Photo | A Circumzenithc Arc – Alan Sealls
The sky smiles upon you, figuratively and literally. As long as we are not stuck under a blanket of dull stratus clouds, this is the time of year when it’s easy to spot what looks like a smile, or what many people call an upside-down rainbow, in the air.
It is not a rainbow, but it is a rainbow impersonator, with the same seven colors. It’s way high in the sky, directly above your head, and it forms when it is not raining.
This smile is called a circumzenithal arc, or a circumzenith arc, or if you really want to be cool like a meteorologist, you can call it a CZA for short. It is an arc of color that partially encircles the zenith (the point directly above your head). Because this happens overhead, few people notice, and many have never seen one. We spend a lot of time looking down at our devices. I guess there is no real reason to look straight up on a regular basis, if you are not a meteorologist. You certainly don’t want to make a habit of looking at the sun when it is high. That’s dangerous for your eyes.
A circumzenith arc forms in the same way that a rainbow forms, except it’s ice crystals rather than water droplets that separate light into the seven colors of the spectrum and create an arc. The ice crystals are in cirrus clouds, miles and miles above the ground, where the temperature can easily be 40 to 50 degrees below 0. If you’ve seen the ring around the sun, called a halo, or the bright spots on either side of the sun, known as parhelia, then you’ve noticed that these and a circumzenith arc all occur in the direction of the sun. A rainbow only happens in the direction of your shadow.
When’s the best time to look? Take a sunny day, when there is a very thin veil of high clouds that does not blot out the sun. Those would be cirrus clouds. We get extra cirrus clouds as a byproduct of airplane vapor trails when planes burn fuel that releases water vapor. You see cirrus clouds any time of year, but the dry, colder air of winter on the Gulf Coast limits the puffy cumulus clouds that would otherwise block a portion of your sky view in the summer.
A circumzenith arc may last seconds, minutes or even a couple of hours. It can appear, disappear and then reappear, all depending upon the clouds. It may be faint or bright or neon electric. An arc tells you the clouds are high and cold. If nothing else, it’s a pretty sight. Even if you don’t see it as a smile, a circumzenith arc should make you smile since it comes along with calm weather. If you call a circumzenith arc an upside-down rainbow, does that make a rainbow a frown? Of course not! That’s worth a smile, too.
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