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The enlightening image below is of a lightning strike slowed down at 10,000 frames per second. It can be seen that the most intense flash produced from the lightening occurs in the direction from the ground up. Why does this final "ground-up" strike occur and why is it so much brighter and faster than the initial part of strike heading towards the ground?

enter image description here

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4 Answers 4

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Does lightning strike from the sky down, or the ground up?

The answer is both. Cloud-to-ground lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast - in about one-millionth of a second - so the human eye doesn't see the actual formation of the stroke.

Source: National Severe Storms Laboratory

The reason is that when cloud-to-ground strike approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field and the "downward leader" strike creates bridge for the "return stroke"; this per the wiki page for Lightning.


Cloud to cloud and Intra-Cloud Lightning

enter image description here

Might be worth also noting that cloud-to-ground is not as common as Cloud to cloud (CC) and Intra-Cloud (IC):

Lightning discharges may occur between areas of cloud without contacting the ground. When it occurs between two separate clouds it is known as inter-cloud lightning, and when it occurs between areas of differing electric potential within a single cloud it is known as intra-cloud lightning. Intra-cloud lightning is the most frequently occurring type.

Ground-to-Cloud

enter image description here

Appears that ground-to-cloud is possible, though normally only a result of a man-made object creating "unnatural" electric potential, and is the least common type of lightning.

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+1 thanks for your answer. I'll wait a few days to accept in case I get more specific answers on why the return flash is brighter and faster however. –  Geodude Apr 27 at 6:57
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Appears that the reason is that when cloud-to-ground strike approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field and the "downward leader" strike creates bridge for the "return stroke"; this per the wiki page for Lightning. Also, appears that ground-to-cloud is possible, though normally only a result of man-made object creating "unnatural" electric potential. –  blunders Apr 27 at 7:06
    
although i do agree that cloud-to-ground lightnings are rarer, it seems a bit bold to affirm that they are "normally only a result of a man-made object creating "unnatural" electric potential". Do you have any sources for that affirmation? –  plannapus Apr 28 at 5:26
    
@plannapus: See Upward-moving lightning from TV towers, skyscrapers and other tall structures - also, FYI, you mean ground-to-cloud, not cloud-to-ground, right? –  blunders Apr 28 at 5:42
    
ah no i misread your sentence as talking about cloud-to-ground and not ground-to-cloud, my apologies. –  plannapus Apr 28 at 6:00

Lightning happens when the potential difference between the clouds and the grounds becomes too large. Once the voltage reaches a critical strength, the atmosphere can no longer act as an electrical insulator. First, a stepped leader is created at the base of the cloud which is a channel through which electrons in the cloud can travel to the ground. But while moving towards the ground, it searches for the most efficient(minimum electrical resistance) route possible. It does so by traveling 50-100 meters at a time then stopping for about 50 microseconds, then traveling another 50-100 meters. In this process it also branches out looking for the best route. As the stepped leader gets close to the ground, a positively charged traveling spark is initiated on some tall object (trees, towers etc) on the ground. The traveling spark moves upward and eventually connects with the stepped leader. Once the stepped leader and the traveling spark have connected, then electrons from the cloud can flow to the ground, and positive charges can flow from the ground to the cloud. Once the stepped leader and the traveling spark have connected, then electrons from the cloud can flow to the ground, and positive charges can flow from the ground to the cloud. This is known as return stroke. But this flow unlike the flow from up has a well defined shortest route now. This massive flow of electrical current occurring during the return stroke combined with the rate at which it occurs (measured in microseconds) rapidly superheats the completed leader channel, forming a highly electrically-conductive plasma channel. The core temperature of the plasma during the return stroke may exceed 50,000 K, which makes it shine so bright.

In your image above as well, you can see a lot of time is lost in looking for the route, whereas the return stroke just runs through the well defined channel and hence is much faster.

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"...and positive charges can flow from the ground to the cloud." Can you elaborate? What exactly is positevely charged and flowing? Positrons? –  Evgeni Apr 27 at 14:28
    
@Evgeni This is from semiconductor physics but I expect the principle is the same; holes are the positively charged objects. An absence of an electron where there should be once can (in the right medium) behave as it is were a particle (with a mass etc) –  Richard Tingle Apr 28 at 12:32

I'm pretty sure that lightning can go either way. We did the calculation in physics class, a bit of a back of the envelope calculation to be sure, but we came out with an electric field of 300,000 V/m being the electric field needed for lightning to form.

What that means is, an electric field is created between the clouds and the ground by a build up of electrons on one of the surfaces. I believe that the electron build up can occur on either side. Once the field strength gets high enough, electrons begin to "leak" from one side of the air-gap (air does not normally conduct electricity unless it is highly charged) and that is what lightning is. You see the same affect in a capacitor that is overcharged, only instead of a thin sheet of paper, you are inside the insulator between plates. That is why you get a lot of noise (thunder of course), see a lot of light, and wireless communications and some electronics are affected.

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Not quite - it's not so much that the charge leaks, as that the air ionises and becomes a plasma, which is highly conductive. So the charge flows. I'm not sure at exactly what stage this happens, though - whether it's just for the return stroke or whether the initial "feelers" use this mechanism too. –  Simon W May 5 at 12:17

In the video, the downward strike created an ionized path. The brightness was less as the current was less because it was travelling through un-ionized air. The upward strike took the ionized path. The upward strike's current (brightness) was much greater as the electrons flowed along the ionized path as it had the least resistance. The current equals the voltage divided by the resistance. The less the resistance, the higher the current.

The apparent speed difference is because the upward strike took a more direct path. The camera shows a two dimensional image and does not show the motion of the lightning away from or towards the camera. In a video when you see an object coming directly towards or away from the camera, you see the object get larger or smaller. The size differences in the video are not large enough to show the direction towards or away from the camera.

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+1!!!! I was sure that that it was a depth issue, but didn't know for sure. –  Neo Apr 28 at 20:25

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