This was taken at Red Rock Lakes National Wildlife Refuge (http://go.usa.gov/jrkY) in Montana seven years ago this week. Fogbows form the same way rainbows do. A small fraction of the light entering droplets is internally reflected once and emerges to form a large circle opposite the sun. Fogbows are formed by much smaller cloud and fog droplets than the raindrops that form rainbows.
Photo credit: James “Newt” Perdue/U.S. Fish and Wildlife Service