All rainbows would be completely round except that the ground gets in the way of completing the circle. However, if viewed from a high altitude, such as an airplane, circular rainbows can been seen when the angle between the sun, the water droplets, and the plane is between the 40° and 42°. In this case, the rainbow is horizontal, meaning that it is parallel to the ground and therefore not blocked by the ground. This is quite a sight!