How does lightning find the shortest path to Earth?

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Offline darwineuron

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[diagram=314_0]...what the shortest path IS from up there in the clouds to down here on earth?

It's all very well to say it follows the shortest path, but how does it know this? Clearly it's non-random, or else lightning conductors would not work. And that's another thing- over what range do lightning conductors work? If a single house standing in a farm has a lightning conductor, how large a farm will it 'protect'? Assuming a farm with grass and occasional cattle? (I suppose this will depend on the distance of the from the ground.. and from the clouds.).

I have this view of the tip of the lightning bolt as some sort of peak in the charge mass, looking downward onto the oppositely charged charge-landscape (complete with cattle ;) below it, and aiming for the peak. The question is, how does it 'see' this landscape and compute the peak?

« Last Edit: 24/01/2016 09:46:32 by chris »


Offline neilep

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How does lightning know...
« Reply #1 on: 01/02/2008 15:35:51 »
Hi darwineuron...and WELCOME here.

Great question..(and great picture too...all in the first post !!...).I say it's great because I asked something similar ages ago...... [:)]'s always nice to get a fresh perspective on things.

I will allow a proper person, who actually knows what they are talking about, to respond but I do recall that although you think it starts from the sky and comes down...I believe the lightning bolt actually starts from the ground and goes up !!...

It just happens so fast that you think it's come from the sky first.
....This clearly must inevitably lead to a correlation with regard to your specific question.
« Last Edit: 01/02/2008 15:37:33 by neilep »
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How does lightning know...
« Reply #2 on: 01/02/2008 17:47:43 »
It takes the 'easiest' path rather than the shortest path.
What does this mean?
The air will only ionise and start a spark when the Electric field is high enough once the air has ionised along the whole path, a small trickle of current quickly becomes many millions of Amps. IF the cloud and Earth were both uniformly horizontal planes then the field would be the same everywhere and  if the air in between had totally uniform temperature, humidity etc, then the lightning would occur at any random place.
However, high objects on the ground distort the field and produce local regions of high electric field strength. This is where the air is most likely to become ionised.  Lightning does, in fact,  often strike the same place twice for this reason.

So-called lightning conductors consist of a sharp point at the top of a rod  and a thick copper strip connected to a stake, buried well in the ground. But they do not just 'take the lightning strike'; they are sticking up in the air and, on the face of it, would be the target of many strikes and lead to regular and costly replacement of copper strip. However, they work in a much cleverer way than that. Because of their sharp point, they develop a very strong field around the tip and produce a small 'corona' discharge around them long before the field between cloud and ground is enough to create a full blown strike. This corona produces a small current of ions which form a conducting 'dome' over the mast or tower, turning it into a high, rounded conductor instead of a sticky uppy pointy thing. The overall effect is to reduce the field between cloud and tower so you are much less likely to get a full strike. Dead clever eh?
I notice there are still patents and improvements being published as, in some parts of the world, lightning is a real problem.


Offline Pumblechook

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How does lightning know...
« Reply #3 on: 04/02/2008 15:58:47 »
It is not quite the same as simple electric current flow as has been said it is to do with air breaking down and ionising.  But if you drove two earth spikes 3 metres apart into the ground  and joined them with a thick copper wire  300 metres long most of the current would flow through the wire from one spike to the other rather than through the much short route through the earth.  It flows mainly through the lowest resistance not the shortest route.