A video is being widely shared on social media (and the "weird news" sections of more traditional media) claiming to show the image of an impossibly large city rising above the fog in the city of Foshan (佛山), Guangdong province, China.
Some have said this is an example of a fata morgana, a type of mirage where light is bent though the atmosphere in such a way to create the illusion of buildings on the horizon. This is utterly impossible in this case, as fata morgana only creates a very thin strip of such an illusion very close to the horizon, and appears small and far away. It does not create images high in the sky.
No expert has actually looked at this video and said it was a fata morgana, and they won't because it's obviously not.
The second and more common type of "floating city" illusions is with buildings that are simply rising up out of clouds or low fog, and hence appear to be floating above them. This has led to "floating city" stories in the past, with this recent example, also from China.
This is simply a photo of building across the river, but when cropped it appears like they are floating, which led to all kinds of wild stories of "ghost cities". This actually came from mistranslations of the original news reports, where local people (who knew exactly what they were looking at) were simply marveling at how pretty the scene looked, with the buildings appearing to float above clouds.
The image actually looks quite like this older image taken from a video published in June 2015.
Here we can see the image is more obviously faked in some way. The "floating" building on the right goes in front of the mountains. This means it cannot be some kind of natural phenomena. The image is either a digital composite, or it was filmed through a window or other sheet of glass.
While the two examples are not identical, there's enough similarities to think that they were faked in the same manner.
Here's an example of someone posting a reflection on Weibo, and calling it a "Mirage" (海市蜃楼) http://weibo.com/1560923874/Bjzjrjlqd. On Weibo it's quite common to see the word "mirage" used to describe a photo of buildings in cloud. i.e. it's generally used figuratively.
Cities in clouds are popular subjects in China. While some videos are obviously just a city in clouds, they are described as "海市蜃楼", or mirage. An image search for this word gives many other examples, largely of buildings rising above clouds, but also some Fata Morgana, as well as many artistic interpretations.
In the decades following “Vanderdecken’s Message Home" in 1821, few other than the most superstitious (or most intoxicated) of sailors actually feared the Flying Dutchman, but real fata morganas made their way into the blossoming mass media of America. In 1871, the Sentinel of Santa Cruz, California reported a fata morgana that made a steamer appear “four or five stories high,” while other schooners played about beautifully in the mirage. Some 20 years later in Buffalo, New York, 20,000 people gathered to witness a fata morgana on Lake Ontario. Though Toronto was over 50 miles away, “its church spires could be counted with the greatest ease” through the mirage, reported Scientific American.
At work here is some basic physics. When the sun heats up the atmosphere above the ocean, it creates a gradient of temperatures: Near the surface, it’s still relatively cool because the water is chilling that air, but sitting above that is a layer of warmer air. Now, light doesn’t always travel in a straight line. When it hits a boundary between two layers of the atmosphere that are different temperatures (and therefore different densities), it bends and travels through the new layer at a different angle. This is known as refraction. The change in the light’s angle of travel depends on the difference in density between the two layers.
How does bending light create a mirage? The rest of the effect is caused by how your brain works. When light hits your eyes, your brain assumes it arrived there in a straight path between you and the object reflecting the light. So if light is bent on its way toward you, your brain will think the object is where it would be if the light’s path was straight. This is why when you are looking down on a surface of water, objects under the surface will appear to be in a different spot than they actually are—just ask a spear fisherman…if you happen to know a spear fisherman. The human brain doesn’t automatically account for the refraction. (Interestingly, the brains of some birds like ospreys do correct for the effect so that when they dive headlong into the water after a fish, they are right on target.)
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 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.