Mount Kato

The vibrant band of colors you are seeing in the thin, wispy clouds is an optical phenomenon known scientifically as a circumhorizontal arc. It is also frequently (though inaccurately) nicknamed a “fire rainbow.”

What Causes It?
Unlike true rainbows, which are created by the reflection and refraction of light through liquid rain droplets, a circumhorizontal arc is technically an ice halo. It requires a very specific set of atmospheric conditions to form:

High-Altitude Ice Crystals: The phenomenon occurs in high-altitude cirrus clouds (the thin, feathery clouds seen in your image). At these high elevations, the clouds are made up of tiny, flat, hexagonal (six-sided) plate ice crystals.

Prismatic Refraction: When these plate-shaped crystals drift horizontally through the air, they act like millions of tiny glass prisms. Sunlight enters the vertical side face of the crystal and exits through the flat bottom face. As the light passes through, it bends (refracts) and separates into its component colors, producing the rainbow spectrum.

A High Sun Angle: This alignment only works when the sun is exceptionally high in the sky—strictly 58 degrees or higher above the horizon. Because of this steep angle requirement, circumhorizontal arcs can only be seen during the summer months in certain latitudes, making them a relatively rare and special sight!