Benjamin Franklin showed scientists how to detect the electric charge of a thunder cloud. Now Earle Williams is showing how an observer at a single site can monitor lightning strokes around the world. What's more, he may be able to use his data to track changes in Earth's global temperature, he says.
"It's one of the few ways we can measure a global property from a single station," notes Dr. Williams, a researcher at the Massachusetts Institute of Technology. He presented his work during a symposium to commemorate progress in the field at the American Association for the Advancement of Science here last month.
Over the past three years, Williams has gathered the most comprehensive data yet assembled on global lightning activity. He notes there is a connection between the frequency of lightning and global temperature. He hopes to use changes he detects in that frequency to track changes in global temperature. Over time, this could show any trend toward a man-made global warming.
The science of atmospheric electricity has come a long way since Franklin founded it 2-1/2 centuries ago. Since Franklin's day, scientists have confirmed that we live in a vast electrical circuit. Earth's surface acts as an electrical conductor. About 80 kilometers above us, the ionosphere - an atmospheric region loaded with electrically charged particles - acts as a second conductor. The air between acts as a slightly leaky electrical insulator.
Between the ionosphere and the ground there's an average difference of 250,000 volts. However, we scarcely notice the 1,000-ampere current - 10 times a typical household's electrical supply - that gently flows between the ionosphere and ground in fair-weather regions. What we do notice is the 100-fold more powerful currents that flash at up to half the speed of light between the ground and a thunder cloud in lightning strokes. Differences of hundreds of millions of volts between ground and cloud drive these awesome discharges. Thunderstorms are the electrical generators that keep the global electric current flowing.
Even though Franklin didn't know the magnitude of these currents and voltages, he did know he was playing with powerful electrical forces when he sent a kite toward a thunderhead and drew sparks from an attached key in 1752. Several years earlier, Franklin established his fame as an electrical scientist when he designed the crucial experiment to prove thunder clouds are electrified. In this, an observer would stand in a shelter with an iron rod projecting through the roof. The observer could then draw sparks from the rod if a passing thunder cloud were electrically charged.
Williams's modern research echoes this early work. Where Franklin's experiment detected electrical charge, Williams is detecting lightning-generated radio waves - something Franklin didn't know existed. The two global conductors - the ground and ionosphere - guide these waves as they travel around the world. Williams detects them with an aluminum ball on a 10-meter pole set up in a Rhode Island forest. A computer program that simulates how the atmosphere transmits the radio waves can pin down the location of a lightning bolt to a few hundred square kilometers half a world a away.
Another effect Franklin didn't suspect is the displays that shoot upward from powerful thunderstorms. Mysterious blue jets sometimes burst skyward. Upward traveling radio waves create so-called elves that appear as expanding rings of light as high up as the ionosphere. Spectacular red displays called sprites often appear at levels a bit lower than the elves.
David Sentman from the University of Alaska at Fairbanks, who is pursuing these phenomena, told the symposium there is no satisfactory explanation for them. It seems to take exceptionally powerful thunderstorms to excite elves and sprites. These tend to be storms where lightning comes from the top of a thunderhead to the ground rather than flashing between the ground and the cloud bottom as is usually the case. This top-to-ground lightning is, itself, a mystery.
Dr. Sentman observed that "lightning is still one of the most mysterious natural forces." That being the case, Franklin still has the last word: "Let the experiment be made," he reputedly said.