<a href="https://en.wikipedia.org/wiki/Balmer_jump" rel="nofollow">https://en.wikipedia.org/wiki/Balmer_jump
I learned just a few days ago of what probably originated the joke.
You have a point. It's possible that Munroe being a physist might've known about Balmer Jump... though that's still quite a stretch to assume it was the source of the comic.
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A completely baseless claim ... "probably" means > .5 probability, but there is no evidence to support that; it's sheer speculation.