A phenomenon that can range in appearance from a simple and fairly common circle of light around the Sun or the Moon to a rare event in which the whole sky is webbed by intricate arcs. Atmospheric halos are caused by tiny, flat ice crystals in the atmosphere that refract and reflect incoming light. Round solar halos with a radius of 22° happen more often than rainbows, and in Europe and parts of the United States can be seen on average twice a week. Within a solar halo, on opposite sides of the Sun and at the same altitude, may be two bright spots known as parhelia, sundogs, or mock suns. Lunar halos may contain parselenae, or mock moons, but these are usually seen only in polar regions and within five days of a full moon.
