It refers to the optical phenomena produced by refracting light from the sun as the light through hexagonal columnar or hexagonal plate ice crystals. This kind of ice crystals exist in the high cloud. Red light has a longer wavelength and a small refraction angle; purple light has a shorter wavelength and a large refraction angle. Due to the different refraction and dispersion, and the observer can see the optical phenomena. When the cloud contains a large number of evenly distributed hexagonal column or plate ice crystals, the observer can observe the sun on its circumference with inner ring (close to light source side) is a warm color ring and the outer ring (far from light source side) is a cool color ring, forming a halo. In addition, different angles of light passing through ice crystals will produce different dispersion. When the light passes through the 60° angle of the ice crystal, it will cause a refraction angle of about 22°, so that the observer can observe the color circle at a position of about 22° from the light source, which is a 22° Halo. Or a small halo; when the light passes through the 90° angle of the ice crystal, it will cause a refraction angle of about 46°, so that the observer can observe the color ring at a position of about 46° from the light source, which is a 46° halo ( 46° Halo) or large halo. Due to the small refraction angle of the small halo, the light of each color is relatively concentrated, so the halo phenomenon is more obvious. The color contrast is low, and even appears white when a large refraction angle.