The Upside Down Rainbow

The upside down rainbow is not a camera trick nor does it have anything to do with rain. It is a rare natural phenomenon known as a circumzenithal arc. Unlike a rainbow, the sky has to be clear of rain and low level clouds for it to be seen. Here is an explanation:

The arc only appears when sunlight shines at a specific angle through a thin veil of wispy clouds at a height of around 20,000 to 25,000 feet. At this altitude the cirrus clouds are made of ice crystals, the size of grains of salt.

Meteorologists say the clouds must be convex to the sun with the ice particles lined up together in the right direction to refract the light. This results in the sunlight bouncing off the ice crystals high in the atmosphere, sending the light rays back up and bending the sunlight like a glass prism into a spectrum of colour.

Well, if that didn’t make sense, then here is a pictorial representation:

I have to admit that I don’t completely understand it myself but I guess it is a rare enough occurrence to make it noteworthy. I have certainly never seen one myself.

Circumzenithal arc facts:

• In a circumzenithal arc, the colors are in reverse order from a rainbow, with violet on the top and red at the bottom.
• The arc usually vanishes quickly because the cirrus clouds containing the ice crystals shift their position.
• Ice particles in high cirrus clouds occur all year round, but circumzenithal arcs are usually obscured by lower level clouds.
• Circumzenithal arcs are so named as they go around the zenith – the point in the sky directly above the observer- rather than the sun.

Just for the fun of it, here are some real rainbows:

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