Circumhorizontal Arc

We saw one of these in the sky while driving through the mountains in Colorado. I've never seen one before (a little research said they are very rare).

This one is not mine as I didn't have my camera but looks the same.

These clouds are caused by light passing through cirrus clouds. They only occur when the sun is very high in the sky (more than 58° above the horizon) and the hexagonal ice crystals that make up the cirrus cloud 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.

I'd add this to my short list of cool things to try to find in the sky:

Sundogs or Ice Halos (can be seen in day or my favorite at night with a full moon)
Moonbows (I saw one of these once at night with a full moon rising on distant storm)

Have any of you seen some rare natural phenomenon?