It has been noted that although the earth is flat, distant continents thousands of miles away remain unseen. This is due to the fact that the atmosphere is not perfectly transparent. There is a limit to human sight before all lands are faded and obscured by the thickness of the atmosphere.
Atoms and molecules are not transparent and so distant objects will be faded with distance. For example, notice how these distant mountains tend to fade out and become discolored with distance. That's because the atmosphere is not perfectly transparent. When you look through the atmosphere you are looking through a fog of atoms and molecules. If the earth had no atmosphere those distant mountains would be as clear and sharp as the foreground.
Viewing distance into the horizon is directly correlated with pressure, gas constant, temperature, humidity, and pollution. At sea level the average air density is 1.2250 kg/m^3. This sort of density will allow a viewing distance of around 25 miles across the horizon. In New York pollution and humidity are at such high levels that viewing distance is limited to 15 miles.
At higher altitudes the air density drops sharply, allowing the viewer to see far away lands before they are obscured by a blue-white sky. It is for this reason that an observer standing on Mt. Everest can see other mountains hundreds of miles away. Such pristine conditions are rare on the earth, however, only existing at high altitudes and in fidged environments.
Why can we see farther when we increase our altitude?
When you increase your altitude you are changing your perspective lines in relation to the earth, pushing back your vanishing point. The vanishing point, beyond which no man can see, is created when his perspective lines approach each other at a certain angle smaller than the eye can see. If you increase your height you are changing your perspective lines and thus can see further before all sight is lost to the vanishing point.
Usually it is taught in art schools that the vanishing point is an infinite distance away from the observer, as so:
However, since man cannot perceive infinity due to human limitations, the perspective lines are modified and placed a finite distance away from the observer as so:
The vanishing point acts as the liming point of all vision, as all bodies beyond it are too small and squished into the surface to see with the naked eye.
The same effect is found on a 3D video game which assumes a flat surface. When you increase your altitude you can see farther because you are so much higher than everything else. Your computer's resolution is better able to see something below you than off on the horizon where the pixels are linearly squished.
When you increase your altitude on a plane you have broadened your perspective lines and have pushed your new vanishing point backwards into the distance.