Uncovering the mechanisms of lightning varieties

University Park, Pa. – The mechanism behind different types of lightning may now be understood, thanks to a combination of direct observation and computer modeling reported by a team of researchers from New Mexico Tech and Penn State.

"Our explanation provides a unifying view of how lightning escapes from a thundercloud," the researchers report in the April edition of Nature Geoscience.

Most people see lightning strikes that go from clouds to the ground, but some lightning goes upward, forming blue jets and gigantic jets. Perhaps the most dangerous lightning appears as "bolts from the blue" – lightning that begins upward, but then moves sideways and then downward to hit the ground as much as three miles from a thunderstorm.

About 90 percent of lightning occurs inside clouds and is not visible to the casual observer. The researchers wondered if lightning that appears within clouds and the lightning that escapes upward or downward shared the same development mechanisms.

"With the help of colleagues from New Mexico Tech, we were able to build a model of lightning and apply it to the various types of lightning," says Jeremy A. Riousset, graduate student in electrical engineering, Penn State. "Thanks to their observations and measurements, we know how lightning like 'bolts from the blue' happen. We know they develop like normal intracloud lightning before escaping the thundercloud at upper levels and branching toward the ground."

They also discovered that upward and sideward lightning events occurred shortly after normal downward lightning bolts occurred or intracloud lightning produced a local charge imbalance in the cloud.

Harald E. Edens, graduate student in physics, New Mexico Tech, working with Paul R. Krehbiel, professor of physics; Ronald J. Thomas, professor of electrical engineering, and William Rison, professor of electrical engineering, all at New Mexico Tech; and Mark A. Stanley, consultant, obtained detailed pictures of "bolts from the blue" using New Mexico Tech's Lightning Mapping Array, a three-dimensional lightning location system that uses multiple measurement stations to capture and time the VHF signal of the lightning. The Lightning Mapping Array can map lightning within clouds, something that normal optical photography or videography cannot do.