Late Sunday afternoon the pale, milky cirrostratus clouds that had been filtering the sun all day thickened up and drew themselves together into patchy sprays of white. Curving tails of drifting ice crystals dropped down into the dryer air below, producing the iconic cirrus mares’ tails.
These are some of my favorite clouds. In this case, waves of moist air came pouring off the top of a cold front skirting the coast to the west. That front will probably give us some rain on Tuesday.
It was sheer good luck that I happened to glance straight up at this precise moment. It faded into view as I watched, and for less than thirty amazing seconds it glowed brightly: the rare, elusive, often fleeting circumzenithal arc!
Wow! I had seen this colorful skybow before, but never captured a photo of it. Camera ready, I snapped away. I was able to get several good shots before the arc faded out.
Circumzenithal arcs are among the brightest, most colorful of skybows. They are caused when sunlight hits flat ice crystals at a low angle, refracting the rays down and passing them out through a vertical side face. To form a CZA, the crystals must be very well aligned, almost all floating with their widest faces horizontal. This can only happen if the air flow within the cloud is very smooth and free of turbulence.
Because the sun angle must be low, CZAs only happen when the sun itself is low in the sky. Because they always happen near the zenith, where the clouds move past most quickly, they are usually very fleeting. One is lucky indeed to see it, and even luckier to get a photo.
The CZA has an even more colorful cousin, the circumhorizon arc, which can only happen when the sun is quite high in the sky. With the summer months coming, CHAs are becoming possible. If I can catch one I’ll certainly share it here!
On the whole, it was a very satisfying cloudscaped evening.