A halo develops when light enters one side of a columnar ice crystal and exits through another side. The light is refracted when it enters the ice crystal and once again when it leaves the ice crystal.
Altocumulus undulatus clouds at Abruzzo national park, Italy. This cloud formation consists of parallel bands of cumulus clouds, occuring when wind shear affects a layer of altocumulus Photograph: Science Photo Library / Rex Feat
The Icelandic cloud formations are just beautiful, so remember to look up at the endless skies while you are here on vacation. Great for world class cloud photographing. This photo is from Búðardalur, Iceland, by Ben Cowden.