Yes, we'd define "clear air" to be something that's visually distinct in the same way we've always done. As I said common sense can be applied to the rules.
We can have VAR setup to catch referee and linesmen mistakes with the understanding if the "mistake" was past a reasonable tolerance of human vision such as the millimetres then we acknowledge that's just outside the realm of human beings and carry on.
VAR will make mistakes. Linesmen will make mistakes. VAR will make less but also has the other drawbacks people have been talking about regarding speed/fans etc. So really what the debate here is if you're willing to trade off the accuracy of VAR that doesn't provide the player an advantage to gain the flow of the game back, and if you are, why would you do that? If the player didn't gain an advantage then it doesn't matter where he is stood, essentially.
Cheers, and yes mind is blown a bit, must swot up on physics.