# how fast does lightning travel

how fast does the end of the lightning bolt travel?

I don’t know, but is there anyone who can confirm that lightning begins from the ground and why?

It’s really, really fast.

OK, our old friend EB says that a cloud-to-ground bolt is made up of two strokes: a leader stroke and a return stroke. A leader stroke, which carries a negatively charged from the cloud to ground (except if the structure about to be hit is rather high). The leader stroke begins inducing a positive charge on the ground. The return stroke, carrying the positive charge, moves its way up to meet the leader. They meet at about 50m above ground, short-circuiting the cloud to the ground causing "a highly luminous return stroke of high current (to pass) through the channel to the cloud. "

Now, as for speed:

So speed = height of cloud / 90 ms. Your mileage may vary.

What’s this? Are you adding microseconds to milliseconds without a 3-decimal shift? Show your work. Almost no lightening where I live, and I don’t think any ever conducts to the ground here, but I wanna know how fast I gotta move if it does.

Ray (electrical engineer ignorant of the subject)

Oops. I’m mixing my units. And anyway the actual bolt apparently occurs in the 70 microseconds.

Now for my patented Wild Ass Math… The height of a cumulonimbus cloud does not exceed 2km. So 2km in 70us = 30 million m/s. Or about 0.1c. That sound right?

Damn, I was hoping I’d be the first to notice. Plus I get busted by a guy with a metric prefix. How a propo. “Show your work” sends chills up my spine…

Alpha is that for greased or plain ol’ unlubricated?

Signitorily yours, Mr John
" Pardon me while I have a strange interlude."-Marx
ARROW? Officer, I didn’t even see any Indians

Nanobyte,

Suffice it to say - you can’t run that fast. However, take it from me - a guy who was nearly struck by lightning in a parking lot once - there is an advanced warning signal.

Here’s my story. I was camping once when I was about 16 years old and had decided to take a walk around the lake where we were staying. Near the end of the walk, it started to rain heavily. I decided to take a short-cut across a parking lot to get back to my camp site. About halfway across the parking lot I noticed that, in spite of the fact that I was soaking wet, all the hairs on my arm started to stand on end… Instinctively, I fell flat to the ground and lightning struck a small tree about 20 yards away in a landscape island. The first thing I noticed was being covered in toothpicks… the tree almost vaporized… The next thing I noticed was the ringing in my ears and the realization that I could no longer hear the rain or the thunderclap of that last flash of lightning. My hearing finally returned about an hour later, but the ringing persisted for a few days.

I’ve heard from other near misses that they too noticed a strong electrostatic field just before the strike. I figure I had about 2 seconds of reaction time, but of course time seems to move at a different rate in those situations, so who knows…

How fast? C

(i.e., the speed of light)

I don’t know how much I believe it, but I was told in school that electricity moves at almost the speed of light which would be 186,000 mps. I was also told that when enough current has built up to overcome the resistance of the air seperating the Earth and the clouds, that it would still move at near the speed of light.

Do I need to go kick my old science teacher’s ass or is this right?

(I know I owe my spelling techer an ass woopin’ so don’t comment if I misspell anything.)

dkgreath,

You wrote:

No. Actually the ‘flow’ electric current is quite slow - on the order of inches per minute in a wire, but probably not too radically different in air.

The things that do move fast in a lightning strike are (1) LIGHT, which travels at basically… the speed of light [though remember we’re not dealing with a vacuum, here] and (2) ENERGY. To understand how the energy is transferred at incredible speeds, consider this analogy.

Imagine I have a one mile long, very hard, very rigid rod. I place one end of this rod about 1/8th inch from a bell and stand at the other end of the rod. Now I strike this end of the rod with a hammer - ignoring friction and compression effects on the rod, the energy is transfered almost instantaneously to the other end of the rod and rings the bell.

This is very similar to the way electricity works, except the rod is replaced by electrons in whatever conductive medium you’re dealing with - in this case, charged air.

You’re best to not conceptualize electricity as moving, but rather pushing.

This from NASA’s lightning research web site:
(Well worth the visit just for the lightning primer).

http://thunder.msfc.nasa.gov/

With the initial breakdown of the air in a region of strong electric fields, a streamer may begin to propagate downward toward the Earth. It moves in discrete steps of about 50 meters each and is called a stepped leader. As it grows, it creates an ionized path depositing charge along the channel, and as the stepped leader nears the Earth, a large potential difference is generated between the end of the leader and the Earth. Typically, a streamer is launched from the Earth and intercepts the descending stepped leader just before it reaches the ground. Once a connecting path is achieved, a return stroke flies up the already ionized path at close to the speed of light. This return stroke releases tremendous energy, bright light and thunder. Occasionally, where a thunderstorm grows over a tall Earth grounded object, such as a radio antenna, an upward leader may propagate from the object toward the cloud. This “ground-to-cloud” flash generally transfers a net positive charge to Earth and is characterized by upward pointing branches.

Greath:

I’ll overlook the spelling, but. . .uh. . .in an electrostatic situation, it is not “current” that “build[s] up”, but rather electric potential (voltage).

You don’t consider light to be energy? And some of the energy is in the form of sound (thunder), which travels at only 738 mph at sea level.

What can I do with this, unless you calibrate “almost instantaneously”?

Well, when push comes to shove. . .

“Electricity” (I didn’t think we used that term anymore, actually.) pushing is ‘potential’/‘voltage’.

“Electricity” moving is ‘current’/‘amperage’.

Now, about electricity pulling. . .

Which leg would you like it on?

Ray (If that charged you too much, just coulomb down a little.)

Nano,

You wrote:

Yeah, you caught me oversimplifying again… I should know better, with you around. I was trying to differentiate between the fairly benign light energy and the fairly destructive EMF (Electro Motive Force)…

Damn. I though I could slip that one quietly through the net, too… The energy moves at different rates through different media and is dependent on a number of different factors like friction, compression, etc…If the material were perfectly elastic and rigid, and there were no other mitigating factors such as friction, then the energy would be transmitted exactly simultaneously (i.e. faster than the speed of light). In the real world, I’m sure it’s significantly slower…

Trying to bring this back around to the topic… Nickrz wrote:

Again, this “return stroke” is referring to the EMF, not the voltage or current. I’m not sure how they arrive at that “close to the speed of light” business, but I think it’s a bit deceptive. It makes people think that the velocity of this energy is somehow related to the speed of light and constrained by the same limits, but this is not the case. I think they probably use “the speed of light” because most people can conceptualize this as something that seems instantaneous, but intellectually they know it’s not…

I have never heard of a single energy vector that propagates faster than light, and every “exactly instantaneous” phenomina I have every heard of is a misapplication of classical mechanics on the quantum level. Maybe you could enligten us with a few examples?

EMF stands for electro magnetic frequency which is a very broad description of “light” from power waves through cosmic rays. And the EMF from the lighinging is a side effect of the moving charge, and the recombination of electrons and ions, and is not a seperate phenomenon. The channel ionization happens in the initial stroke with the stepped leaders and streamers. Most of the current discharge and heat generatrion happens in the return strokes.

I have heard discovery channel say 4,700m/s is “nearly the speed of light.” The speed of light in a vaccuum is 299,792,458m/s. Its not possible for the E field change to propagate faster than this, or FTL communication would have been commonplace years ago.

Doh.
EMF stands for electro magnetic field.
So, I’m an idiot.

Falcon wrote:

I think Electro Magnetic Field must be the more modern use of this TLA, however I have several text books from my Electrical Engineering studies that refer to EMF as Electro Motive Force.

You were not listening. I said that a perfectly elastic, perfectly rigid material in an environment without friction could transmit energy faster than the speed of light. No such material or medium exists… You don’t even have to go to the quantum level before the analogy breaks down in the real world.

Actually electro-magnetic field is OSHA mumbo jumbo, I was obviously thinking you meant EMR when I wrote my response.

The common units on EMF are umm…VOLTS

How bout an example? If you have some idea on moving energy FTL, we all want to hear it.

How fast does lightning travel? It depends if there is a cop around with radar.