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Last Friday was World Smile Day but Mother Nature got out ahead of it with this little grin in the sky. That’s not really what it’s called, but it’s a lot easier than saying the real name – circumzenithal arc – isn’t it?
Jess Bargen was out in Kentville, N.S., last Thursday when she spotted the lovely colours high above her.
The circumzenithal arc or CZA is one of the brightest and most colourful members of the halo family. Its colours, ranging from violet on top to red at the bottom, are purer than those of a rainbow because there is much less overlap in their formation.
I’ve heard people refer to it as an upside-down rainbow, but this arc isn’t a rainbow in the traditional sense because it’s a result of light passing through ice, instead of rain.
You’ll only spot one of these when the sun is very high in the sky – more than 58 degrees above the horizon. The shape of the ice crystals is also very important. The hexagonal ice crystals that make up the cirrus clouds must be shaped like thick plates and their faces have to be parallel to the ground. The light enters at a 90-degree angle and is refracted by the tiny plates of ice. When light enters through a vertical side face of this type of ice crystal and exits the bottom face, it refracts or bends in the same way light passes through a prism. If these crystals are aligned just right, the whole cloud lights up in a spectrum of colours.
Jess mentioned there were also a couple of sundogs in the sky that day. I was not surprised. If you ever see sundogs, look high above the sun. The same type of ice crystals that causes them are responsible for the CZA.
Another good reason to remember to look up!
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Cindy Day is the chief meteorologist for SaltWire Network.