What causes a rainbow in the clouds?

Ever noticed a rainbow tint in some of the clouds? Of course, it’s not actually a rainbow, but there’s a reason why you see a colorful tint to the clouds.

Photo from Jody Reese in Brigantine 

Iridescent clouds (or “rainbow clouds”) occur when sunlight diffracts from water droplets in the atmosphere. Iridescent clouds usually accompany thunderstorms. These clouds are rare. The cloud must be thin and have lots of water droplets or ice crystals of about the same size. When that happens, the sun’s rays encounter just a few droplets at a time.

Have you ever noticed a rainbow cloud stretched out like an arc? This is known as “Circumhorizonal Arc.” Clouds are made of layered ice crystals, shaped like hexagonal plates. The sunlight enters a vertical side and exits the flat bottom, then the ice refracts the light separating the colors.

Only when the light source’s elevation exceeds 58 degrees does the Circumhorizontal Arc appear, meaning the sun has to strike the cloud and ice crystals at the perfect angle. For this reason, rainbow clouds typically only occur from Spring to early Autumn.

It’s always important that you wear protective sunglasses when looking up at these beautiful cloud formations!

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