Circumzenithal Arc Over Norfolk, Virginia
February 09, 2007
This colorful circumzenithal arc (CZA) was part of a magnificent display of arcs and parhelia observed during the late afternoon of November 27th (2006), in Norfolk, Virginia. The CZA and parhelia were photographed within minutes of each other. A faint upper tangential arc [not shown] was also visible for a short while. Later that afternoon, just after sunset, a faint "sundog pillar" appeared to the north of the setting point; according to Dr. Les Cowley, this strange sight may have been the result of an overlapping parhelion and subparhelion, giving rise to the appearance of a parhelion pillar (another possibility is a variety of Lowitz arc).
The CZA forms only when Sun is less than 32.3° high, and its appearance changes as the Sun sets: near the upper altitude limit the arc is faint and diffuse and almost in a zenithal position, but as the Sun sets the arc moves away from the zenith (remaining symmetrically convex towards the sun), and it becomes narrower, while its angular extent increases somewhat. I think that the Sun's altitude was between 25 and 30 degrees at the time this photograph was taken. The cirrus cloud crystals from which this CZA resulted dispersed shortly thereafter, and the lovely arc disappeared from sight. CZAs form when sunlight passes through the uppermost basal faces of oriented, hexagonal plate crystals and then exits through vertical side faces. These two faces are perpendicular, causing the colors to disperse more widely that for smaller "prism angles". Consequently, there's little overlap between the colors, and the hues are very pure -- purer than those of a rainbow.