Halo (Optical Phenomenon)

A halo is an optical phenomenon produced when light, typically from the Sun or Moon, interacts with ice crystals suspended in the atmosphere, producing displays that range from colored or white rings to arcs and spots, some appearing close to the light source and others elsewhere in the sky or even in the opposite direction. Most halos form in high cirrus or cirrostratus clouds around 5 to 10 kilometers up, though ground-level diamond dust can produce them in very cold conditions; the ice crystals act like small prisms and mirrors, reflecting and refracting the light and sometimes splitting it into colors. The most familiar form is the 22-degree halo, a ring around the Sun or Moon with a radius of about 22 degrees, roughly the span of an outstretched hand at arm's length; other forms include light pillars and sun dogs. Because the cirrostratus clouds that produce halos often precede an approaching frontal system, a halo has long been read as a sign that rain may fall within the next day.