The sun is near the horizon, and the rainbow curves upwards on the sides. Normal rainbows surround the sun. This one definitely didn't have the sun at its center. The sun is outside the rainbow in the picture. After a little research, I think that what I saw was a Circumzenithal Arc. This type of rainbow or halo is created by ice crystals in cirrus clouds and seems to surround the zenith of the sky.
I should write some interesting and thoughtful closing sentence here, but I can't think of anything. Oh, well.