Lightning produces substantial radio frequency radiation that can be used to image the developing breakdown channels inside a storm, using interferometer or time-of-arrival techniques. We have recently developed a time-of-arrival system that reveals the structure and development both of individual lightning discharges and of the overall lightning activity in a storm. The system measures the arrival time of impulsive radiation events in an unused TV channel (e.g., Channel 3, 60-66 MHz) at a countywide array of stations, using GPS clocks at each station. The instrument has been used to study the relatively infrequent lightning in the storms of central New Mexico as well as the nearly continuous electrical activity in large, supercell storms of the western Great Plains.

As often happens when a new observational system is developed, the mapping array has quickly become a valuable research tool, in this case for studying lightning and storm electrification processes, to a degree not originally anticipated. Examples have been the finding that supercell and other large hailstorms electrify in a manner that is inverted in polarity from that normally observed in storms, the discovery of characteristic lightning-free regions or `holes' in tornadic storms, the ability of lightning mapping observations to detect strong convective surges in storms, and an understanding of lightning `bolts from the blue'.