Why are rainbows curved?

Rainbows are curved because round raindrops send sunlight back to your eyes at a steady angle. All the drops that line up at that angle form a circle around the point opposite the sun. From the ground, you usually see only the upper arc.

Each person is looking at a different set of droplets, so your rainbow is not quite the same as the one beside you. From high above the ground, with enough mist below, people can sometimes see the full ring of color.