This is an optical phenomenon caused by ice crystals in cold cirrus clouds located high (5–10 km,
or 3–6 miles) in the upper troposphere. Light is reflected and refracted by the ice crystals and
may split up into colors because of dispersion, similarly to a rainbow. (from the Wikipedia)
Saguaro cactus (Carnegiea gigantea) in the foreground.
Photographed in the Sonoran Desert north of Mesa, Arizona USA.