I have encountered this claim while searching for sources answering " Can we see the curvature of earth from the top of world's tallest building? ".
Wikipedia article on horizon claims (with no references), that compared to a geometrical model, the the apparent curvature of Earth is less than expected due to refraction of light in the atmosphere.
Thinking about the problem, I fail to see why this could be so. It is easy to see the refraction affects a distance of the apparent horizont, but I fail to see how it could affect its curvature, as the refraction seems to have the same effect in all directions, because your distance to horizon is the same in all directions.
Is that claim just bogus and there is no such effect, or is the effect real?