system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do we hear the slow build up of clacking when lightning strikes and the the big boom. Assuming sound is a wave but originates at a single line in the sky why isn’t it always a boom instead of a slow crackle and then a boom?
gpt
First of all, lightning is _not_ a "single line in the sky". Lightning is more similar to a network of roots, each seeking out a path of low resistance. When this is happening in the sky, that can take quite some time and there doesn't even have to be a boom at all. When one of those "roots" finds its way to the ground, however, it rapidly discharges, making that path one of very low resistance, so the rest of the charge quickly follows. You can see this happening in real time. First, a lot of small forking arcs will form, until one of them hits the ground. Then all that electricity quickly drains into that one bolt that leads to ground. That's where you get those really big booms. Or, well, at least that's the most obvious way.

There isn't really a single good answer to your question, though, because lightning comes in a lot of flavors and can behave in a lot of different ways, depending on the conditions. So, without knowing what you are talking about exactly, it's a bit tricky to figure out the exact cause for your observations. But understanding that lightning is virtually never just a single bolt should go a long way to clearing this up, I'd imagine.