We were driving across the scrubby and barren central valley on the way to Fresno, me and some of my California family. We’d been talking about the weather. My family all knew I liked the weather, and the sky, and the combination of those two things. I talked about it all the time. Just as I’d been doing for the last ten minutes, regaling them with stories about the changeable weather and unpredictable crazy clouds in the skies above my familiar Midwestern home, how every hour, every minute, seemed to bring a new and different kind of weather, and how exciting it was to watch for what was going to happen next — because I knew it would! — and be surprised and awestruck when it invariably did.
So when cousin Bonnie interrupted to ask, “Well, David, what about right now?” I looked out the window at the clear cloudless central California skies, and realized I wasn’t in Kansas any more. Or Missouri. For all I knew, these inland California skies of theirs had seen no clouds in a hundred years. Maybe I was wrong about the ever-interesting sky. Maybe this was the only weather my family ever saw.
But I stuck my head out the window anyway, looking all around for some sort of miracle. Then unexpectedly, there it was.
“Rainbow!” I shouted. “Directly overhead!”
They didn’t believe me, of course. But sure enough, directly overhead was a curved arc of prismatic color forming a faint semicircle above the mid-morning sun. The sky which had seemed crystal clear actually had a thin layer of ice-crystal cirrus clouds spread across it, thin enough to be invisible to the eye, everywhere except right above us.
The rest of the passengers were as astonished as I was, and my statements about the sky were proved true, and we all agreed that the weather must be delightfully unpredictable no matter where you were in the world.
These little weather imps, impromptu colored flags and circles and signs in the sky, often appear in thin cirrus clouds and may last for part of an hour, sometimes only for a few scant moments. Maybe that’s why we see them so seldom. They seem to appear in otherwise featureless, uninteresting skies, like playful little imps, as if to make sure we’re on our toes.
What we saw that day wasn’t a rainbow, but if I’d shouted “circumzenithal arc!” it probably wouldn’t have gotten people’s attention. The circumzenithal arc that we saw is part of a group of optical phenomenon caused by light cascading through ice crystal clouds, breaking the sunlight into colors as it passes through the crystals’ tiny prisms, each crystal shape playing a different type of trick with the sunlight. Unlike rainbows, these prismatic displays are formed by light passing through the crystals, not reflecting through liquid raindrops. Where rainbows are seen opposite the sun, optical phenomenon generally occur near the sun.
There are many different crystal shapes, and sometimes more than one way for refraction to occur through any one shape. Most of the possible prismatic effects appear in a circle around the sun. The Parhelic Circle includes all the different effects of crystal shape and sunlight refraction. I managed to catch this photo in 1997, an unusual display where nearly the whole parhelic circle appeared, with a circular halo, an upper arc, and two sun dogs on either side. The location of clouds within this circle, and the type of ice crystal shape in the cloud (or mix of shapes), means we usually see only one or two of these possibilities at any one time.
Some ice crystals are shaped like #2 pencils, long thin six-sided lozenges, which tumble through the air in all directions as they fall. Halos are one result of light passing through these shapes.
One of my other favorites is the “sun dog”. The two bright spots on either side of the sun in the parhelic circle are examples. They often appear in partly overcast skies as the sun drops towards the horizon at dusk, sometimes one, sometimes both.
The ice crystal that creates sun dogs is shaped like a flat six-sided dinner plate. The light is refracted from one of the six sides to another, so it is only visible when the plate is aligned in that direction. As these plates fall through the air, they tend to lay horizontally rather than tumbling end over end, and thus the refracted light is strongest in the horizontal direction. Appearing 22 degrees right or left of the real sun, sometimes only one of these “mock suns” is visible, but if you see one, be sure to check the other side of the sun for the other. Often one or both sun dogs occur embedded within a larger halo.
But the optical events represented in the parhelic circle are just some of the many varied and unusual appearances I’ve had the fortune to see. Some of them I have yet to find an official name for. Unpredictable situations of sun, cloud, temperature and ice crystal shape produce an infinite variety of displays.
They don’t stop at night, either, for the full moon can also produce halos, sun dogs and iridescence, often rivaling the sun’s effects in brightness.
Though they usually appear in light cirrus clouds, that’s not always the case, and this colorful sky-embedded art does seem to hold a showing when you least expect it, with no announcement ahead of time.
So whether the weather you watch is in California or Kansas City or anywhere else in the world, keep an eye out for these playful weather imps. There might be one over your head right now just begging to be noticed. Go outside and look!