They tuned in to VHF signals the kind that produce static on non-cable TV given off as lightning searches for a path to strike. Each receiver records the time and location of every step of the discharge, mapping the lightning channel in fine detail from beginning to end, inside and outside the thunderstorm. The details are then combined with the 3-D lightning mapping to create a comprehensive picture of the inside of a thunderstorm. It acts as sort of a signature of a tornadic storm. Lightning hole Lightning-free hole is visible in top image, which shows mapped lightning discharges from a severe supercell thunderstorm on June 29, 2000.
Bottom panel is the mapped lightning overlaying the radar reflectivity.
Note that the lightning-free hole is directly over the tornado. These can cause more damage than negative strikes and are responsible for starting the majority of lightning-triggered fires. Overlaying a storm s lightning history on the radar picture led New Mexico Tech s Paul Krehbiel and his colleagues to discover the lightning holes in Oklahoma thunderstorms in 1998. He believes that the charge in the thunderstorm leading to lightning gets thrown out of the updraft, which can reach speeds of 100 mph, and falls with the prevailing wind through the storm to the east side of the hole. That is where the LMA has found a higher concentration of lightning.