Hiker films fire rainbow over Vorarlberg

He was hiking on the Lünersee, a mountain lake, on July 5 when he noticed and filmed the colorful sky spectacle, also known as the fire rainbow. “At first I thought it was just my imagination, because I’m colorblind, right. I’m hallucinating because of the effort of hiking and the heat,” the man said.

When he got home, he sent the footage (which has only just been published, note) to his friends and searched for it on the Internet. So he finally figured out that the clouds shimmering in rainbow colors were “a rare phenomenon,” the man continued.

Circumhorizontal arcs resemble a horizontal rainbow and are therefore colloquially called rainbow clouds. Although the glow resembles a rainbow (same spectral colors, note), this is not the case, because in rainbows the sunlight is refracted by liquid raindrops (and not by ice flakes).

Ice crystals are responsible for the shine
The rainbow colors of a circumhorizontal arc are created by the refraction of the sun’s rays as they strike nearly vertically ice crystals floating in high cirrus or cirrostratus clouds. Depending on the size and orientation of the ice crystals and the angle at which the light strikes the crystals, sometimes whitish and sometimes colored halos appear in the sky in the form of circles, arcs, or even columns.

A circumhorizontal arc can only occur when the sun is higher than 57.8 degrees above the horizon. Therefore, rainbow clouds cannot be observed at locations further north than 55 degrees north latitude or south than 55 degrees south latitude.