A sundog (also known as parhelion, from the Greek para "beside" and helios "sun") is an optical phenomenon that manifests as one or two bright luminous spots located approximately 22° on either side of the sun, at the same height above the horizon. They frequently display a reddish hue on the edge nearest the sun and a bluish tint on the opposite side, and may be accompanied by a 22° solar halo. Sundogs are produced by the refraction of sunlight passing through flat hexagonal ice crystals (plate type) suspended in cirrus or cirrocumulus clouds. As these crystals fall slowly, they orient themselves with their flat faces horizontal, acting as prisms that deflect light by a minimum of 22°. Unlike the halo, which requires randomly oriented crystals, sundogs need this preferential horizontal orientation. Sundogs are most intense and frequent when the sun is low on the horizon (sunrise and sunset), as the refraction geometry is optimal. As the sun climbs higher, sundogs move away from the 22° halo position and lose intensity. In antiquity, the appearance of multiple "suns" was interpreted as an omen, and Roman and medieval chroniclers documented numerous sightings. Today they are known to indicate high cirrus clouds, frequently associated with the approach of a warm front.