period, they monitored the current pulses to the individual rods. Amazingly, all strikes to the array went to the blu0nt-ended rods. Only the 12.7 and 19 mm rods were struck, with most of the strikes going to the 19 mm rods.
The authors explained this surprising result with an analysis of what happened to the electric field in the area just above the rod's tip. The pointed rods began to emit streamers (corona) at much lower ambient field levels than the blunt rods, However, as the streamers left the immediate region of the tip, the field level fell rapidly, and the streamer failed to propagate upward. The 51 mm rods were presumably limited by the fact that they required much higher ambient fields to produce the initial streamers.
Of course, these results don't prove that the unstruck rods would not have received discharges if they had been the only available terminators. They simply show that the 12.7 mm and 19 mm blunt rods were “more attractive” than the others.
Finally, R. Kithil, of the National Lightning Safety Institute, and V. Rakov, the University of Florida, submitted a critical analysis of NFPA 780, the National Fire Protection Association's “Standard for the Installation of Lightning Protection Systems.” Although not a technical research paper, the report recommends additions and updates to certain areas of the standard, and it looks at other documents and standards that discuss lightning protection systems.
To order a copy of the conference proceedings on CD-ROM, go to www.sae.org/calendar/ace/bookstore.htm#proceed.