I had never seen or heard of one of these before. But this past Sunday around noon, I saw this rainbow colored cirrus cloud directly ahead in the sky as I drove south on the freeway. I could see it for several miles as I drove. It seemed to me to be a very strange phenomenon. Was it the northern lights I’ve heard about? No, those are at night. Was it the combination of my sunglasses and some coating on the windshield playing tricks on me? I took my sunglasses off and it was still there. I really, really wished I had my camera with me to take a pictures!
Well, today I asked Google to tell me if I was dreaming or this was a know condition. It turns out that what I saw was real and is scientifically called a “Circumhorizontal arc.” Here are some examples:
And one of the most dramatic over the Washtington-Idaho border two years ago:
Have you ever seen one of these?
It turns out that this rainbow cloud effect occurs when “the sun is high in the sky, at least 58° above the horizon, and can only occur in the presences of cirrus clouds. The phenomenon is quite rare because the ice crystals must be aligned horizontally to refract the high sun. The arc is formed as light rays enter the horizontally-oriented flat hexagonal crystals through a vertical side face and exit through the horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, makes the entire cirrus cloud shine like a flaming rainbow.”
Here are several more examples:
Rainbow Cloud on Flickr – Photo Sharing!
rainbow cloud | KOMO-TV – Seattle, Washington | YouNewsTV
The Cloud Appreciation Society
What was that strange glowing cloud? | KOMO-TV – Seattle, Washington | News
And here are a couple of related phenomena:
Noon, Saturday 3 June. Over the fields of northern Idaho [pictured in this post above], a rare and beautiful sight could be seen. Known as a circumhorizontal arc, it forms as sunlight is refracted through hexagonal ice crystals in cirrus clouds several kilometres above the ground. Sunlight enters a near-vertical face of each crystal and leaves from a horizontal face at the bottom. “Effectively the crystals act as a 90-degree prism for the passing rays,” says Evelyn Hesse of the light-scattering group at the University of Hertfordshire in Hatfield, UK.
Circumhorizontal arcs occur when the sun is at more than 58 degrees above the horizon. “They are most unusual, and this is an impressive example,” Hesse says.