Visual sighting distance depends a lot on the conditions.

Variable 1) nearer the equator the sunlight is stronger and objects can be made out much further than in areas which receive less direct sunlight.

Variable 2) the thickness of the air. Generally speaking the higher you go the thinner the air becomes and the further you can see.

Variable 3) how close the air is to it’s dewpoint. If the air is significantly above it’s dewpoint the water vapor in the air won’t coalesce and if the air is significantly lower then the water vapor drops out of the air. When the air temperature is close to its dewpoint haze and mist forms.

Variable 4) How much smoke/dust is present.

As a general rule sighting distances in mountainous areas are longer than usual as the air is thinner and less polluted and the air is often lower than the dewpoint.