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July 24, 2007 4:57 AM   Subscribe

Lightning. Why does it always go to ground?

I had to drive through a storm a few days ago, complete with heavy rain, thunder and lightning strikes. I got to thinking - why does lightning (or electricity) always seek the earth? Why doesn't lightning stay in the cloud? Why does it have to "go somewhere"? Why don't I ever see lightning damage? I figure that all that energy striking a tree has to do it some damage, but compared to the amount of lightning storms I've seen, I've only ever seen one potentially lightning damaged tree.

I realise that I should probably have paid more attention in physics class at school, but it's a bit late for that now. So I'm turning the AskMeFi to educate me instead.

Please be warned that I never was able to grasp much of physics, so bonus points are available to anyone who can explain it in simple terms. :)
posted by Solomon to Science & Nature (14 answers total) 1 user marked this as a favorite
Simple terms are all I got baby.

But seriously, I think this is simple yet eloquent:

How does lightning form?
Rapidly rising air in a thunderstorm interacts with rapidly falling air within the thunderstorm to create separately positive and negative charged areas within the cloud. Air acts as an insulator, but when the charge builds up to a level that exceeds its ability to act as an insulator, the result is a spark we see as lightning. The lightning equalizes the positive and negative charged areas.

posted by dreamsign at 5:00 AM on July 24, 2007

You need to re-think your model: it doesn't go to the ground, it comes from the ground.
posted by handee at 5:03 AM on July 24, 2007

Ack, doesn't address the ground part, does it? The earth is usually negatively charged compared to the atmosphere.

Actually, looking at some of the later info in that first link, it isn't as good as I thought it was This one is better. You may find the different kinds of lightning, and the mechanisms for each, particularly interesting. I think positive lightning is just crazy neat.

On preview: handee, you might want to check that link, too. You're talking about 5% of all lightning in your statement.
posted by dreamsign at 5:04 AM on July 24, 2007

Here's a tree. Dead trees tend to catch fire. In live trees, the lightning can boil the sap and the tree can explode.
posted by MtDewd at 5:05 AM on July 24, 2007

On the first part of your question, lightning that doesn't reach the ground is the most common type of lightning. Lightning follows the path of least resistance to the place where it can rebalance negative and positively charged regions in clouds, and, more often than not, that's another cloud.

As far as why you don't see more lightning damage, it's just a matter of there being lots of trees and nowhere near enough lightning strikes to damage enough trees for lightning struck trees to be common. Couple that with the fact that lightning kills some trees it hits (leading to their being cut down), so it means even fewer damaged trees survive. But lightning certainly will damage a tree.
posted by dseaton at 5:08 AM on July 24, 2007

Think of static electricity, when you shuffle across carpet in your socks. Why doesn't it stay in your body? Electrical charges want to be equal, so it equalizes by making a spark.

posted by blue_beetle at 5:39 AM on July 24, 2007

First off, lightning does not always go to/from the ground.

It /does/ always go between areas of a high difference of electrical difference. It's nothing but a big spark, the same kind that tickles you in cold/dry weather.

Kid version: When one place rubs against another and kidnaps a bunch of electrons, after a while the difference is big enough that the electrons stage a revolt and go back home.
posted by cmiller at 6:08 AM on July 24, 2007

Dreamsign's second link is pretty interesting. I had no idea there were so many different forms of lightning, or about the existence of sprites and elves and so forth (not being a shoemaker). BTW, the link suggests that emissions from the ground play a role not just in positive lightning (the 5%), but in all lightning that has an earthly component.

Re. lightning strikes on trees, what I want to is, are rubber trees pretty much immune??
posted by Clyde Mnestra at 6:21 AM on July 24, 2007

Note the "streamer" that comes off the monument but doesn't make contact with the actual bolt. Before lightning hits the ground, these charged streamers flow up while the same comes down from the cloud. The ones that connect is where the majority of the charge flows through and, hence, you get a bolt. This happens in a fraction of a second.

So yes, cloud/ground lightning is kind of a two way thing.

See "several stages" here.
posted by mkn at 6:56 AM on July 24, 2007

Opposites attract.
posted by caddis at 7:09 AM on July 24, 2007

As people have been saying, lightning doesn't always go towards the ground. In fact, most lightning occurs between clouds. Understanding a bit about the basic process will help answer a lot of your questions, actually. For reasons that are only partially understood, clouds aquire a large negative charge during thunderstorms. When you shuffle your feet on a carpet and go up to another person, you are causing the same sort of charge seperation. To shock another person, you have to get pretty close to them before a spark will appear, but the more you shuffle your feet, the longer the distance can be. This comes from the fact that having a net charge causes you to also have an electric field. At some point, the electric field can become strong enough to actually strip electrons off of the molecules in the air. Electricity is just electrons moving, so the air can now conduct electricity. The spark is just electricity flowing along this channel of now-conductive air. Thunderstorms are amazing because the charge they aquire is strong enough to cause a spark several hundred feet long. The target of cloud-ground lightning is usually something tall because it is easier to ionize as little air as possible and often metal because it is easier to generate a strong electric field near metal. Differences in the charge of clouds or even different parts of the same clouds, combined with the relative closeness compared to the ground, make cloud-cloud lightning much more common.

By the way, when lightning strikes the ground, it fuses the dirt together to make something called a fulgurite. They can look pretty cool.

The wikipedia page is quite good.
posted by Schismatic at 7:39 AM on July 24, 2007

strangely enough the top two google hits when searching '"cloud to cloud" lightning' say the exact opposite: "Cloud-to-cloud lightning is the most common of lightning. This form lights up the sky and the clouds. If you do not see the bolt of lightning but it illuminates the cloud this is called in-cloud." and "Cloud-to-Cloud Lightning: A rare event, it is lightning that travels from one cloud to another.".
posted by jepler at 7:40 AM on July 24, 2007

Try the University Corporation for Atmospheric Research lightning page.
posted by snarfodox at 4:42 PM on July 24, 2007

If you happen to be cruising north to south, or south to north along the Eastern seaboard, in an airplane at 40,000 feet on a summer evening, there is a fair chance of seeing how prevalent cloud lightning is, just by looking down. Living in Atlanta, and traveling frequently on business to the Northeast, I've flown over major weather systems moving over the Appalachian Mountains dozens of times. Many times, I've witnessed dozens of lightning bolts in clouds every second, for several hours, while flying down from New York or Boston to Atlanta. It's an amazing show, like a field of fluffy cotton, into which thousands of sparkly strobe lights have been embedded, serenely dancing beneath you.
posted by paulsc at 4:48 PM on July 24, 2007 [1 favorite]

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