A solar halo (also called a 22° halo) is a luminous ring around the sun formed by the refraction of sunlight through hexagonal ice crystals in cirrus or cirrostratus clouds. The ring has an angular radius of 22° (roughly the span of an outstretched hand at arm's length) and appears as a bright circle with a slightly reddish inner edge and a white outer edge. It is one of the most common atmospheric optical phenomena, observed on average 100+ days per year in many locations.
The 22° halo forms because light enters one face of a randomly oriented hexagonal ice crystal and exits through an alternate face, with a minimum deviation angle of 22° — light cannot be deviated less than this, so it concentrates at 22°, creating the bright ring. More complex halo phenomena include sundogs (parhelia — bright spots at 22° to the left and right of the sun, formed by horizontally oriented plate crystals), circumzenithal arcs (a bright, rainbow-coloured arc near the zenith — often called "the most beautiful atmospheric phenomenon"), tangent arcs, and light pillars.
The folk weather saying "ring around the sun means rain is near" has a solid meteorological basis: cirrus and cirrostratus producing halos are often the advance guard of an approaching warm front, with rain following 12–36 hours later. However, not all halos precede rain — cirrus can appear without frontal activity. Solar halos are best observed wearing sunglasses and blocking the sun with your hand. On Earth, they are visible in any climate; similar halos have been observed in the atmospheres of Mars and Jupiter. Full halo displays with multiple arcs are rare and spectacular, with the most complex observations cataloguing over 20 distinct arcs simultaneously.