Under special atmospheric conditions, we can sometimes see much farther than the curvature of the earth would normally allow us. Atmospheric refraction usually makes distant objects appear elevated. This is why we can sometimes see, for instance, a boat that would otherwise be hidden by the curving bulge of water between it and the observer. My question is: Why does atmospheric refraction only affect the boat in this case? Why does the boat appear to be elevated but not the bulge of water behind which it is located?

  • $\begingroup$ "Why does atmospheric refraction only affect the boat in this case?" Does it only affect the boat? $\endgroup$ – probably_someone Nov 15 '19 at 15:11
  • $\begingroup$ No. If refraction is strong, it may also affect the water (extending the apparent horizon line) or other objects that are even further away. But if the boat is elevated, why isn't the bulge of water in front of it elevated as well (hiding the boat despite refraction)? I guess that this is just a misunderstanding. $\endgroup$ – Hepper Nov 15 '19 at 16:13
  • $\begingroup$ Refraction is a change in the angle of incoming light, not just the height at which it's coming from. Just as if you're looking at a hill (i.e. a piece of ground at an angle) sloping up away from you, this means that objects further away have more apparent change in their elevation. $\endgroup$ – probably_someone Nov 15 '19 at 17:34
  • $\begingroup$ Let's say we have two boats. We have a direct line of sight to one of them, but the other one is hidden below the horizon. Refraction makes the latter visible. But it doesn't noticeably affect the apparent elevation of the first boat (to which we have a direct LOS), even though we'd expect the temperature gradient and thus the refractive index to be relatively uniform over such a body of water. Why is that? $\endgroup$ – Hepper Nov 15 '19 at 17:48

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