September 10, 2010 | By Eric Sorensen | Comments Off
Categories: Earth sciences, Electrical Engineering, Meteorology, Physics
Tags: bolts, electron avalanche, Eric Sorensen, lightning, Patrick Pedrow, plasma, Science, thunder, video, Washington State University, WSU
A loud, rousing thunderstorm is one of the rare treats in Washington State, the Lower 48’s last-place finisher for lightning strikes per square mile. One blew through Pullman a few nights ago, slamming doors in a late-summer fit of meteorological melodrama.
But as we’re fond of saying here at Discovery, when nature fails, there’s always the Internet, and few storms have lit up our browser more impressively than this bit of slow-motion footage of lightning strikes over Rapid City, SD. Unlike lightning in real time, the video makes it easy to see that lightning’s light—the bolts—not only go down but up. And not only do they strike the same place twice, they often follow the same route.
We ran this by Patrick Pedrow, associate professor in the School of Electrical Engineering & Computer Science, and got some helpful Lightning 101.
First, the basics: Lightning runs between large, separated pockets of positive and negative charges, some of which can accumulate on the earth and on protruding objects, like the towers in the video. Now and then, the charged pockets combine through what is called an “electron avalanche” and affiliated phenomena. A free electron is accelerated away from a negative charge and towards a positive charge, and if the separated charge is large enough, the accelerated electron strikes and ionizes an atom or molecule. Now there are two free electrons, each of which is accelerated and can have collisions that make even more free electrons—and an electron avalanche.
Electron avalanches convert the air around them into a highly conductive channel of neutral and charged particles called plasma. Some channels can be less than a centimeter in diameter. When one “touches” the earth or a raised object, thousands of amperes of current flow through the channel to neutralize previously separated regions of charge and a very hot bright channel—a lightning bolt—forms.
Even after the glow disappears, the ionized channel remains and can act as a conduit for more currents and subsequent lightning bolts.
“The human eye integrates all of this emitted light together,” says Pedrow, “and the typical observer reports only one ‘lighting strike.’ But as shown in the film clip there can be a large number of charge pockets being ‘drained’ in one lightning event.”
Video courtesy Tom A. Warner of ZT Research