A 22° halo (known also as moon or solar halo) is an optical phenomenon, forming a circle with a radius of approximately 22° around the moon or sun, produced by reflected/refracted light interacting with millions hexagonal ice crystals suspended in the cirrus or cirrostratus clouds (troposphere, 6-13 km high).
No light is reflected towards the inside of the ring, giving it the impression of a big hole in the sky. As the sailors say, when the 22° halo appears in the sky, a storm is approaching.
In clear sky conditions the view of this phenomenon is spectacular: the one in the photo (taken in Thaba-Tseka, Lesotho) is a lunar halo, much rarer than the solar halo.