This question already has an answer here:
How far can you see through an atmosphere like the one we have on Earth? The protagonist inhabits a huge world, like the inside of a sphere, a Dyson-sphere, with an approximate circumference defined by the circle of Earth’s orbit. The world is magical and thus atmosphere is retained on the inside of the sphere, uninhibited by the laws of physics. In fact, a breathable atmosphere is available in up to 30 kilometers altitude. But as the surface curves upwards (albeit very slightly) instead of downwards as on Earth, it would be possible to see farther than here, if the atmosphere does not obscure the view. Theoretically, there could be a lot of air between two points of observation! (Note that this particular world is divided into "smaller" segments by a form of curtain-like dividers, but some of the segments are much larger than, say, the surface of our own planet.) I’m thinking about the way water filters out more and more light as the mass of water between light source and place of observation grows. Does that same mechanism affect atmospheric air?
Added explanations in response to comments: The (inner) surface of the sphere is mostly uninhabited wastelands, barren rock with traces of water and lichen, and with a thin layer of breathable atmosphere. Inhabited areas consist of "islands", made up by hundreds of segments, divided by "solar curtains" that seem like fog or mist, but actuav´lly philters out sunlight. The central "sun" is covered, but allows for relatively thin stalks to drop towards inhabited segments, like elongated snails eyestalks, so that every segment in practicality has their own sun. There are no moons or stars, but a few other heavenly bodies. Thus one segment may experience day simultaneously with a neigbouring segment experiencing night. Also, weather and climate may vary drastically as a being passes a curtain between segments. All of this is possible because of magic. ("Gods" play an important part here.)