A rare phenomenon commonly described as a ‘rainbow cloud’ was recently spotted above Tokyo and pictures of it went viral on social media.

On June 26th, reports of an iridescent cloud visible above Tokyo started being shared on Twitter, and soon, photos of the intriguing phenomenon made their way to the popular social media platform as well. It was as the rumors described, a multicolored cloud that oddly resembled a rainbow, only in reality this optical phenomenon was closer to an ice halo than a rainbow.

Commonly known as a “fire rainbow” because it often looks like a multicolored flame in the sky, because of the shape of the fragmentary cirrus clouds it form in, this sky attraction is actually a circumhorizontal arc. Interestingly, it is a rather common phenomenon in some parts of the world, like the United States, where it can be seen several times each summer in any one place, and very rare in others, like in northern Europe.

“Apart from the presence of ice-containing clouds in the right position in the sky, the halo requires that the light source (Sun or Moon) be very high in the sky, at an elevation of 58° or greater,” Wikipedia explains. “This means that the solar variety of the halo is impossible to see at locations north of 55°N or south of 55°S. A lunar circumhorizon arc might be visible at other latitudes, but is much rarer since it requires a nearly full Moon to produce enough light.”

The circumhorizontal arc forms when sunlight enters horizontally-oriented, flat, hexagonal ice crystals in clouds. The ice particles act as a prism, and the light is divided according to the wavelength, so the colors are visible.