April 2, 2008 -- Researchers have discovered the secret to making different kinds of lightning. By combining lightning observations and computer modeling, the shapes and directions of lightning can be traced to a layer cake of charges inside thunderheads that govern the ways the super-hot bolts escape from thunderclouds.
There are basically four types of lightning that escape thunderstorms: 1.) downward cloud-to-ground, 2.) sideways-striking bolts from the blue 3.) upward-shooting blue jets and 4.) giant blue jets that bolt up and out of a cloud. Until now there was no theory that explained all of these sorts of lightning.
"What we wanted to do is see if we could try and explain lightning going upwards and to the ground in a very similar way," said Penn State electrical engineering graduate student Jeremy Riousset. He is the co-author of a paper describing their unified lightning theory published in the latest issue of Nature Geoscience.
The secret to all the forms of lightning boils down to the layers of charged air inside thunderheads.
According to their models, video footage of lightning storms and 3-D radio imaging of internal lightning inside New Mexican thunderheads by New Mexico Tech researchers, most tall, lightning-producing thunderhead clouds have up to four layers of alternating positive and negative charges stacked up like cake layers inside of them.
"The important thing to remember is that as an electrical storm develops, it creates layers of charges," Riousset explained.
A big difference in the intensity of the two oppositely charged layers in the lower part of a cloud can propagate a lightning bolt in a downward direction -- toward the ground.
A blue jet is created when an imbalance between the two middle layers generates internal lightning. A large surplus of electrons in one of these layers then shoots the bolt upward and out of the cloud.
Sideways-striking "bolts from the blue" can hit the ground miles from the thunderhead. They appear to be created when a very intense negative charge layer is formed near the ground and positive layers get out of kilter. The bolt of lightning then slips out the side of the cloud and continues diagonally towards more distant ground.
Giant blue jets shoot up from doubly tall thunderheads that generally only form in the tropics. A very intense negative charge in the gut of such a thunderhead can send a charge upward toward a less intense positive layer and overshoot it into space.
"It's always easier to go upwards," said Riousset.
That's because the thinner air moves electricity better, he explained.
But more significant than all these details is how the new work makes sense of lightning in a way that's not been done before, said Earle Williams, a lightning research scientist at the Massachusetts Institute of Technology in Boston.
"Lightning has always been seen as something chaotic," Williams told Discovery News.
But more than two centuries since Benjamin Franklin started studying lightning, there is finally a theory that explains the phenomenon -- in all its varied forms.
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