arcing involves ionizing air, right? I would have expected that the breakdown voltage of air increases with altitude, due to decreasing density of air.

It's actually the opposite (to a point, I think). Higher mean free path means more time to accelerate, then hit and ionize something else. That's how fluorescent tubes maintain discharge a meter long at measly voltages.

Radiation doesn't help either, as sibling commented - it creates ions making the initial breakdown easier, though it shouldn't make a difference for a continuous flow that involves innumerable ions.

Here ya go:

https://www.youtube.com/watch?v=MIqK45nxcTo

He even explained it with balls and magnets!

I would guess that it could be due to having more ions in air due to higher radiation levels.

Yes, and when the cathode heater burns out, they stop conducting, as they depend on the thermionic emission of electrons (and the absense of gases).

Considering most of humanity lives within an hour's drive of the ocean, it's not a bad assumption.

I'm interested why you are being downvoted, as you bring up a good point that aligns with what kortex was saying in their post.

edit: ahh, now I see it's because you're factually incorrect. It's only 1/3rd of humans that live within 100km of oceanic coasts.

https://science.nasa.gov/earth-science/oceanography/living-o...