Rainbows are commonly described as arcs, but in reality they are full circles.

A rainbow forms when sunlight enters raindrops, reflects inside them, and exits at a precise angle of about 42 degrees relative to the direction opposite the Sun. From the observer’s perspective, all of the illuminated raindrops that send light to the eye lie along a circular band in the sky.

If nothing blocked your view, the rainbow would appear as a complete circle centered on the point directly opposite the Sun.

Most of the time we see only the upper half because the lower portion of the circle lies below the horizon and is hidden by the ground. When observers gain enough elevation, the entire ring can become visible.

Pilots, skydivers, and hikers looking down into mist or clouds occasionally report seeing the full circular rainbow surrounding the antisolar point.

What we usually call a “rainbow arc” is therefore just the visible portion of a much larger circle.