Rainbow cloud: How a circumhorizontal arc forms


KNOXVILLE (WATE) – Did you happen to notice a strange, horizontal-looking rainbow cloud in the sky on Monday? Many people contacted the WATE 6 On Your Side Storm Team asking about it.

It looked like a rainbow, but we didn’t have any rain Monday. These really aren’t rainbows. They are called circumhorizontal arcs.

They form by light passing through wispy, high-altitude cirrus clouds. They are rather rare because they are only seen when the sun is very high in the sky, more than 58 degrees above the horizon. Another important factor is that the hexagonal-shaped ice crystals within the high, thin cirrus clouds need to be thick plates with their faces parallel to the ground. So, as you can imagine, this is often a difficult task to get these events to occur at the same time.

When the sun’s light enters through a vertical side of this ice crystal and leaves from the bottom face, it refracts, or bends in the same way that light passes through a prism. If the cirrus’s crystals are aligned just right, the whole cloud lights up in a spectrum of colors.

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