Circles

Complete Parhelic Circle

Imaged at Riga, Latvia on April 9, 2010 by Ivo Dinsbergs who assembled the montage from 14 separate images.

The parhelic circle is everywhere at the same altitude as the sun. It is noticed less often than it might be because it is white and therefore not readily distinguished from cloud streaks. Certain parts do in fact have faint colours but it takes practise to see them.

Plate crystals, column crystals and Parry oriented columns form the parhelic circle.

The circle has more ray paths contributing to it than any other halo.

The simplest path is an external reflection from a near vertical ice face but this is not necessarily the major component.

Below are HaloSim simulations for thin plate crystals filtered to allow only a specific number of interactions with crystal faces.

Single external reflection - Only a weak parhelic circle relatively near to the sun.

Passage through 2 faces - No parhelic circle can be formed. Notice also that at this sun elevation no sundog is formed either!

3 faces - Ray paths with a single internal reflection from a vertical face form most of the parhelic circle. Rays enter through the crystal top basal face and leave through the lower one.

4 faces - A faint circle. The 120 degree parhelion is most evident.

5 - 15 faces - A weak contribution away from the sun. And the sundog is formed! The latter is the result of an even number of reflections between the top and lower basal faces.

Crystal regularity, thickness and tilt all influence the relative contributions of different rays. The point is that the parhelic circle is no simple halo.

Iimage �Ivo Dinsbergs, shown with permission