Most people think of rainbows as colorful arcs stretching across the sky, but in reality, rainbows are full circles. The ground usually blocks the lower half from view, which is why we typically only see a semi-circular shape. However, if you’re at a high elevation with the right lighting, such as in an airplane, you can sometimes see the full circular rainbow.

Rainbows form when sunlight is refracted, reflected, and dispersed inside water droplets in the atmosphere. The light enters each droplet, bends, reflects off the back, and then bends again as it exits. This process directs light back toward the observer at a fixed angle, about 42 degrees from the direction opposite the sun. The circle is centered on this “anti-solar point,” and the complete rainbow exists around it.

Because the Earth’s surface gets in the way when viewed from the ground, we only see the upper portion of the circle. The higher you go, the more of the circle becomes visible. Pilots and mountain climbers occasionally report seeing full-circle rainbows, especially when the sun is low and behind them.

Learn more:

• If Rainbows Are Circular, Why Do We Only See Arches?
• National Geographic – Rainbow