A circumzenithal arc is a striking and relatively rare optical phenomenon that appears high in the sky, often resembling an upside-down rainbow or a multicolored “smile” above the Sun. It forms when sunlight enters horizontally oriented, flat, hexagonal ice crystals in high altitude cirrus or cirrostratus clouds. As the light passes through one face of the crystal and exits through another, it is refracted or bent at a specific angle, creating a bright, colorful arc centered around the zenith, the point directly overhead. A closely related phenomenon is the circumhorizontal arc, which appears much lower in the sky, often well below the Sun, and runs roughly parallel to the horizon; sometimes called a “fire rainbow,” it forms when sunlight enters vertically oriented hexagonal plate crystals and exits through a side face, producing a vivid, rainbow-like band of color.

Unlike typical rainbows, circumzenithal arcs are caused by refraction rather than reflection, and they tend to be more vivid and sharply defined, with brilliant bands of color. For this phenomenon to occur, the Sun must be relatively low in the sky no higher than about 32 degrees above the horizon and the ice crystals must be properly aligned. Circumhorizontal arcs, by contrast, require the Sun to be much higher in the sky, at least about 58 degrees above the horizon, which makes them more common in summer months at mid-latitudes. Although they can be brief and easy to miss, circumzenithal arcs are among the most colorful and visually stunning atmospheric optics one can witness, with circumhorizontal arcs offering a similarly brilliant display when conditions are right.

I captured this instance of a circumzenithal arc from my backyard while in the pool on July 25, 2025. Had I not been in the pool floating around and looking high in the sky I might have missed it.

Click on any of the images below to go to the gallery for a better view and additional information.