Rights: University of Waikato Published 3 April 2017 Download


Craig Rodger

Lightning is a really nice example of a terrestrial plasma, very short lived but beautiful to see, a vast blast of energy. What it is really is just an electrical spark, and you can make an electrical spark fairly easily, but the electrical sparks that most people make are sort of like this long, or if you’re really extreme, that long.

A lightning discharge is normally measured in kilometres, so it’s a gigantic electrical spark carrying a gigantic current zapping the atmosphere – heating it up and causing this tube of plasma to run from the top of the thundercloud down to the ground.

In order to form lightning, you need to have a large electric field. So what happens in practice is you have a thundercloud that’s full of charge, and eventually, that charge needs to go to ground – it needs to dissipate itself.

If the electric field gets strong enough, a current will flow through that space. That current is the lightning flash, so if you just set up too much charge difference, you’ll get a spark. If you set up too much charge difference over some kilometres, you get a spark that is a lightning flash.

So you’ve got this huge pulse of plasma being formed with a big current on it, and that radiates radio waves out into space, producing the sferic.

So a sferic is a radio wave signature, and it sounds like [clicks his fingers]. It’s a click. We call it a sferic as a contraction of the word atmospheric.

And the thunder is a consequence of the production of the plasma. If you take a big chunk of atmosphere and heat it up really, really fast, that expels a whole lot of atmosphere quickly – boof – and that shock wave is the thunder.

Associate Professor Craig Rodger, University of Otago, Department of Physics
Image of lightning strike over Nelson, courtesy of Rick Kiessig
Stephen Witherden
Michael Schollum
Richard Mayston
Helgi Arnar Alfreðsson
Footage of night-time storm with lightning strikes, courtesy of James Insogna Creative Commons 3.0 license
Phillip Bloom