When sunlight and raindrops combine to make a rainbow, they can make a whole circle of light in the sky. But it’s a very rare sight. Sky conditions have to be just right for this, and even if they are, the bottom part of a full-circle rainbow is usually blocked by your horizon. That’s why we see rainbows not as circles, but as arcs across our sky.
When you see a rainbow, notice the height of the sun. It helps determine how much of an arc you’ll see. The lower the sun, the higher the top of the rainbow. If you could get up high enough, you’d see that some rainbows continue below the horizon seen from closer to sea-level. Mountain climbers sometimes see more of a full-circle rainbow, though even a high mountain isn’t high enough to show you the whole circle.
Pilots do sometimes report seeing genuine full-circle rainbows. They’d be tough to see out the small windows we passengers look through, but pilots have a much better view from up front.
Bottom line: Can you ever see a full-circle rainbow in the sky? Yes, but they’re most often seen by pilots, who have a good view of the sky from the wide front windows of a plane.