Video of the "floating city" above a town in China.



Video of a mysterious cityscape in the clouds hovering over a Chinese city has gone viral this week. And explanations for this startling video range from a secret NASA project to an elaborate hoax to an actual atmospheric phenomenon.

Experts hesitate to say the video is real. "It looks almost too good," says Peggy LeMone, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado.

"[But,] if it is real, it's called a superior mirage, which just means it's an upward projecting mirage," says Jill Coleman, an atmospheric scientist at Ball State University in Muncie, Indiana. It could be something called a fata morgana, she says, which is a certain kind of atmospheric mirage.

To get a fata morgana, you need cold dense air near the ground with a layer of warmer air above it, Coleman says. This is called a temperature inversion, since it's the reverse of what usually happens in the atmosphere. They usually occur over large bodies of water since the air tends to be relatively cooler close to an ocean or lake surface, but can form over land too.

This kind of layering will bend light rays as they pass from one air mass into the next. Those light rays are bent in such a way that our brains are tricked into thinking an object is higher than it really is. (Learn about other weird atmospheric phenomena.)

And since a person's perspective can alter the appearance of a fata morgana, the further away from an object a person is, the taller the object will appear, says Coleman.

That's likely why in the video, "the city looked like it was floating on cloud nine," she says. It would probably have looked very different to someone standing in the center of the city.

A fata morgana is why people can sometimes see ships "flying" through the sky or a "wall" of water dominating the horizon. In fact, this type of mirage could be how the myth of the Flying Dutchman—a ghost ship that sails the high seas—got its start. And according to historian Tim Maltin, a fata morgana could have contributed to the sinking of the Titanic.

Coleman looked at atmospheric conditions over Jiangxi and Foshan, China, during the time the video was supposedly shot. "They did have temperature inversions going on during that time period," she says. And some of the buildings in the city below the clouds look similar to what shows up in the sky. So Coleman thinks the video could be real.

Fata morganas can be quite common in certain parts of the world as long as conditions are right, says LeMone.

They're common in polar regions, says Teresa Wilson, a graduate student in physics at Michigan Technological University in Houghton, in an email. "But [they] can happen anywhere." People have even seen fata morgana in the Strait of Messina between Italy and Sicily.

LeMone hasn't ever seen one in person, but she's seen the reverse—a reflection of the sky on the road—plenty of times. That kind of mirage is known as an inferior mirage because the light rays bend in such a way that our brain thinks an object is lower than it actually is.

"You can see some pretty cool things in the atmosphere," LeMone says. "You just have to watch for them." (Learn about "hole punch" clouds.)