Have you ever seen a “rainbow” in the sky? Netizens yesterday shared a lot of photos of a beautiful “rainbow”, which was in fact a rare circumzenithal arc according to the city’s weather bureau.

The circumzenithal arcis an optical phenomenon similar in appearance to a rainbow, but belonging to the family of halos arising from refraction of sunlight through ice crystals, generally in cirrus or cirrostratus clouds, rather than from raindrops.

The arc is located at a considerable distance above the sun and at most forms a quarter of a circle centered on the zenith. It has been called “a smile in the sky”, as its first impression is that of an upside-down rainbow.