Wednesday, 3 August 2011

Lightning explained?

@pruthvii1991 asked on Twitter what exactly is happening in the video embedded on the 15th May post, 'Lightning strikes'.  I'm always happy to get questions and to try to give an answer.  Of course the physics of lightning is still being explored, but here is an entry level explanation.

In brief, lightning strikes are not a single electric arc but a series of discharges in a fraction of a second. (Hence the flickering that you perceive.)  Initially the cloud and the ground send out streamers or fingers which are 'searching' for the best route to a place of the opposite charge.  These tentative fingers of lightning are caused by a large voltage gradient from the clouds to the ground.  You might not see them with the naked eye but in the plane video they were visible.  Sometimes they find nothing and retreat.  You can actually see these streamers in almost every photo you ever see of 'forked lightning' (as in images at this site which has 15 fantastic lightning photos).

As you can see in this photo from another source (see below), the fainter forks are the ones that failed to connect, and they quickly cool.  However, once a  tentative connection is made, the heated and ionised air becomes a good electrical conductor a huge current flows through one (or sometimes a few) paths.  The current heats the air to incandescent white which you see as the lightning strike.

A large flying electrical conductor like a plane can find itself as part of the route and might complete the connection from cloud to cloud or cloud to ground.  In the case of the featured video, the plane appears to have been in both routes.  The electrical current flows harmlessly through the metal skin of the plane and everything inside is safe because the skin is so good at conducting electricity.  The plane acts as a 'Faraday cage'.

When you hear thunder it is the result of the sudden heating and cooling of the air.  As the air is heated it expands rapidly, and as soon as the current in the lightning bolt stops, the air cools and collapses back to where it came from.  The resulting shock wave (which we perceive as sound) is the thunder clap.

If you have ever been close to a lightning strike (say less than 100m away) the sound is almost unbelievable and it seems to happen at the same time as the lightning flash!  If you are further away you see the lightning first and then hear the sound later.  For every kilometre between you and the lightning there is a delay of about 3 seconds because light travels so much faster than sound.

Finally - just for fun - see a page of 20 surprising 'facts' about lightning at this link.


Pruthvi said...

Wow ! That is a perfectly logical explanation. But after reading this few more doubts popped into my head.
1) The heat generated by the electrons colliding with the air molecules causes the air to be ionized or possibly plasma.Its the electrons at the tip of the lightning which should sacrifice their energy .Air is a bad conductor of electricity so under very windy conditions is it possible that the path of the lightning bolt is dependent on the wind ?? If the plasma generated is displaced due to wind does the lightning strike deviate from its original path ?

Plasma Engineer said...

That is a good question. This answer is an informed guess. The electrons in the lightning travel at a speed close to the speed of light whereas the wind travels much slower than sound. From the point of view of an electron (or ion) in the lightning, the air is moving so slowly that is is effectively stationary. In principle the answer to your question is that wind does affect it, but in practice it affects it very little. (That's my best guess.)

p.s. Don't rely on my logic as absolute truth, but as analogy with a hint of truth. I've noticed that in physics the 'truth' depended mostly upon my age and the particular version that had been fed to me at that particular level of schooling. As an adult I see how that helped me to understand how things developed through history. When I was 16 I would have thought I understood but would have had an older version of the 'truth' than I have now. Now I know for sure that I understand nothing, but that the mystery itself is rather beautiful.