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Name: Francine
Status: student
Grade: other
Country: USA
Date: Summer 2013


Question:
If the following statement is true, then how does lightning go sideways?


Replies:
"Lightning develops as a result of a concentration of charge on the ground and an opposite charge in the cloud. A stepped leader starts from the ground and a leader meets it a few hundred meters above the ground. This ionizes a path for a lightning stroke, which goes up to the cloud from the ground (electrons stream from the cloud to the ground) and energy is transferred from the cloud to the ground.

David Cook Argonne National Laboratory


Hi Francine,

Thanks for the question. The statement below does not appear to be false, but it is not the entire story. In understanding lightning and electrical phenomena, one must use the concept of electric potential or electric potential energy. In brief, the idea is that electrons flow from high potential energy to low potential energy. The decrease in potential energy may occur in the electrons move vertically, horizontally, or a combination of both. I should mention that the concepts of electric potential and electrical potential energy are not usually discussed until high school physics (at the earliest). Even my college students have trouble grasping these concepts.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell


Francine,

In the paragraph you quoted, I was writing about cloud to ground (CG) lightning.

The same process can happen between two areas of opposite charge within a single thunderstorm (intracloud) or between two clouds (intercloud). When that happens, lightning can travel sideways (horizontally). This is actually very common. There are approximately twice as many inter or intracloud lightning flashes as there are cloud to ground lightning flashes. Intercloud flashes are most easily seen. Many intracloud flashes are not visible to us on the ground; they're hidden within the inside of the cloud. However, you can often see the glow of intracloud flashes taking place in the cloud without actually seeing the flash itself.

David R. Cook Meteorologist Atmospheric and Climate Research Program Environmental Science Division Argonne National Laboratory


Francine:

Good thinking! The definition is good for lightning that strikes the ground. As you pointed out, the definition does not fit everything we see. It is not that good a definition.

Basically, lightning travels between areas of opposite charge. It these areas are close enough horizontally, say in two adjoining clouds, the lightning will jump between the clouds rather than fight all that air resistance to get to the ground.

There is a rather rare and interesting form of lightning called ball lightning which seem to do just about anything it likes. But I will leave it to you to investigate that!

Hope that helps. Bob Avakian Tulsa, OK


Hi Francine G.,

While up on the Mount Washington Observatory I witnessed this cloud to cloud phenomena first hand. The meteorologists (there were 7 of them, all researchers) explained that the charges really do not care where they attempt to reach equilibrium of charges - they just do!

Lightening may strike ground to cloud, cloud to ground or cloud to cloud. Ice crystals rub together in the turbulence to form charges. Those charges may be unequal in many local events ( in each cloud). There may be low clouds that are less charged than higher ones, these may create "leaders".

However the scene plays out there are some basic rules: 1) a charge inequality is formed that is electronegative in nature 2) a potential charge of extreme (for humans) voltage (pressure) and amperage (volume) may develop; 3) those charge inequalities will seek to become equal via; 4) any means available, including anything in the way and 5) always seeking the least resistive path.

If the least resistive path is from cloud to cloud, sideways - the charges will move sideways.

Thanks! Peter E. Hughes, Ph.D. Milford, NH


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