What looks like an upside-down rainbow is most likely a circumzenithal arc, an optical phenomenon related to halos.
Circumzenithal arcs are formed by sunlight refracting through ice crystals in cirrus clouds high in the atmosphere. Unlike rainbows, which are caused by refraction in water droplets, circumzenithal arcs need these hexagonal ice crystals to be oriented in a specific way.
Here's a breakdown:
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Ice Crystals: The upper atmosphere contains cirrus clouds, which are made of tiny, hexagonal ice crystals.
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Sunlight and Refraction: When sunlight enters these ice crystals, it is refracted (bent). The specific angle of refraction separates the light into its constituent colors, just like in a rainbow.
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Crystal Orientation: The key difference between a rainbow and a circumzenithal arc lies in the orientation of the crystals. For a circumzenithal arc to form, the ice crystals must be oriented horizontally. Sunlight enters the top face of the crystal and exits through a side face.
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Upside-Down Appearance: This specific refraction and the crystal orientation cause the arc to appear above the sun (near the zenith) and, thus, give it an "inverted" or upside-down appearance. The colors are also purer and more vibrant than those in a regular rainbow.
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Association with Halos: Circumzenithal arcs are often associated with other halo phenomena, such as sun dogs and circumhorizontal arcs, all of which are caused by light interacting with ice crystals in the atmosphere.
In short, you aren't seeing an upside-down rainbow, but a circumzenithal arc. It's a beautiful, relatively rare atmospheric phenomenon created by sunlight refracting through horizontally-oriented ice crystals high in the atmosphere.
