Further informationSometimes, you won't see an entire half-circle rainbow and you'll just see a small section of one. This happens when the light hitting the droplets in the air only hits a smaller section of water droplets.
Sources & links
"horizon." Merriam-Webster Online Dictionary. 2009. Merriam-Webster Online. 8 November 2009 “About Rainbows.” The National Center for Atmospheric Research & the UCAR Office of Programs. University Corporation for Atmospheric Research. 8 Nov. 2009 “Optical Illusions: Can rainbows make a full circle?” Weather Wiz Kids. 8 Nov. 2009