Cloud Prism?

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On Saturday, Kirk and I rode down to Washington for the day. We got a late start and, as a result, we were on the Lougheed just past the Haney bypass, near 11:30am, or somewhere close to then. I first noticed the cloud (??) on the straight stretch where the road becomes single lane until it widens again before Mission. At first I thought the sun was reflecting strangely on my visor, casting a prism on the surface. But when I lifted it to check the phenomenon was still there to see with my bare eyes. I kept thinking it would disappear quickly so didn’t pull over to dig out the camera from my tailbag. Now I wish I had because it didn’t go away quickly.

It was like someone was shining a light through one of those little glass prisms and a rainbow effect was on a spot in the sky, on some thin cirrus clouds. It was beautiful, and like nothing I’d ever seen before. I noticed Kirk looking up at it too, and when we finally stopped at a light we both said “Do you see….?”

It was like a rainbow, but it wasn’t. And it stayed for a long time. We watched it all the way through Mission, and all along the Mission bypass. All the way through Abbotsford and across the border and into Sumas. It was like a rainbow coloured spot in the sky, but so strange.

I finally googled it this morning and discovered the following. What we saw was real, and scientifically called a “Circumhorizontal arc.”

These photos from Wikipedia are exactly what we were seeing!

That last one was over the Washtington-Idaho border a few years ago, and highlighted in a National Geographic posting

The arc isn’t a rainbow in the traditional sense—it is caused by light passing through wispy, high-altitude cirrus clouds. The sight occurs only when the sun is very high in the sky (more than 58° above the horizon). What’s more, the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground.

When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus’s crystals are aligned just right, the whole cloud lights up in a spectrum of colors.

According to Wikipedia, 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.

It was a neat visual treat and now I know we weren’t seeing things!

When we turned into the mountains it went out of sight and I forgot about it until later in the day when we were coming home and the clouds were creating more of a solid layer. When a system is coming in it’s not unusual to see a halo around the sun, but today we received a second, although much less spectacular treat. Less spectacular enough that Kirk didn’t even notice it. But today’s halo was a faint rainbow. Much like the following image from Wikipedia.