Circumzenithal arc

(Section 3.2.3.1.5)

A circumzenithal arc (previously known as the upper circumzenithal arc) is a near-zenith, colourful half-circle with a vertex at about 48° above the Sun. It has bright colours with red on the lower, outside of the arc and violet on the upper, inside. This arc occurs only when the elevation of the light source (for example, the Sun) is less than 32°. At this solar elevation, it has the smallest radius. By decreasing the radius, the arc will steadily enlarge. When the angular elevation of the light source is about 22°, the circumzenithal arc touches the 46° halo, if visible. These two features are increasingly separated with a light source further from 22°. The arc may be visible without the 46° halo also being present. When there are parhelia, there is a high probability that a circumzenithal arc will be visible, especially at Sun elevations between 15° and 25° because both are caused by plate-shaped ice crystals that are oriented horizontally.