this atmospheric phenomenon is known as a circumhorizontal arc, which occurs when the sun is at least 58° above the horizon and the hexagonal ice crystals which form cirrus clouds become horizontally aligned.
(photos x, x, x, x, x, x, x. see also: more circumhorizontal arcs, asperatus clouds, mammatus clouds, polar stratospheric clouds and cloud iridescence)
(via mcavoy)