Swiss Halos
Mountain diamond dust halos imaged by Alistair Adams in the Melchsee-Frutt region of Switzerland.
 ©Alistair Adams, shown with permission
Atmospheric
Optics

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A white parhelic circle, almost a solid bar in intensity, sweeps horizontally from the sun off-image at left. At left is a blindingly bright sundog from horizontal plates and a weaker 22° halo.

Inside the 22° halo and inclined upwards is a rare helic arc from Parry oriented crystals.

At right, horizontal column crystal supralateral and infralateral arcs intersect in a cusp at the parhelic circle.   The brightening of the supralateral arc at the picture’s top is a rarer Parry supralateral arc.

Halos are labelled in the 100 million ray HaloSim ray tracing at left.




The cusp at the parhelic circle is a sure way to distinguish supralateral/infralateral arcs from the rarer 46° halo.   Colours of these halos are widely spread because the dispersion is through crystal faces inclined at 90° compared with the 60° of halos closer to the sun.




Halo displays do not come better than this.  At the risk of slightly impairing the magic, the crystals might not have been entirely natural. Crystals of exceptional optical quality form slowly downwind of the nuclei emitted by snow blowers and one was operating about 5 km distant. However, we do not know whether it was a contributor.