That matters because a clear horizon provides a strong visual reference.
When an unusual light appears against an open sky, there are few nearby objects with which the eye can compare its size or distance.
A bright shape can therefore appear much more substantial than it actually is.
The absence of buildings or nearby objects can make it difficult to judge perspective.
A distant phenomenon may look close.
A relatively small optical effect may appear enormous.
A light source whose actual position is difficult to determine may seem to hover somewhere between the sky and the horizon.
When Atmospheric Layers Distort What We See
Atmospheric refraction is capable of producing some remarkably strange visual effects.
When light travels through air with different temperatures and densities, its path can bend slightly.
Normally, these changes are too subtle to notice.
Under certain conditions, the effect becomes obvious.
This can produce mirages in which distant objects appear to float, stretch, or become distorted.
