Round rainbow can be seen mostly by pilots and occupants of an aircraft when flying above a cloud layer. When the sun shines at such an angle that you can see the airplane's shadow, the shadow will be encircled by a perfect rainbow. Actually, there will be two concentric rainbows, but the second one with the inverse color sequence is sometimes hard to see. What makes it most interesting is the fact that this rainbow cannot be seen by anybody else in the air or on the ground even if they see your shadow against a cloud. You are so special that it exists only for you. If you are an AOPA member, you can read more about it by following this link.
Here's a club for those pilots who have actually encountered a round rainbow in the sky. If you were fortunate enough to capture it on camera, send your pictures to us at email@example.com and we will post it here. Please provide as much detail as you remember: date and place, altitude, aircraft type, type of camera, any special photo techniques.