How does lightning "know" the shortest path to the earth?

I recognize that its final path may not be the absolute shortest, but it’s awfully close. You never see lightning go parallel to ground, for example (or, do you?). So how does it “know” which direction to take, and following one of the shortest paths at that? Is there some gradient that a lightning bolt is following?

Thanks!

it finds the least resistance paths to dissipate all of its energy. multiple paths that get established in a flash by the forces of nature.

Same way water knows where the weakest place is in a pipe.

Lightning can & does go every which a way…

Like said, path of least resistance.
Some times the path seems strange but, I have been hit twice so I speak from a place of pain… Bawahahahah

Would it be an apt analogy to say “the same way water knows which direction to flow”?

edit: Ah, beated to it by GusNSpot

Why is the path of least resistance invariably perpendicular to Earth’s surface (or very close to it)?

Air is a bad conductor. The ground is a comparatively good one. Therefore the path of least resistance will be the one that goes through the shortest possible distance through the air.

So it literally sends out “feelers” to figure that out? How does it “know” the ground is a particular direction given that it (the ground) may be a long way distant? Or is the resistance lower along a “resistance gradient” in the direction of the ground?

Thanks to all . . . give me time . . . I am a slow learner.

There is a voltage gradient in the direction of the ground, not sure how that translates into resitance.

As to the second question, most lighting is actually between clouds and not between cloud and ground. So it is a less preternatural “knowledge” of where the ground is than it might appear at first glance: if it did somehow know to aim for the groun, it has a pretty bad aim since most lightning doesn’t reach the ground.

This question got me interested and I found this explanation on a website:

“A step-like series of negative charges, called a stepped leader, works its way incrementally downward from the bottom of a storm cloud toward the Earth. Each of these segments is about 150 feet (46 meters) long. When the lowermost step comes within 150 feet (46 meters) of a positively charged object it is met by a climbing surge of positive electricity, called a streamer, which can rise up through a building, a tree, or even a person. The process forms a channel through which electricity is transferred as lightning.”

See if this helps. Imagine a spherical, pressurized vessel filled with water, like a boiler. The water pressure exerts an equal amount of pressure on the inside of the sphere in all directions (ignore gravity.)

Now you drill a hole at one spot on the surface of the boiler. The pressurized water will shoot out of that hole. How does the water “know” to go through the hole? It doesn’t, but the walls of the boiler resist the water pressure, and the hole lets it through. The water will flow from the place of very high pressure to the place of low pressure. So when a water molecule happens by chance to go shooting off in the direction of that hole, it will go through, otherwise it will bounce off the inside wall of the boiler.

Another way for the water to get through, if there are no holes, is to build up greater and greater pressure until the vessel bursts at its weakest point (say along a seam.) Pressure wants to equalize itself and will do so when given the opportunity.

Now electricity doesn’t work exactly the same way, but “pressure” can be analogous to electrical potential in some ways. Think of the electrical charge building up between the ground and the clouds as pressure, and the atmosphere as the walls of the vessel. If the pressure becomes too large for the vessel to contain, the electrical current will flow from high pressure to low pressure. (The pressure, or potential, between two points is what we measure as voltage.)

Much obliged for all your help.

Not that I know anything about it (save my intuitive sense of what it might mean!), but is the process that lightning “uses” anything like simulated annealing?

Yes, lightning can travel from cloud to cloud and never touch the ground.

I would expect someone with a username like ‘KarlGauss’ to know more about electro-magnetism.:wink:

You shold look into positive vs negativestrikes andsprites and elves.

[quote=“friedo, post:10, topic:629077”]

(ignore gravity.)

last time I did that I almost floated away.

Not really. The feelers change the state of the air, creating ionised paths, which will be followed by the main strikes. So the paths don’t itterate to a better solution, but take the first solution found.

The AI analogue is sort of good, since the standard way of expressing the base problem in AI is hill climbing. With an altimeter, but no map, how do you find the highest point? Clearly you try to keep walking uphill, until you are at a spot where all the directions lead downwards. However you don’t know if the hill peak you are standing on is the highest point in the mountain range. Simulated annealing is one tactic to try to rattle the solution about so that you skip past local maxima, and have a better chance of finding a higher peak. Lightning simply wanders about always following the steepest path until it hits the ground. The path it follows isn’t optimal and doesn’t improve. The path is defined by the vector of electrical potential.

I was going to suggest it’s more like a gradient search but you’re explenation is much better.

I am no expert on this, but, as I understand it, when a sufficiently high level of charge has built up in a cloud it will induce a charge of the opposite sign in the ground (or buildings, or whatever) below: a capacitance effect. Inevitably, by the coulomb inverse square law of electrostatic attraction, this will be greatest in the bit of ground closest to the cloud, i.e., directly below it. Then, when the lightning actually does strike, it is not just a matter of the charge in the cloud pushing down, as it were, to the ground, it is more that the oppositely charged ground immediately below is pulling the charge down towards it. That, I think, is why strikes are (more or less) straight down.

All this stuff about finding the path of least resistance through the air helps explain why a typical bolt is not completely straight and vertical, but the OP wants to know why (when the strike is between cloud and ground) it tends toward the vertical. I think the reason is the one I have given: because the oppositely charged ground immediately below the charged cloud attracts the bolt towards itself.

I’ve seen lightning fork repeatedly across the sky horizontally without ever getting to the earth. Or did you mean “ground” in an electrical sense?

Either way, it’s like any other spark of static electricity between differently charged bodies, except lightning is freaking huge, and the chain of spark can fork or even turn right angles along the way.

Normal air isn’t conductive. The cloud sets up a potential difference with earth. The shorter the distance, the higher the electric field. Hence, the field in the shortest path causes breakdown of the air first, and that’s where lightning travels.

Ps here’s a nice video of the streamers

http://www.youtube.com/watch?v=7kI1d7DMbco&feature=youtube_gdata_player