South Pole Halos - Sunwards

South Pole 11th January 1999.

The sun circles the snow and ice of the polar plateau at a near constant altitude of 22º.

Marko Riikonen's fisheye lens view is 180º wide encompassing the zenith and half the horizon.

Mouse over the "Halo key" to identify the halos.

Low level diamond dust ice crystals produce the halos; some nearby crystals show up as specks and streaks. The crystals are plate and column shaped. All orientations are present with the notable exception of that of Lowitz.

Halos frequently viewed singly in more temperate skies blaze all together with unacustomed brilliance. A 22º halo circles the sun, topped by an upper tangent arc. Well above that shines a circumzenithal arc. Bright sundogs lie on a complete parhelic circle.

Twice as far from the sun as the 22º halo lie the rare supralateral and infralateral arcs. Atop the upper tangent arc is an intense suncave Parry arc.

A 46º halo shows faintly near the parhelic circle where it is less obscured by brighter arcs.

But for halo hunters, the real gems are the very rare arcs here visible all together. The brightenings on the supralateral and infralateral arcs are Parry supra- and infralateral arcs made by rays passing between the end and sloping side faces of Parry oriented crystals. These are the same ray paths that in singly oriented columns form 'ordinary' supra- and infralateral arcs. The arcs are alternatively known as Tape arcs.

The thin diagonal cross through the sun and extending high towards the zenith is a heliac arc. To make it, sunlight has to reflect from the external (and sometimes the internal) faces of crystals with exquisitely precise Parry orientations.

Tangential to the heliac arc at its furthermost extension is the antisolar arc (also called the subanthelic arc), formed by precisely oriented Parry columns of superb optical quality.

Some very rare arcs (subhelic, Wegener, Hastings) labelled on this image are better visible on other views and are described there. And those views show other rare arcs too.

	Heliac arc Rays in the blue path reflect externally from the lower slanting side face of a Parry oriented crystal. In the second, purple path rays enter the uppermost horizontal face, reflect internally from an upward slanting side face and then leave through a downward slanting side face.

	Antisolar arc (subanthelic arc) Rays follow the same path as that of the purple heliac arc ray but there is an additional internal reflection off an end face. Other much more complicated ray paths also form this arc. The arc passes through the antisolar point and was first explained by R.A.R. Tricker in 1979.