The ring is caused by sunlight or moonlight being diffracted as it passes through cirrostratus clouds that are usually at altitudes above 20,000 feet. Cirrostratus clouds are composed mostly of small ice crystals that spread out into a thin layer. They are sheet-like, and the sun and moon can be seen through them easily. Halos are most commonly seen as a white ring around the sun or moon, but sometimes they can appear as a rainbow-colored ring with red on the inside and going to blue-white on the outside. This is seen more often around the sun than around the moon. Halos most commonly form at a 22-degree radius. A more rare halo is the great halo, which forms at a 46-degree radius.