Exceptional Diamond Ring Parhelic Circle Display
September 01, 2016
Shown above is a wonderful parhelic circle display as observed over South Berkshire, England, on August 16, 2016. The parhelic circle is the huge white halo parallel to the horizon and passing through the Sun -- in the form of a spectacular diamond ring, with the Sun as the diamond. To indicate the size of the parhelic circle, this photograph was taken with a fisheye lens having a field of view of 100 x 67 degrees. A standard 50 mm lens would encompass only one-tenth of this image. Sightings of segments of the parhelic circle aren't particularly uncommon, but it's very rare to see the complete circle. This was the first such episode for me in over 40 years of watching and photographing the sky.
A parhelic circle is formed from reflection of sunlight by the nearly vertical face of a hexagonal plate ice crystal. Because it results from reflection rather than refraction, the circle appears white. The brilliantly colored arc above the Sun is a relatively common upper tangent arc, here exhibited as part of a circumscribed 22 degree halo, although the entire halo itself isn't visible. The upper tangent arc is formed by sunlight refracting through a hexagonal columnar ice crystal, and as a result it exhibits the colors of the visible spectrum. Also evident are the 120 degree parhelia (at 10:00 and 2:00 on the parhelic circle), together with the faintly-colored, right-most 22 degree parhelion or sundog. Look for it here on the parhelic circle, a little outside where the 22 degree halo would be seen.
A computer simulation of this display using Halosim 3.6 ray tracing software by Michael Schroeder and Les Cowley is shown at bottom. This simulation was achieved by defining a population of hexagonal ice prisms consisting of 4 percent horizontal plates aligned within 0.5 degrees, 41 percent horizontal columns aligned within 0.3 degrees and 55 percent randomly oriented crystals; 10 million rays were plotted. Note that some of the very faint arcs suggested in the simulation weren't actually visible.
Photo Details: Canon 6D DSLR camera; 15 mm fisheye lens; f/11; 1/1000 sec. exposure; manually underexposed one stop; ISO 100; minor additional post-processing contrast enhancements through Adobe Lightroom. Photo taken at 12:52 p.m.