A complex mirage or fata morgana is a display that involves multiple images, alternately expanded and compressed vertically, often giving the impression of buildings, cliffs, etc. where no such objects exist. The name is traditionally used in Italy for the vivid mirages seen across the Strait of Messina.
“What does a mirror look at?”
– Frank Herbert, Chapterhouse: Dune
It occurs when two layers of air of different temperatures meet. The basic, or inferior mirage of the sort we see in summer on the roads, arises when the cold air above begins to warm as the heat rises from the hot road surface. This causes the boundary where the layers meet to appear to shimmer like water when viewed from a certain angle. This is due to light refraction, or bending really, and instead of seeing the road, we see the reflection of the blue sky which appears like water.
When these different layers of air meet in the sky, and the boundary is curved, rather than level, the effect not only produces a mirror image, but can also act as a lens to magnify anything that lies many miles beyond the horizon. When this happens and there are several layers of alternating cold and warm air, the images are superimposed one upon the other, creating a multi-layered shimmering vision. The ground air must be cooler than that of the levels above to create a true fata morgana, so this is more commonly seen at sea or around coastal areas. This effect has ‘created’ cities in the deserts and mountain ranges, phantom ships at sea, and more latterly UFOs.
“Gussie, a glutton for punishment, stared at himself in the mirror.”
– P.G. Wodehouse, Right Ho, Jeeves
I’ve also read that the same (or a similar) refraction process takes place when viewing a sunrise or sunset over the sea – that, in the case of the sunset (and the converse for the sunrise), we continue to “see” the sun for some time, after it has passed beyond the horizon.
That is indeed an actual thing that occurs every day.