[FROM THE PAGE]: "The colorful phenomenon, known officially as a circumhorizon arc, occurs when sunlight strikes cirrus clouds 窶" the kind that typically look like cotton candy and form very high in the sky 窶" at a certain angle.
"Cirrus clouds are composed entirely of ice crystals because they're so high up," said 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."