Halos are caused by the light of the sun or moon passing through a very thin layer of cirrus (ice-crystal) clouds in the upper atmosphere. The ice crystals refract the light of the moon, similar to the way water droplets in the lower atmosphere can refract sunlight to produce a rainbow. Just like a rainbow, strong halos can have bands of color in them, due to slightly different refractive properties of the ice crystals for different colors. Essentially, halos ARE rainbows caused by primary refraction in ice crystals.
There’s an old weather saying: “ring around the moon means rain soon.” There’s truth to this saying, because high cirrus clouds often come before a storm.
Source: Curious About Astronomy
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