In the case of a fata morgana mirage, light reflecting from a distant object, such as a ship, is bent downward as it passes through the colder, denser air near the surface of the ocean (or sometimes cold land, particularly ice). But your brain places the object where it would be if the light came to you in a straight path—higher than it actually is. This bending effect can even work with the curvature of the Earth if conditions are just right, which is why some fata morgana images can actually be refracted cities and ships from beyond the horizon.
The opposite situation is what produces mirages like an oasis of water in the desert. In this case, a hot layer of air just above the surface bends light rays up toward your eyes, causing your brain to perceive things as much lower than they actually are. The “false desert oasis” is actually the sky. This kind of mirage is known as inferior, while the fata morgana, which places objects higher than they actually are, is superior.
A Fata Morgana is a complex form of superior mirage that is seen in a narrow band right above the horizon. It is an Italian term named after the Arthurian sorceress Morgan le Fay, from a belief that these mirages, often seen in the Strait of Messina, were fairy castles in the air or false land created by her witchcraft to lure sailors to their deaths. Although the term Fata Morgana sometimes is applied to other more common kinds of mirages, true Fata Morgana is different from both an ordinary superior mirage and an inferior mirage.
Fata Morgana mirages significantly distort the object or objects on which they are based, often such that the object is completely unrecognizable. A Fata Morgana may be seen on land or at sea, in polar regions, or in deserts. It may involve almost any kind of distant object, including boats, islands, and the coastline.
Often, a Fata Morgana changes rapidly. The mirage comprises several inverted (upside down) and erect (right side up) images that are stacked on top of one another. Fata Morgana mirages also show alternating compressed and stretched zones.
Like other superior images, a Fata Morgana occurs because rays of light are bent when they pass through air layers of different temperatures in a steep thermal inversion where an atmospheric duct has formed. If the thermal inversion is strong enough (greater than 10 degrees C in 100 meters), the curvature of the light rays within the inversion layer is stronger than the curvature of the Earth (called “super-refracting”), thus trapping them. This is the duct. It will guide the rays along the surface of the earth rather than shooting out into space. An observer needs to be within or below an atmospheric duct in order to be able to see a Fata Morgana.
In calm weather, a layer of significantly warmer air may rest over colder dense air, forming an atmospheric duct that acts like a refracting lens, producing a series of both inverted and erect images. A Fata Morgana requires a duct to be present; thermal inversion alone is not enough to produce this kind of mirage. While a thermal inversion often takes place without there being an atmospheric duct, an atmospheric duct cannot exist without there first being a thermal inversion.
A Fata Morgana is most commonly seen in polar regions, especially over large sheets of ice that have a uniform low temperature. It may, however, be observed in almost any area. In polar regions the Fata Morgana phenomenon is observed on relatively cold days. In deserts, over oceans, and over lakes, however, a Fata Morgana may be observed on hot days.
A Fata Morgana may be observed from any altitude within the Earth’s atmosphere, from sea level up to mountaintops, and even including the view from aircraft.
A Fata Morgana may be described as a very complex superior mirage with more than three distorted erect and inverted images. Because of the constantly changing conditions of the atmosphere, a Fata Morgana may change in various ways within just a few seconds of time, including changing to become a straightforward superior mirage.
A Fata Morgana superior mirage of a ship can take many different forms. Even when the boat in the mirage does not seem to be suspended in the air, it still looks ghostly, and unusual, and what is even more important, it is ever-changing in its appearance. Sometimes a Fata Morgana causes a ship to appear to float inside the waves, at other times an inverted ship appears to sail above its real companion.
In fact, with a Fata Morgana it can be hard to say which individual segment of the mirage is real and which is not real: when a real ship is out of sight because it is below the horizon line, a Fata Morgana can cause the image of it to be elevated, and then everything which is seen by the observer is a mirage. On the other hand, if the real ship is still above the horizon, the image of it can be duplicated many times and elaborately distorted by a Fata Morgana.