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Created 8-Oct-11
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This colorful phenomenon, known officially as a Circumhorizon Arc, occurs when sunlight strikes cirrus clouds — the kind that typically look like cotton candy and form very high in the sky — at a certain angle.

"Cirrus clouds are composed entirely of ice crystals because they're so high up," says NASA's Rob Gutro. "The higher up you go, the colder it gets and they're beyond the freezing point."

The clouds' ice crystals act the same way crystals in a sparkly bracelet or ring might. When light hits the crystals' facets at a certain angle, it is separated out into all the colors in the spectrum.

However, because the light show depends on the sun's angle, these cirrus rainbows can be seen only at certain times of the year in certain spots on the globe.
"It depends on the position of the observer," Gutro told OurAmazingPlanet.

The sun must be at an angle above 58 degrees, so the further from the equator you are, the shorter your window to see a circumhorizon arc, according to the website Atmospheric Optics.

In Seattle, the sun is high enough to produce the colorful spectacles from May through August. Travel south to Houston, where the sun stays higher in the sky for a longer portion of the year, and you can catch a glimpse from March through September.

This shot was taken on June 12, 2011 @ 1:13 PM.

Nikon D7000, Nikkor 28-300mm @ 180mm, 1/1000 sec @ f/6.3, ISO 125, Handheld, Center Weighted. — in Anacortes, WA.

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