To see a rainbow make a perfect, full circle, you'd need to see it from a plane in the air. This happens for very small droplets, such as a fine mist, or just after a rain shower when the air is just moist. You need very small water droplets in order for a rainbow to form. If you are flying a little above the cloud cover, keep a lookout for the small shadow of your plane on the clouds. The clouds are tiny water droplets, so chances are you may see a small circle of color around the shadow of the airplane.

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