Circumhorizontal arc at Angel's Landing

My brother, his wife, and I were hiking the strenuous and perilous Angel's Landing trail in Zion National Park earlier this week.

Physics lessons abound in work (elevation gain is 1500 ft), friction (footing), tension (chains), equilibrium (precarious balance), etc. But after returning from Angel's Landing to Scout Lookout, my brother directed my attention to a spectrum in the cirrus clouds above.

I had never seen such a color band in the sky. It was clearly the interaction of sunlight and cirrus cloud ice crystals. But it was at a much greater angle from the sun than the oft-seen 22° halo. The colors were spread wider and were more deeply saturated.

Upon returning to the connected world (which is a small world, indeed, when you're at the mercy of AT&T), I googled "cirrus cloud rainbow" and discovered that what we were seeing was a circumhorizontal arc (CHA).

A circumhorizontal arc lies 46° from the sun and is observed only when and where the sun is more than 58° above the horizon. We get more of them in the States than they do in Europe.

The heart of CHA optics is the hexagonal ice crystal. For CHA, we need plates rather than rods. Sunlight enters a vertical side (rectangular) face and exits the bottom (hexagonal) face.

Of course, the different colors in the sunlight are refracted by different amounts. The dispersion of colors is fairly wide in CHA.

For more images of this atmospheric grooviness, you can check out this filtered gallery, or image-search "circumhorizntal arc." (Remember that an image-search will give you many results that are incorrectly tagged.) My favorite CHA image is the "fire rainbow" photograph taken near the Idaho/Washington border some years ago. It caused enough of a stir to get its own Snopes article!

My travel shots and landscapes from the trip are here.