Rare natural spectacle

Hiker films rainbow of fire over Vorarlberg

(Bild: kameraOne (Screenshot))

A hiker in Vorarlberg was recently treated to a fascinating natural spectacle. In the sky near the municipality of Vandans, a colorful luminous phenomenon could be admired - a so-called circumhorizontal arc ...

He was hiking at Lünersee, a mountain lake, on 5 July when he noticed and filmed the colorful sky spectacle, which is also known as a rainbow of fire. "At first I thought I was just imagining it because I'm colorblind, or that I was hallucinating due to the exertion of hiking and the heat," the man described.

Back at home, he sent the photos (which have only now been published, note) to his friends and researched them on the Internet. This is how he finally found out that the clouds shimmering in rainbow colors were "a rare phenomenon", the man continued.

A hiker observed a rainbow of fire in the sky over Vorarlberg.(Bild: kameraOne (Screenshot))

Circumhorizontal arcs resemble a horizontal rainbow, which is why they are colloquially referred to as rainbow clouds. Although the luminous phenomenon is reminiscent of a rainbow (the same spectral colors, note), it is not one, because in rainbows the sunlight is refracted by liquid raindrops (and not by ice flakes).

Ice crystals responsible for the luminous phenomenon
The rainbow colors of a circumhorizontal arc are created by the refraction of sunlight when it falls almost vertically onto 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 hits the crystals, sometimes whitish, sometimes colored halos appear in the sky in the form of circles, arcs or even columns.

A circumhorizontal arc can only form when the sun is higher than 57.8 degrees above the horizon. Therefore, rainbow clouds cannot be observed in places that are more than 55 degrees north or more than 55 degrees south.