You explain to them refraction causes an optical illusion and what you're really seeing is mirage and not the actual object and they just roll their eyes and think you're making stuff up to fill a hole in the conspiracy.
Which is ironic because they explain away the curve you see on planes by saying plane windows have a fisheye effect.
I mean, it's not that hard to figure out the Earth is round via the same method Aristotle used thousands of years ago.