This photo may look like the beginning of a nuclear bomb explosion or a magic portal opening in the sky, but it's a rare rainbow pileus cloud captured by Peter Lowenstein.
From NASA's Astronomy Picture of the Day:
Pictured, behind this darker cloud, is a pileusiridescent cloud, a group of water droplets that have a uniformly similar size and so together diffract different colours of sunlight by different amounts. The above image was taken just before sunset when it was noticed by chance by a photographer in Murambi East, near Odzi Valley and the Mtanda Range of Zimbabwe. Also captured were unusual cloud ripples above the pileus cloud. The formation of a rare pileus cloud capping a common cumulus cloud is an indication that the lower cloud is expanding upward and might well develop into a storm. In this case, however, only a few minutes after the colourful cloud was noticed, it disappeared.