It's almost like the step leader trailed a wire from the cloud to the ground. So, once the step leader reaches the ground, there's a path from cloud to ground of ionized (conducting) air. Essentially, the step leader is just leaving bits of charge in the air as it goes along, like Hansel and Gretel's breadcrumbs. Ionized air is just air that itself is charged. It rockets down to the ground extremely quickly, and sometimes will split up and such on its way (this is why lightning forks, more on that in a second).Īs the step leader goes through the air, the air gets ionized.
This "charge packet" is called the step leader. What happens is that eventually the charge builds up so much that a "packet" of charge gets spontaneously ejected. That's why electricity (charge) flows well through wires, but doesn't jump out of the wire through the air and zap you: the wire conducts, but the air doesn't. For charge to flow, there needs to be a conducting path, otherwise it sort of just gets stuck. The problem is that there isn't really a path for them to follow. As you may know, negative charges repel each other, so the charge "wants" to get to the ground, which is neutrally charged. We aren't totally sure why that happens, but it does. What happens in a lightning strike is that, like I said before, a whole bunch of negative charge collects at the bottom of a cloud.
Confusingly, it's correct to say that lightning travels downwards or upwards, depending on what you mean by "traveling." That's where the idea that "lightning travels upwards" comes from. If you watch a high-speed video of a lightning strike, though, the flash travels from the ground up to the cloud. So, the charge travels down from the cloud to the ground. Lightning happens because the bottom of the cloud accumulates a strong negative charge, and eventually releases some of it in the form of a lightning bolt.
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