A ring or arc of light appearing to surround the Sun or Moon, resulting from refraction and reflection of sunlight or moonlight by ice crystals in cirrus or cirrostratus clouds. The commonest solar and lunar halos have an angular diameter of 44°. The edge of the halo shows a prismatic effect, with blue light refracted to the outer edge and red to the inner. As a result of the preferential refraction of light to the halo’s edge, the sky within a complete halo often appears darker than that outside. Lunar halos can be seen clearly only when the Moon is bright, typically within five days of full.