Lake Michigan Circumzenithal Arc, David Wehnes of Milwaukee, Wisconsin & David Westman witnessed it while sailing. Image by David Westman. ©David Westman, shown with permission.

The mast nicely shows that this most beautiful of the halo arcs does indeed (partially at least, see the Kern arc) circle the zenith (so does the related circumhorizon arc!) and is sufficiently far overhead that it is noticed less often than it deserves.

The CZA's origin via refraction through plate crystals determines its geometry. The sun's rays enter a near horizontal upper face and leave through one of the six near vertical side faces.

The farthest the arc can stray from the zenith is when the sun is very low. The CZA can then be over 30 degrees from directly overhead.

But then it is very faint: (1) because the crystal face subtends only a minute cross-section to the incoming rays and (2) most rays are externally reflected at these grazing angles of incidence.

The brightest CZAs are at a compromise solar elevation, some 15 to 25 degrees.


About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed