While technically not rare, seeing a complete 360-degree rainbow is pretty rare in practice from the ground.
Here's why:
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Rainbows are always full circles: A rainbow isn't actually an arc; it's a complete circle. The phenomenon results from the refraction and reflection of sunlight within raindrops.
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The horizon obstructs the view: From our vantage point on the ground, the Earth itself blocks the lower portion of the rainbow. We typically only see the upper arc.
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Ideal conditions are needed: To witness a full 360-degree rainbow, you need to be elevated, such as in an airplane or on a high mountaintop, with the sun behind you and rain or mist below.
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"Glories" and Pilot Sightings: Pilots often see full circular rainbows, sometimes referred to as "glories," surrounding their aircraft's shadow on clouds below. These are essentially full rainbows viewed from above.
In summary, the physics of rainbows dictates that they are always full circles. The difficulty lies in finding an unobstructed viewpoint to witness the complete 360-degree phenomenon. So, while rainbows themselves are relatively common, observing a full circle is a rare and stunning experience.
