Yes, a halo rainbow, specifically referring to a circumzenithal arc (often called an inverted rainbow), is an extremely rare optical phenomenon.
Here's why:
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Specific Conditions Required: Circumzenithal arcs require a very specific set of atmospheric conditions to form. These include:
- High Cirrus Clouds: They need the presence of high-altitude cirrus clouds composed of ice crystals.
- Sunlight Angle: The sun must be at a specific angle (relatively low in the sky, typically less than 32.2 degrees above the horizon) for the light to refract properly through the ice crystals.
- Crystal Alignment: The ice crystals within the cirrus clouds need to be aligned in a particular orientation to refract the sunlight into the vibrant colors of the arc. This alignment is often horizontal.
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Rarity Compared to Regular Rainbows: Common rainbows are formed by water droplets and are seen more frequently after rainfall when the sun is behind the observer. Halo rainbows require more particular atmospheric conditions making them significantly rarer.
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Confusion with Other Phenomena: It's worth noting that the term "halo rainbow" can be confusing. Ordinary halos (rings around the sun or moon) are relatively common, but the circumzenithal arc (the inverted rainbow) is the truly rare phenomenon.
Therefore, seeing a "halo rainbow" (meaning a circumzenithal arc or inverted rainbow) is a rare and special sight due to the very specific atmospheric conditions necessary for its formation.
